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Theory of Intelligent Design, the best explanation of Origins » Molecular biology of the cell » Mitosis and Cell Division

Mitosis and Cell Division

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1 Mitosis and Cell Division on Thu May 07, 2015 5:08 am


Cellular reproduction: Mitosis

16 cell-cycle regulators are essential. If one is missing, the cell-cycle is not completed. Therefore, the regulation of the cell cycle is irreducibly complex:

CDK2  No reduplication, normal duplication, needed for duplication in absence of CDK1
Separase (Xl) No centriole disengagement, impaired duplication
Spliced Sgo1 (Mm) Precocious centriole disengagement
p53 (Mm, Hs) Amplification
CHK1 (Gg, Hs) No centrosome amplifi cation upon DNA damage
PLK1 (Hs) No reduplication in S phase-arrested cells
PLK2 (Hs) No reduplication in S phase-arrested cells
MPS1 (Hs, Mm, Sc) No reduplication (Hs, Mm; reports differ); normal
duplication (Dm); no spindle-pole-body duplication
BRCA1 (Hs, Mm) Premature centriole separation and reduplication
in S-G2 boundary (Hs); amplification (Mm)
Cdc14B (Hs) Amplification
PP2 (Dm) Centrosome amplification Overexpression: prevents reduplication
Nucleophosmin/B23 (Mm, Hs) Amplification
CAMKII (Xl) Blocks early steps in duplication
CDK1 (Dm, Sc) Amplification
Skp1, Skp2, Cul1, Slimb (SCF Complex) (Dm, Xl, Mm, Hs) Blocks separation of M-D pairs and reduplication,  increased centrosome number 

Mitotic cell division is the most fundamental task of all living cells. Cells have intricate and tightly regulated machinery to ensure that mitosis occurs with appropriate frequency and high fidelity. 1

According to the third tenet of the cell theory, new cells originate only from other living cells. The process by which this occurs is called cell division. For a multicellular organism, such as a human or an oak tree, countless divisions of a single-celled zygote produce an organism of astonishing cellular complexity and organization. Cell division does not stop with the formation of the mature organism but continues in certain tissues throughout life. Millions of cells residing within the marrow of your bones or the lining of your intestinal tract are undergoing division at this very moment. This enormous output of cells is needed to replace cells that have aged or died. Although cell division occurs in all organisms, it takes place very differently in prokaryotes and eukaryotes. We will restrict discussion to the eukaryotic version. Two distinct types of eukaryotic cell division will be discussed in this chapter. Mitosis leads to production of cells that are genetically identical to their parent, whereas meiosis leads to production of cells with half the genetic content of the parent. Mitosis serves as the basis for producing new cells, meiosis as the basis for producing new


The kinetochore is a network of protein complexes that assembles on centromere regions of chromatin and acts as the connection point between chromatids and the spindle microtubules that segregate them into daughter cells. Kinetochores must sense microtubule attachment and determine whether this attachment of sister chromatids is to the same or different spindle poles. The stable engagement of a kinetochore is regulated by the Aurora B kinase, the activity of which has been shown to reduce the affinity of several kinetochore proteins for microtubules. A single unattached or incorrectly attached kinetochore is sufficient to trigger the spindle assembly checkpoint and halt progress into anaphase, preventing cell division. 1  Once all of the chromosomes in a cell preparing to divide are correctly bioriented and satisfy the spindle assembly checkpoint, chromosomes must be pulled apart into daughter cells. An essential function of the kinetochore is to couple chromosome movement to microtubule depolymerization. Kinetochores are able to track depolymerizing microtubule ends and harness the energy released during microtubule depolymerization to move chromosomes to opposite spindle poles.

Kinetochores attach sister chromatids to the Spindle

Following the assembly of a bipolar microtubule array, the second major step in spindle formation is the attachment of the array to the sister-chromatid pairs. Spindle microtubules become attached to each chromatid at its kinetochore, a giant, multilayered protein structure that is built at the centromeric region of the chromatid (see picture below)

The kinetochore. see below (A) A fluorescence micrograph of a metaphase chromosome stained with a DNA-binding fluorescent dye and with human autoantibodies that react with specific kinetochore proteins. The two kinetochores, one associated with each sister chromatid, are stained red. (B) A drawing of a metaphase chromosome showing its two sister chromatids attached to the plus ends of kinetochore microtubules. Each kinetochore forms a plaque on the surface of the centromere. (C) Electron micrograph of an anaphase chromatid with microtubules attached to its kinetochore. While most kinetochores have a trilaminar structure, the one shown here (from a green alga) has an unusually complex structure with additional layers.

In metaphase, the plus ends of kinetochore microtubules are embedded head-on in specialized microtubuleattachment sites within the outer region of the kinetochore, furthest from the DNA. The kinetochore of an animal cell can bind 10–40 microtubules, whereas a budding yeast kinetochore can bind only one. Attachment of each microtubule depends on multiple copies of a rod-shaped protein complex called the Ndc80 complex, which is anchored in the kinetochore at one end and interacts with the sides of the microtubule at the other, thereby linking the microtubule to the kinetochore while still allowing the addition or removal of tubulin subunits at this end.

Regulation of plus-end polymerization and depolymerization at the kinetochore is critical for the control of chromosome movement on the spindle. Kinetochore attachment to the spindle occurs by a complex sequence of events. At the end of prophase in animal cells, the centrosomes of the growing spindle generally lie on opposite sides of the nuclear envelope. Thus, when the envelope breaks down, the sister-chromatid pairs are bombarded by microtubule plus ends coming from two directions. However, the kinetochores do not instantly achieve the correct ‘end-on’ microtubule attachment to both spindle poles. Instead, detailed studies with light and electron microscopy show that most initial attachments are unstable lateral attachments, in which a kinetochore attaches to the side of a passing microtubule, with assistance from kinesin motor proteins in the outer kinetochore. Soon, however, the dynamic microtubule plus ends capture the kinetochores in the correct end-on orientation

Another attachment mechanism also plays a part, particularly in the absence of centrosomes. Careful microscopic analysis suggests that short microtubules in the vicinity of the chromosomes become embedded in the plus-end-binding sites of the kinetochore. Polymerization at these plus ends then results in growth of the microtubules away from the kinetochore. The minus ends of these kinetochore microtubules are eventually cross-linked to other minus ends and focused by motor proteins at the spindle pole.

Bi-orientation is achieved by trial and error

The success of mitosis demands that sister chromatids in a pair attach to opposite poles of the mitotic spindle, so that they move to opposite ends of the cell when they separate in anaphase. How is this mode of attachment, called bi-orientation, achieved? What prevents the attachment of both kinetochores to the same spindle pole or the attachment of one kinetochore to both spindle poles? Part of the answer is that sister kinetochores are constructed in a back-to-back orientation that reduces the likelihood that both kinetochores can face the same spindle pole. Nevertheless, incorrect attachments do occur, and elegant regulatory mechanisms have evolved to correct them. Incorrect attachments are corrected by a system of trial and error that is based on a simple principle: incorrect attachments are highly unstable and do not last, whereas correct attachments become locked in place. How does the kinetochore sense a correct attachment? The answer appears to be tension .

When a sister-chromatid pair is properly bi-oriented on the spindle, the two kinetochores are pulled in opposite directions by strong poleward forces. Sister-chromatid cohesion resists these poleward forces, creating high levels of tension within the kinetochores. When chromosomes are incorrectly attached—when both sister chromatids are attached to the same spindle pole, for example—tension is low and the kinetochore sends an inhibitory signal that loosens the grip of its microtubule attachment site, allowing detachment to occur. When bi-orientation occurs, the high tension at the kinetochore shuts off the inhibitory signal, strengthening microtubule attachment. In animal cells, tension not only increases the affinity of the attachment site but also leads to the attachment of additional microtubules to the kinetochore. This results in the formation of a thick kinetochore fiber composed of multiple microtubules. The tension-sensing mechanism depends on the protein kinase Aurora-B, which is associated with the kinetochore and is thought to generate the inhibitory signal that reduces the strength of microtubule attachment in the absence of tension. It phosphorylates several components of the microtubule attachment site, including the Ndc80 complex, decreasing the site’s affinity for a microtubule plus end. When bi-orientation occurs, the resulting tension somehow reduces phosphorylation by Aurora-B, thereby increasing the affinity of the attachment site.

How tension might increase microtubule attachment to the kinetochore. ( see picture below ) These diagrams illustrate one speculative mechanism by which bi-orientation might increase microtubule attachment to the kinetochore. A single kinetochore is shown for clarity; the spindle pole is on the right. (A) When a sisterchromatid pair is unattached to the spindle or attached to just one spindle pole, there is little tension between the outer and inner kinetochores. The protein kinase Aurora-B is tethered to the inner kinetochore and phosphorylates the microtubule attachment sites, including the Ndc80 complex (blue), in the outer kinetochore as shown, thereby reducing the affinity of microtubule binding. Microtubules therefore associate and dissociate rapidly, and attachment is unstable. (B) When bi-orientation is achieved, the forces pulling the kinetochore toward the spindle pole are resisted by forces pulling the other sister kinetochore toward the opposite pole, and the resulting tension pulls the outer kinetochore away from the inner kinetochore. As a result, Aurora-B is unable to reach the outer kinetochore, and microtubule attachment sites are not phosphorylated. Microtubule binding affinity is therefore increased, resulting in the stable attachment of multiple microtubules to both kinetochores. The dephosphorylation of outer kinetochore proteins depends on a phosphatase that
is not shown here.

Following their attachment to the two spindle poles, the chromosomes are tugged back and forth, eventually assuming a position equidistant between the two poles, a position called the metaphase plate. In vertebrate cells, the chromosomes then oscillate gently at the metaphase plate, awaiting the signal for the sister chromatids to separate. The signal is produced, with a predictable lag time, after the bi-oriented attachment of the last of the chromosomes.
Multiple Forces Act on Chromosomes in the Spindle

Multiple mechanisms generate the forces that move chromosomes back and forth after they are attached to the spindle, and produce the tension that is so important for the stabilization of correct attachments. In anaphase, similar forces pull the separated chromatids to opposite ends of the spindle. Three major spindle forces are particularly critical, although their strength and importance vary at different stages of mitosis. The first major force pulls the kinetochore and its associated chromatid along the kinetochore microtubule toward the spindle pole. It is produced by proteins at the kinetochore itself. By an uncertain mechanism, depolymerization at the plus end of the microtubule generates a force that pulls the kinetochore poleward. This force pulls on chromosomes during prometaphase and metaphase but is particularly important for moving sister chromatids toward the poles after they separate in anaphase. Interestingly, this kinetochore-generated poleward force does not require ATP or motor proteins. This might seem implausible at first, but it has been shown that purified kinetochores in a test tube, with no ATP present, can remain attached to depolymerizing microtubules and thereby move. The energy that drives the movement is stored in the microtubule and is released when the microtubule depolymerizes; it ultimately comes from the hydrolysis of GTP that occurs after a tubulin subunit adds to the end of a microtubule

How does plus-end depolymerization drive the kinetochore toward the pole?  Ndc80 complexes in the kinetochore make multiple low-affinity attachments along the side of the microtubule. Because the attachments are constantly breaking and re-forming at new sites, the kinetochore remains attached to a microtubule even as the microtubule depolymerizes. In principle, this could move the kinetochore toward the spindle pole. A second poleward force is provided in some cell types by microtubule flux, whereby the microtubules themselves are pulled toward the spindle poles and dismantled at their minus ends. The mechanism underlying this poleward movement is not clear, although it might depend on forces generated by motor proteins and minus-end depolymerization at the spindle pole. In metaphase, the addition of new tubulin at the plus end of a microtubule compensates for the loss of tubulin at the minus end, so that microtubule length remains constant despite the movement of microtubules toward the spindle pole 

Any kinetochore that is attached to a microtubule undergoing such flux experiences a poleward force, which contributes to the generation of tension at the kinetochore in metaphase. Together with the kinetochore-based forces discussed above, flux also contributes to the poleward forces that move sister chromatids after they separate in anaphase.

The APC/C Triggers Sister-Chromatid Separation and the Completion of Mitosis

After M-Cdk has triggered the complex processes leading up to metaphase, the cell cycle reaches its climax with the separation of the sister chromatids at the metaphase-to-anaphase transition

Although M-Cdk activity sets the stage for this event, the anaphase-promoting complex (APC/C)  throws the switch that initiates sister-chromatid separation by ubiquitylating several mitotic regulatory proteins and thereby triggering their destruction  During metaphase, cohesins holding the sister chromatids together resist the poleward forces that pull the sister chromatids apart. Anaphase begins with the sudden loss of sister-chromatid cohesion, which allows the sisters to separate and move to opposite poles of the spindle. The APC/C initiates the process by targeting the inhibitory protein securin for destruction. Before anaphase, securin binds to and inhibits the activity of a protease called separase. The destruction of securin at the end of metaphase releases separase, which is then free to cleave one of the subunits of cohesin. The cohesins fall away, and the sister chromatids

In addition to securin, the APC/C also targets the S- and M-cyclins for destruction, leading to the loss of most Cdk activity in anaphase. Cdk inactivation allows phosphatases to dephosphorylate the many Cdk target substrates in the cell, as required for the completion of mitosis and cytokinesis. If the APC/C triggers anaphase, what activates the APC/C? The answer is only partly known. As mentioned earlier, APC/C activation requires binding to the protein Cdc20. At least two processes regulate Cdc20 and its association with the APC/C. First, Cdc20 synthesis increases as the cell approaches mitosis, owing to an increase in the transcription of its gene. Second, phosphorylation of the APC/C helps Cdc20 bind to the APC/C, thereby helping to create an active complex. Among the kinases that phosphorylate and thus activate the APC/C is M-Cdk. Thus, M-Cdk not only triggers the early mitotic events leading up to metaphase, but it also sets the stage for progression into anaphase. The ability of M-Cdk to promote Cdc20–APC/C activity creates a negative feedback loop: M-Cdk sets in motion a regulatory process that leads to cyclin destruction and thus its own inactivation.

Unattached Chromosomes Block Sister-Chromatid Separation:
The Spindle Assembly Checkpoint

A spindle assembly checkpoint mechanism  ensures that cells do not enter anaphase until all chromosomes are correctly bi-oriented on the mitotic spindle. The spindle assembly checkpoint depends on a sensor mechanism that monitors the strength of microtubule attachment at the kinetochore, possibly by sensing tension. Any kinetochore that is not properly attached to the spindle sends out a diffusible negative signal that blocks Cdc20–APC/C activation throughout the cell
and thus blocks the metaphase-to-anaphase transition. When the last sister-chromatid pair is properly bi-oriented, this block is removed, allowing sister-chromatid
separation to occur.

The negative checkpoint signal depends on several proteins, including Mad2, which are recruited to unattached kinetochores . Detailed structural analyses of Mad2 suggest that the unattached kinetochore acts like an enzyme that catalyzes a change in the conformation of Mad2, so that Mad2, together with other proteins, can bind and inhibit Cdc20–APC/C. In mammalian somatic cells, the spindle assembly checkpoint determines the normal timing of anaphase. The destruction of securin in these cells begins moments after the last sister-chromatid pair becomes bi-oriented on the spindle, and anaphase begins about 20 minutes later. Experimental inhibition of the checkpoint mechanism causes premature sister-chromatid separation and anaphase. Surprisingly, the normal timing of anaphase does not depend on the spindle assembly checkpoint in some cells, such as yeasts and the cells of early frog and fly embryos. Other mechanisms, as yet unknown, must determine the timing of anaphase in these cells.

Chromosomes Segregate in Anaphase A and B

The sudden loss of sister-chromatid cohesion at the onset of anaphase leads to sister-chromatid separation, which allows the forces of the mitotic spindle to pull the sisters to opposite poles of the cell—called chromosome segregation. The chromosomes move by two independent and overlapping processes. The first, anaphase A, is the initial poleward movement of the chromosomes, which is accompanied by shortening of the kinetochore microtubules. The second, anaphase B, is the separation of the spindle poles themselves, which begins after the sister chromatids have separated and the daughter chromosomes have moved some distance apart.

Chromosome movement in anaphase A depends on a combination of the two major poleward forces described earlier. The first is the force generated by microtubule depolymerization at the kinetochore, which results in the loss of tubulin subunits at the plus end as the kinetochore moves toward the pole. The second is provided by microtubule flux, which is the poleward movement of the microtubules toward the spindle pole, where minus-end depolymerization occurs. The relative importance of these two forces during anaphase varies in different cell types: in embryonic cells, chromosome movement depends mainly on microtubule flux, for example, whereas movement in yeast  and vertebrate somatic cells results primarily from forces generated at the kinetochore. Spindle-pole separation during anaphase B depends on motor driven mechanisms similar to those that separate the two centrosomes in early mitosis. Plusend directed kinesin-5 motor proteins, which cross-link the overlapping plus ends of the interpolar microtubules, push the poles apart. In addition, dynein motors that anchor astral microtubule plus ends to the cell cortex pull the poles apart.

Although sister-chromatid separation initiates the chromosome movements of anaphase A, other mechanisms also ensure correct chromosome movements in anaphase A and spindle elongation in anaphase B. Most importantly, the completion of a normal anaphase depends on the dephosphorylation of Cdk substrates, which in most cells results from the APC/C-dependent destruction of cyclins. If M-cyclin destruction is prevented—by the production of a mutant form that is not recognized by the APC/C, for example—sister-chromatid separation generally occurs, but the chromosome movements and microtubule behavior of anaphase are abnormal. The relative contributions of anaphase A and anaphase B to chromosome segregation vary greatly, depending on the cell type. In mammalian cells, anaphase B begins shortly after anaphase A and stops when the spindle is about twice its metaphase length; in contrast, the spindles of  yeasts and certain protozoa primarily use anaphase B to separate the chromosomes at anaphase, and their spindles elongate to up to 15 times their metaphase length.

Segregated Chromosomes Are Packaged in Daughter Nuclei at Telophase

By the end of anaphase, the daughter chromosomes have segregated into two equal groups at opposite ends of the cell. In telophase, the final stage of mitosis, the two sets of chromosomes are packaged into a pair of daughter nuclei. The first major event of telophase is the disassembly of the mitotic spindle, followed by the re-formation of the nuclear envelope. Initially, nuclear membrane fragments associate with the surface of individual chromosomes. These membrane fragments fuse to partly enclose clusters of chromosomes and then coalesce to reform the complete nuclear envelope. Nuclear pore complexes are incorporated into the envelope, the nuclear lamina re-forms, and the envelope once again becomes continuous with the endoplasmic reticulum. Once the nuclear envelope has re-formed, the pore complexes pump in nuclear proteins, the nucleus expands, and the mitotic chromosomes are reorganized into their interphase state, allowing gene transcription to resume. A new nucleus has been created, and mitosis is complete. All that remains is for the cell to complete its division into two. We saw earlier that phosphorylation of various proteins by M-Cdk promotes spindle assembly, chromosome condensation, and nuclear-envelope breakdown in early mitosis. It is thus not surprising that the dephosphorylation of these same proteins is required for spindle disassembly and the re-formation of daughter nuclei in telophase. In principle, these dephosphorylations and the completion of mitosis could be triggered by the inactivation of Cdks, the activation of phosphatases, or both. Although Cdk inactivation—resulting primarily from cyclin destruction—is mainly responsible in most cells, some cells also rely on activation of phosphatases. In budding yeast, for example, the completion of mitosis depends on the activation of a phosphatase called Cdc14, which dephosphorylates
a subset of Cdk substrates involved in anaphase and telophase.


M-Cdk triggers the events of early mitosis, including chromosome condensation, assembly of the mitotic spindle, and bipolar attachment of the sister-chromatid pairs to microtubules of the spindle. Spindle formation in animal cells depends largely on the ability of mitotic chromosomes to stimulate local microtubule nucleation and stability, as well as on the ability of motor proteins to organize microtubules into a bipolar array. Many cells also use centrosomes to facilitate spindle assembly. Anaphase is triggered by the APC/C, which stimulates the destruction of the proteins that hold the sister chromatids together. APC/C also promotes cyclin destruction and thus the inactivation of M-Cdk. The resulting dephosphorylation of Cdk targets is required for the events that complete mitosis, including the disassembly of the spindle and the re-formation of the nuclear envelope.

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The machinery of mitosis: Kinetechores, centrioles and chromosome pumps

Mitosis-- Movie Narrative 1

Cell division is required for an organism to grow, mature, and maintain tissues. During the mitotic phase, a cell will undergo mitosis to form two new nuclei and then divide to form two new individual cells during cytokinesis.

Mitosis is the process of dividing the duplicated DNA of a cell into two new nuclei. Mitosis is split into distinct stages. The first stage is prophase; the DNA condenses, organizes, and the classic chromosome structure appears. Next comes prometaphase where microtubules attach to the chromosomes. This step is followed by metaphase where the chromosomes align. Metaphase is followed by anaphase where the chromosomes separate. Finally, during telophase nuclear membranes reappear around the two sets of chromosomes. Mitosis is now complete. After mitosis two new cells are formed by a process called cytokinesis.

Mitosis is only one part of what is called the cell cycle. For many eukaryotic cells, a cell is duplicated every 24 hours. Most of the life of a cell is spent in interphase. Interphase consists of three stages called G1, S, and G2.

G1 (or Gap 1) is the first growth stage of interphase. In G1, the cell grows to nearly its full size and performs many of its specific biochemical functions that aid the organism.

Next is the S (or synthesis) phase. This is an important stage, because it is during the S phase that DNA in the nucleus is replicated.

The cell next enters another growth stage called G2 (or Gap 2). It is during G2 that the cell finishes growing. Once the cell has duplicated DNA in the nucleus, and two centrosomes have appeared in the cytoplasm, mitosis can begin. For a typical eukaryotic cell this will last about 80 minutes.

During the first stage of mitosis, called prophase, we first see the classic chromosome structure. This occurs through a condensation process. At the same time, protein strands called microtubules appear from the centrosomes in animals. Finally, a structure found within the nucleus, the nucleolus, disappears.

Next, prometaphase begins when the nuclear membrane is broken down. At the same time, microtubule strands, or spindle fibers, are growing from the centrosomes. These strands attach to a protein structure called the kinetochore. One kinetochore is attached to the centromere of each sister chromatid.

Next comes metaphase. During this stage the sister chromatids align along the center of the cell so that both chromatids face toward opposite poles of the cell.

Now the sister chromatids are ready to be separated. This occurs during anaphase through a shortening of the microtubules attached to the kinetochores. Additionally, the poles of the cell move farther apart and cause increased separation of sister chromatids. At the end of anaphase, the sister chromatids have moved to the two ends of the cell.

Telophase is the final stage of mitosis. It is here the components of the new cells begin to appear. At this point the spindle fibers are broken up. A new nuclear membrane surrounds the chromosomes at the end of each cell. And the chromosomes uncoil and return to an uncondensed state. Mitosis is now complete. The formation of two cells is all that remains.

Following mitosis, the cell undergoes a process called cytokinesis. First the cell is compressed by a contractile ring that divides the cell in nearly equal halves. By now the organelles in the cell have been replicated, and are now divided between the two halves of the cell. This includes mitochondria, golgi bodies, and the rough ER. Plant cells also have chloroplasts. Once split, the two new cells are now fully in the G1 stage of interphase and ready again to begin their growth.

Let’s watch the process one more time. Mitosis begins with prophase. Notice the DNA condensing into chromosomes during this stage. Microtubules appear during prometaphase, and the nuclear membrane breaks down. Metaphase occurs when the chromosomes are aligned at the center of the cell. During anaphase the chromosomes are moving apart. The telophase stage is marked by the appearance of new nuclear membranes. This is the end of mitosis.

Finally, the splitting of the cell occurs during cytokinesis. The two new cells are now ready to grow and perform their specialized functions.

( —At the cellular level, the mitotic spindle apparatus is arguably the most complicated piece of machinery in existence. Its basic function is to isolate and separate the chromosomes during cell division. A group of researchers at the University of North Carolina have been piecing together a model of the spindle and associated proteins which provides a way to visualize in detail exactly what might be going on. The group chose to simulate budding yeast cells because their entire spindle is comprised of only around 40 microtubules (MTs), compared to 100 times that amount in mammalian cells. Over the years the group has contributed to an emerging mechanical picture of the spindle wherein the MTs provide the compression elements, pericentric chromatin the elastic tension elements, and a proteinaceous kinetochore bridges the two polymers together. Their most recent paper, published in Current Biology, provides a new and detailed 3d map of the kinetochore region of the chromosome, and seeks to provides answers to the origins of the seemingly mysterious force that organizes the dividing cell.

Local force generation through the action of motor proteins, like kinesin and dynein, provides an important directional instructor to the spindle. However just as these components are likely not them main source of motility for large organelles like mitochondria, a more holistic view of spindle behavior suggests that a more diffuse, entropic influence constitutes the invisible hand that guides its overall dynamics. One potential source of force generation is provided by the chromosome pumps, if you will, that act like an osmotic contractile gel, expanding and constricting throughout mitosis—perhaps not completely unlike the recently revealed mechanism used in Herpes virus infection. Here the herpes DNA and associated matrix, under extreme pressure, literally blasts itself into the host cell.
The authors note that the entropic elasticity of chromosomal DNA can act to reel the spindle "arms" in to the spindle pole, just as one end of a spring recoils when the other end is pulled to a fxed point. This mechanism of entropic recoil has already been indicated to act in the segregation of replicated DNA in simpler bacteria. One can watch spindle behaviors under the microscope, and see these kinds of mechanisms at work, but they cannot be described very well in paper form by simply detailing lists of binding interactions. So these days, it now seems that in cell biology at least, the model provides the best way to move understanding forward, with parametrically-fed simulations providing the feedstock. Ideally, just like in parametric CAD, if the variables that specify the critical metrics are set up and linked properly, you can change one parameter, and those associated to it seamlessly follow suit without the need to create from scratch.
A Matlab/Simulink model was used to generate population measurements of spindle lengths for 16 left and 16 right kinetochore MTs, where each kinetochore has its own associated MT. The entire model contains spindle pole bodies (centrosomes), linked by the kinetochore MTs, with the main barrel structure comprised of coiled DNA tubes. These tubes are stitched together by cohesin rings and condensin linker molecules. In more recent models, the researchers are looking to constrain the unsolved structural details at the surface of the so-called inner kinetochore plate which resides at the kinetochore-MT interface.

In particular, they are looking to understand the distribution of critical molecules like Cse4 and Ndc80. To this end they created a model based on stochastic growth and shortening of spindles and kinetochore MTs. A cylindrical spindle with a diameter of 250nm was created based on geometry obtained form electron microscopy and tomography. Experimentally, flourophores were placed at the MT plus ends and used to simulate the distribution of Ndc80.
The results of these efforts are not best transmitted in word from. Nowadays collaborations like this are often headed up by artisans that recruit the biology and computer tech experts needed to create a visual simulation. The Harvard Biovisions series, or the illustrative DNA compaction video below are well known examples. The larger public may have to wait a little while to see the fruits of these new labors which seek to make the unseeable biology visible. The mitotic spindle may perhaps be the feature film that many haven't even realized they have been waiting for.

More information: A 3D Map of the Yeast Kinetochore Reveals the Presence of Core and Accessory Centromere-Specific Histone, Current Biology, 26 September 2013.
The budding yeast kinetochore is ~68 nm in length with a diameter slightly larger than a 25 nm microtubule. The kinetochores from the 16 chromosomes are organized in a stereotypic cluster encircling central spindle microtubules. Quantitative analysis of the inner kinetochore cluster (Cse4, COMA) reveals structural features not apparent in singly attached kinetochores. The cluster of Cse4-containing kinetochores is physically larger perpendicular to the spindle axis relative to the cluster of Ndc80 molecules. If there was a single Cse4 (molecule or nucleosome) at the kinetochore attached to each microtubule plus end, the cluster of Cse4 would appear geometrically identical to Ndc80. Thus, the structure of the inner kinetochore at the surface of the chromosomes remains unsolved. We have used point fluorescence microscopy and statistical probability maps to deduce the two-dimensional mean position of representative components of the yeast kinetochore relative to the mitotic spindle in metaphase. Comparison of the experimental images to three-dimensional architectures from convolution of mathematical models reveals a pool of Cse4 radially displaced from Cse4 at the kinetochore and kinetochore microtubule plus ends. The pool of displaced Cse4 can be experimentally depleted in mRNA processing pat1Δ or xrn1Δ mutants. The peripheral Cse4 molecules do not template outer kinetochore components. This study suggests an inner kinetochore plate at the centromere-microtubule interface in budding yeast and yields information on the number of Ndc80 molecules at the microtubule attachment site.


Origin of the cell nucleus, mitosis and sex: roles of intracellular coevolution

The transition from prokaryotes to eukaryotes was the most radical change in cell organisation since life began, with the largest ever burst of gene duplication and novelty. According to the coevolutionary theory of eukaryote origins, the fundamental innovations were the concerted origins of the endomembrane system and cytoskeleton, subsequently recruited to form the cell nucleus and coevolving mitotic apparatus, with numerous genetic eukaryotic novelties inevitable consequences of this compartmentation and novel DNA segregation mechanism. Physical and mutational mechanisms of origin of the nucleus are seldom considered beyond the long-standing assumption that it involved wrapping pre-existing endomembranes around chromatin. Discussions on the origin of sex typically overlook its association with protozoan entry into dormant walled cysts and the likely simultaneous coevolutionary, not sequential, origin of mitosis and meiosis.

The problem of nuclear origins therefore requires understanding coevolution of about 27 cell components and how they became functionally interlinked into the fundamentally novel eukaryotic life cycle

Peroxisomes, mitochondria, centrioles, cilia, and Golgi dictyosomes must also have originated prior to the last common ancestor of all extant eukaryotes.

This radical transformation of cell structure (eukaryogenesis) is the most complex and extensive case of quantum evolution in the history of life haha.

there are three crucial problems for understanding the origin of the nucleus [5]: (1) assembly of endomembranes around chromatin (the DNA-histone complex); (2) evolution of the nuclear pore complex (NPC), which crucially allows a channel between nucleoplasm and cytoplasm; and (3) origin of centromeres and mitotic spindle, without which nuclear chromosomes cannot be stably inherited.

origin of the cell nucleus cannot be understood in isolation from other major innovations of the eukaryotic cell; intracellular coevolution among different cell constituents that interact physically or that profoundly affect selective forces acting on each other is the key to understanding eukaryote origins

Intracellular coevolution of about a 100 novel properties is at the core of understanding eukaryogenesis.

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3 Re: Mitosis and Cell Division on Wed Jun 10, 2015 10:01 pm


Kinetochores (even the Simplest ones) are more complex than the bacterial motor. These irreducibly complex devices are located on the center of the chromosomes,: which is prepared for distribution during mitosis. Such fused chromosomes is called "chromosome metaphase". Kinetochores are placed in the middle of this chromosome. Kinetochore is attached to a DNA sequence: which is called "centromere". Next to this sophisticated and complex "hook" attaches microtubule and drags chromosome to the daughter cells;

"The simplest characterized kinetochore is in budding yeast where 38 structural proteins from various subcomplexes assemble on centromeric DNA to form a single microtubule binding site. The majority of yeast kinetochore proteins are conserved, and it is thought that kinetochores in multicellular eukaryotes are simply repeated units of the budding yeast kinetochore. Yeast kinetochores are therefore ideally suited for biochemical and structural studies that should be applicable to all eukaryotic kinetochores. Although a number of models of kinetochore structure have been proposed based on protein-protein interaction experiments and physical studies of individual subcomplexes, little is known about the architecture of an intact kinetochore and the mechanism by which kinetochores bind to and maintain attachments to dynamic microtubules.

What is the EM structure of a native kinetochore?

We are collaborating with Tamir Gonen’s laboratory at Janelia Farms to perform electron microscopy to elucidate the overall architecture of kinetochores purified from yeast.

How do kinetochores assemble?

We are using minichromosomes to develop assays to monitor the assembly requirements for kinetochores in vitro.

How do kinetochore proteins interact with each other and microtubules?

We are developing MS-crosslinking approaches in collaboration with Jeff Ranish’s lab at the ISB to identify protein-protein contacts within the kinetochore and between the kinetochore and microtubules.;

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4 Re: Mitosis and Cell Division on Wed Jun 10, 2015 10:02 pm



Have you ever wondered how the eukaryotic cell knows when to transition from one phase of its cycle to the next? How does the cell know to begin DNA replication (S phase) or mitosis (M phase), or to enter the two "gap phases" (G1 and G2) that separate them? How are previous phases halted and new phases initiated? What quality-control measures are in place to minimize mistakes?

The cell cycle is under tight regulatory control, ensuring that it operates in a beautifully systematic fashion. The remarkable molecular mechanisms underlying this control constitute one of the most astonishing processes in molecular biology.

The animation above illustrates the beauty of one of the stages -- indeed, the culmination -- of the eukaryotic cell cycle, involving mitotic cell division. In this and a subsequent article, I will explore these processes in more detail. Before we examine the ingenious mechanisms of cell cycle control, however, I will provide here some context by means of a short description of the different phases involved. The following information regarding the cell cycle can be found in any standard biology textbook.

Introducing the Cell Cycle

cell cycle.png

The eukaryotic cell cycle is divided into four phases, designated G1, S, G2, and M respectively (see the figure to the right). Collectively, the G1, S, and G2 phases are referred to as "interphase" (designated "I" in the figure), and take up approximately 90% of the cell's lifespan.

The Gap Phase 1 (G1 phase) is a period of cell growth that occurs prior to chromosome duplication. During the G1 phase, cells can exit from the cycle and enter a non-dividing state called G0. Alternatively, they can pass a so-called "restriction point" which commits them to the entire cell cycle. This irreversible decision is largely dependent on nutrient availability, and often cell size (for many types of cells, commitment to another round of division requires a critical cell size). External factors known as "growth factors" also play an important role in the regulation of the transition past the restriction point. Many human cells, such as nerve cells, are permanently arrested in the G0 state. Others can be induced to re-enter the cell cycle. Still others, such as skin cells, are constantly dividing. Cells that are in the G0 state have elevated concentrations of cell cycle inhibitors and are marked by an absence of DNA replication enzymes. Cells in the G1 state, conversely, are marked by low concentrations of cell cycle inhibitors and the presence of DNA replication enzymes.

S phase is the stage at which DNA replication takes place, producing two sister chromatids (identical copies of each chromosome). These sister chromatids are tethered together by a protein called cohesion until mitosis occurs. Complex regulatory mechanisms are in place to ensure that the DNA is replicated completely and accurately before the cell is allowed to enter the next phase of the cell cycle. DNA replication is itself an absolutely remarkable process involving many different specialized protein complexes. For an overview of some of the complexities associated with DNA replication, see my previous articles on the subject here, here, here, here, here, and here.

The Gap Phase 2 (G2 phase) is a second phase of growth prior to the separation of sister chromatids. During this phase, the mitotic spindle (i.e., the apparatus responsible for driving chromosome segregation) begins to form. Cellular content also increases further at this stage (this content will later be distributed between the two daughter cells). A cell cycle checkpoint occurs at this phase, and cell cycle progression can be temporarily arrested or paused if there is any chromosome damage, such as a double-stranded DNA break, until the damage is repaired.

M phase is the final stage of the cell cycle. During this phase, two major events occur: mitosis (segregation of sister chromatids into the opposite sides of the cell), and cytokinesis (division of the cell to produce two daughter cells). Chromatids attach to the mitotic spindle (made up of microtubules that radiate from microtubule organizing centers called centrosomes) at protein structures present on the chromatids called kinetochores. The mitotic spindle pulls the sister chromatids apart, creating two identical chromosome clusters, which will go on to populate the two resultant daughter cells. As is the case for S phase and the G2 phase, there are checkpoints in place to halt cell cycle progression if there are any problems such as chromosome mis-segregation or improper attachment of chromosomes to the spindle.

The Stages of Mitosis

There are a number of different mitotic stages. These are: prophase, prometaphase, metaphase, anaphase and telophase. During prophase, the chromatin condenses into the familiar chromosome structures in which the chromatin becomes visible under the microscope. During prometaphase, the nuclear membrane disintegrates and breaks into membrane vesicles. The kinetochores form during this stage, and become attached to the microtubules that radiate from the centrosomes at the spindle poles. During metaphase, the condensed chromosomes line up in the middle of the cell, driven by motor proteins (kinesin and dynein) associated with the microtubules. During anaphase, the chromosomes break up and the sister chromatids are pulled to the opposite poles of the cell. Finally, during telophase, which occurs at the same time as cytokinesis, two daughter nuclei are formed and the chromosomes unravel back into their original expanded chromatin formation. To better visualize the process of mitosis, I recommend viewing the video animation embedded at the top of this article. Here is another animation worth watching.


There is no question that the processes described above are truly astounding. In fact, when you consider the amazing regulatory control of just the microtubule-associated motor proteins (that are involved in a number of processes during mitosis, including the aligning of chromosomes along the cell's equator during metaphase), the complexity of the system is taken to a whole new level. Later, I will discuss the even more astonishing molecular mechanisms that underpin the cell cycle's regulation and quality control -- processes that typify the sheer engineering prowess of biological machines.

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The mechanism by which chromosomal DNA molecules are held together: entrapment within cohesin rings?

mysterious has been the trigger for what is arguably the most dramatic and one of the most highly regulated events in the life of a eukaryotic cell, the sudden disjunction of sister chromatids at the metaphase to anaphase transition. 1

Work in our lab has shown that sister chromatids are held together by a multi-subunit complex called cohesin whose Smc1 and Smc3 subunits are rod shaped proteins with ABC-like ATPases at one end of 50nm long intra-molecular anti-parallel coiled coils. At the other ends are pseudo-symmetrical hinge domains that interact to create V shaped Smc1/Smc3 heterodimers. N- and C-terminal domains within cohesin’s third subunit, known as α kleisin, bind to Smc3 and Smc1 ATPase heads respectively, thereby creating a huge tripartite ring whose integrity is essential for holding sister DNAs together. A thiol protease called separase opens the cohesin ring by cleaving its α kleisin subunit, which causes cohesin’s dissociation from chromosomes and triggers sister chromatid disjunction.


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6 Mitosis and Cell Division on Sun Jul 12, 2015 9:23 am


Mitosis and Cell Division

The five phases of mitosis and cell division tightly coordinate the movements of hundreds of proteins. 1

Perhaps the most amazing thing about mitosis is its precision, a feature that has intrigued biologists since Walther Flemming first described chromosomes in the late 1800s (Paweletz, 2001). Although Flemming was able to correctly deduce the sequence of events in mitosis, this sequence could not be experimentally verified for several decades, until advances in light microscopy made it possible to observe chromosome movements in living cells. Researchers now know that mitosis is a highly regulated process involving hundreds of different cellular proteins. The dynamic nature of mitosis is best appreciated when this process is viewed in living cells.

In his pioneering studies of mitosis, Flemming noted that the nuclear material, which he named "chromatin" for its ability to take up stains, did not have the same appearance in all cells. Specifically, in some cells, chromatin appeared as an amorphous network, although in other cells, it appeared as threadlike bodies that Flemming named "mitosen." Based on his observations, Flemming had the insight to propose that chromatin could undergo reversible transformations in cells. Today, scientists know that Flemming had successfully distinguished chromosomes in the interphase portion of the cell cycle from chromosomes undergoing mitosis, or the portion of the cell cycle during which the nucleus divides

A cell is shown changing its morphology and reorganizing its chromosomes in four sequential phases of the cell cycle, depicted here as a circle with clockwise arrows spanning each phase. During G1 phase, a cell evaluates whether environmental conditions are suitable for cell division. The cell contains uncondensed DNA inside its nucleus; the DNA looks like a tangled clump of green and orange thread. G1 phase is depicted as a purple, bifurcated arrow. The two branches of the arrow indicate the two pathways the cell might take. The branch that diverges to the left of the circle indicates that, if conditions are unfavorable for division, the cell will exit the cell cycle. The other G1 branch continues in the path of the cell cycle toward S phase. This is the path the cell will take if conditions are favorable for cell division. S phase is next and is depicted with a light blue arrow. During S phase, the DNA replicates, and in the image of the cell in S phase, the DNA appears to occupy more space inside the nucleus. S phase DNA still appears like a tangled clump of thread. G2 phase is depicted with a green arrow. During the G2 phase, the cell prepares for mitosis. Mitosis is depicted as a pink arrow. During mitosis, the nuclear envelope breaks down, DNA condenses, and chromosomes become distinct. The chromosomes separate into two groups, and at the conclusion of mitosis, the cell's cytoplasm divides to form two independent daughter cells. Each new cell contains its own nucleus, and each nucleus contains uncondensed green and orange DNA. M phase is followed by division of the cytoplasm by cytokinesis and this leads to G1 phase, completing the cycle. Interphase is comprised of G1, S, and G2 phases, and is represented by a thick, dark grey arrow under these phases.

With very few exceptions, mitosis occupies a much smaller fraction of the cell cycle than interphase.
The difference in DNA compaction between interphase and mitosis is dramatic. A precise estimate of the difference is not possible, but during interphase, chromatin may be hundreds or even thousands of times less condensed than it is during mitosis. For this reason, the enzyme complexes that copy DNA have the greatest access to chromosomal DNA during interphase, at which time the vast majority of gene transcription occurs. In addition, chromosomal DNA is duplicated during a subportion of interphase known as the S, or synthesis, phase. As the two daughter DNA strands are produced from the chromosomal DNA during S phase, these daughter strands recruit additional histones and other proteins to form the structures known as sister chromatids

The sister chromatids, in turn, become "glued" together by a protein complex named cohesin. Cohesin is a member of the SMC, or structural maintenance of chromosomes, family of proteins. SMC proteins are DNA-binding proteins that affect chromosome architectures; indeed, cells that lack SMC proteins show a variety of defects in chromosome stability or chromosome behavior. Current data suggest that cohesin complexes may literally form circles that encompass the two sister chromatids (Hirano, 2002; Hagstrom & Meyer, 2003). At the end of S phase, cells are able to sense whether their DNA has been successfully copied, using a complicated set of checkpoint controls that are still not fully understood. For the most part, only cells that have successfully copied their DNA will proceed into mitosis.

Chromatin Is Extensively Condensed as Cells Enter Mitosis

The most obvious difference between interphase and mitosis involves the appearance of a cell's chromosomes. During interphase, individual chromosomes are not visible, and the chromatin appears diffuse and unorganized. Recent research suggests, however, that this is an oversimplification and that chromosomes may actually occupy specific territories within the nucleus (Cremer & Cremer, 2001). In any case, as mitosis begins, a remarkable condensation process takes place, mediated in part by another member of the SMC family, condensin (Hirano, 2002; Hagstrom & Meyer, 2003). Like cohesin, condensin is an elongated complex of several proteins that binds and encircles DNA. In contrast to cohesin, which binds two sister chromatids together, condensin is thought to bind a single chromatid at multiple spots, twisting the chromatin into a variety of coils and loops

The Mitotic Spindle Aids in Chromosome Separation

During mitosis, chromosomes become attached to the structure known as the mitotic spindle. In the late 1800s, Theodor Boveri created the earliest detailed drawings of the spindle based on his observations of cell division in early Ascaris embryos

The composition of the spindle fibers remained unknown until the 1960s, when tubulin was discovered and techniques were developed for visualizing spindles using electron microscopes. It is now well-established that spindles are bipolar arrays of microtubules composed of tubulin

In panel A, four chromosomes are attached by their kinetochores to dark green microtubules. The remaining microtubules are light green, and contribute to the oviform shape of the spindle. A pair of cylindrical rods embedded in a yellow circle are labeled the "centrosome." Centrosomes are present at both poles of the spindle. In panel B, a mitotic spindle in anaphase is shown. Microtubules are labeled in green, and chromosomes are labeled in purple. Chromosomes have been pulled towards opposite poles of the spindle. A constricted region at each pole is labeled the "centrosome." Microtubules that are not part of the spindle radiate from the centrosomes in all directions.

and that the centrosomes nucleate the growth of the spindle microtubules. During mitosis, many of the spindle fibers attach to chromosomes at their kinetochores

which are specialized structures in the most constricted regions of the chromosomes. The length of these kinetochore-attached microtubules then decreases during mitosis, pulling sister chromatids to opposite poles of the spindle. Other spindle fibers do not attach to chromosomes, but instead form a scaffold that provides mechanical force to separate the daughter nuclei at the end of mitosis.

Mitosis Is Divided into Well-Defined Phases

From his many detailed drawings of mitosen, Walther Flemming correctly deduced, but could not prove, the sequence of chromosome movements during mitosis

Flemming divided mitosis into two broad parts: a progressive phase, during which the chromosomes condensed and aligned at the center of the spindle, and a regressive phase, during which the sister chromatids separated. Our modern understanding of mitosis has benefited from advances in light microscopy that have allowed investigators to follow the process of mitosis in living cells. Such live cell imaging not only confirms Flemming's observations, but it also reveals an extremely dynamic process that can only be partially appreciated in still images.

During interphase, which is shown in panel a, DNA replication occurs. The panel shows a cell before and after DNA replication. On the left is the cell before DNA replication. It is circular and contains a round, purple nucleus. The outline of the nucleus is labeled the nuclear envelope. Uncondensed green chromatin is contained inside the nucleus, and looks like a wad of green thread. In the cytoplasm at the top of the cell is a single centrosome that consists of a pair of centrioles, which are represented by two perpendicular, purple cylinders, and pericentriolar material, which is shown as a yellow cloud around the centriole pair. An arrow points to a schematic of the cell after it has undergone DNA replication. The nucleus of this cell contains uncondensed green and orange chromatin. The orange represents the newly synthesized DNA, and the green represents the original DNA. The cell has also duplicated its centrosome during DNA synthesis, and two centrosomes and their pericentriolar material are now shown next to each other at the top of the cell.

Panel b shows prophase, which is the first stage of mitosis. Early and late prophase are shown at the top in two illustrations of a cell. During prophase, the two centrosomes migrate to opposite sides of the cell. Some microtubules, which are represented by black lines, emanate from the centrosomes, but the microtubules are not in contact with the chromosomes. Chromosome condensation also occurs during prophase. During early prophase, the chromosomes are depicted as a tangled wad of green and orange threads. The late prophase cell has four distinct chromosomes. Each chromosome consists of a green chromatid and an orange chromatid. The chromatids resemble fat worms and are connected near their middles at centromeres, which are represented by black dots. In the late prophase illustration, a black line pointing to the microtubules is labeled "developing spindle." Also in panel b, two side-by-side photomicrographs show lily cells in early and late prophase. The microtubules in these cells are stained pink, and the chromosomes are stained purple. In the early prophase photomicrograph at left, the chromosomes are just beginning to condense, and the entire circular nucleus looks purple and speckled. The microtubules look like lines and are found throughout the cell's cytoplasm. In the late prophase photomicrograph at right, the chromosomes are distinct and look like noodles inside the nucleus. Empty space in the nucleus, no longer occupied by uncondensed DNA, appears white. The pink microtubules are found throughout the cytoplasm, but are more concentrated around the nucleus.

Panel c shows an illustration and a photomicrograph of cells in prometaphase. During prometaphase, the nuclear envelope breaks down. This allows microtubules to connect with the chromosomes at kinetochores, which are located at the centromeres. The microtubules that are connected to kinetochores are called kinetochore microtubules. A mitotic spindle forms with the centrosomes at the spindle poles. Each of the two sister chromatids that make up a chromosome is connected to an opposite spindle pole by kinetochore microtubules. The mitotic spindle contains both kinetochore microtubules as well as other microtubules that emanate from the centrosomes and connect in the center of the cell. At this stage, sister chromatids remain together, and although the four chromosomes are connected to the spindle, they are not aligned with each other. A photomicrograph of a lily cell in prometaphase shows worm-like chromosomes of various lengths spread throughout the horizontal, pink lines that are the mitotic spindle. Panel d shows an illustration and a photomicrograph of cells in metaphase. The illustration of the metaphase spindle, shown on the right, closely resembles the prometaphase spindle. The difference between the two is the position of the chromosomes: in metaphase, the four chromosomes are lined up one after the other in a vertical line at the midpoint between the two spindle poles on the metaphase plate. A dashed line running vertically through the middle of the cell represents the metaphase plate, which is an imaginary plane perpendicular to the spindle microtubules and midway between the two spindle poles. A photomicrograph of a lily cell in metaphase shows many pink, horizontal spindle microtubules in the middle of the cell. The chromosomes, which resemble purple worms, are lined up in the center of the spindle, equidistant from the two spindle poles. Because plant cells lack centrosomes, the microtubules at each pole have a more diffuse appearance; they don't form a single, discrete point of origin for the spindle as is shown in the illustrated animal cells. Panel e shows an illustration and a photomicrograph of cells in anaphase. In the illustration, sister chromatids have separated, and the chromosomes are being pulled toward opposite spindle poles. The chromosomes are still attached to microtubules by their kinetochores, and the kinetochore microtubules have shortened. When the sister chromosomes separate, one will go to one pole, and the other to the other pole. This way, each daughter cell will receive one copy of each chromosome. Each group of separating chromosomes contains both chromosomes with original DNA and chromosomes with newly synthesized DNA. A photomicrograph of a lily cell in anaphase is similar to that of the lily cell is metaphase. The spindle is composed of many horizontal microtubules. The chromosomes are migrating toward opposite spindle poles; one group of chromosomes is moving to the left pole, and the other group is moving to the right pole. Some microtubules are visible between the separating chromosomes. These interdigitating microtubules are interpolar microtubules that provide stability to the spindle and aid in the separation of chromosomes during anaphase. Panel f has a photomicrograph and an illustration of cells in telophase. During telophase, the chromosomes have completely separated. Nuclear envelopes assemble around the chromosomes, so two new nuclei are formed. In the illustration on the right, the formation of new nuclei is depicted as purple circles with jagged edges enclosing the chromosomes. There are four chromosomes in each new nucleus. In the nucleus on the left, one of the chromosomes is green, and three of the chromosomes are orange. In the nucleus on the right, one of the chromosomes is orange, and three of the chromosomes are green. The centrosomes remain at opposite sides of the cell, but there is no longer a mitotic spindle. In the photomicrograph of a lily cell in telophase, the chromosomes are still distinct, but they are not migrating to the poles; they are clustered in two groups at opposite sides of the cell. Panel g shows a photomicrograph and an illustration of cells in cytokinesis. The illustration shows cytokinesis in an animal cell. Two fully formed nuclei with uncondensed, orange and green chromosomes are present on opposite sides of the cell. In between the two nuclei, the plasma membrane is pinching together and dividing the cytoplasm. Eventually, this pinching will completely divide the cell into two new cells. The photomicrograph shows a lily cell in cytokinesis. Plant cells do not pinch off their plasma membranes the way animal cells do. Instead, a structure called a phragmoplast is used for cell plate assembly. An expanding cell plate eventually forms new cell walls between the two nuclei, and two new cells are formed. Many microtubules are visible in the area between the nuclei in the lily cell; these microtubules are part of the phragmoplast.


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7 Re: Mitosis and Cell Division on Sun Jul 19, 2015 10:29 pm



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New Microscope Collects Dynamic Images of the Molecules that Animate Life 1

A single HeLa cell in metaphase (during mitosis), imaged by a lattice light sheet microscope. Growing microtubule endpoints and tracks are color coded by growth-phase lifetime (credit: Betzig Lab, HHMI/Janelia Research Campus, Mimori-Kiyosue Lab, RIKEN Center for Developmental Biology/Science)


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9 Re: Mitosis and Cell Division on Thu Oct 22, 2015 1:09 pm



Cell Division1

As important as the origination of the first living cell may be, that life is certainly limited if the cell cannot reproduce itself. This is particularly a problem for the evolutionary hypothesis, since evolution requires reproductive processes to operate before it can manifest its posited ability to improve and refine living systems. It is essential that we look for evidences in nature that will tell us about the history of life. Let us begin by considering the mechanisms of cell division in modern bacteria. This will give us useful information for answering the following questions. On a prebiotic earth, and, we will consider these topics in an overlapping fashion, looking first at what cell division entails and working from there to the complexities of modern cellular division.
What is Required for Cell Division?
A cell, any cell before it can divide in a fashion that will increase the meaningfulness of the system, must replicate its contents. The central core memory in the form of DNA (or RNA if such a system is considered possible) must be replicated so that two more or less equivalent copies exist in the cell. These two copies must then be separated from one another in such a way that they can segregate independently to the resultant daughter cells. The parent cell must also make copies of all other critical molecules of which it is composed; otherwise, the cell contents will become diluted by division. These criteria apply regardless of the nature and origin of the systems.
Replication: DNA to DNA
The prokaryote, typified by the colon bacterium, E. coli, has no nucleus, and the cytoplasm does not contain organelles. The bacterial cell itself may be only 1-2 microns in diameter. The DNA molecule it contains is huge by comparison, about 1.4 mm in length, containing 4,639,221 bases. The DNA is thus 1000 times the length of the cell; its hydrated volume (volume when exposed to water outside the confines of the cell) exceeds that of the cell 1000 fold.
A bacterial cell replicates in 45 minutes. During that time it must not only duplicate proteins and other cellular constituents, and replicate the genetic material, but separate the two DNA strands so that one remains with each half of the dividing cell. DNA molecules in bacterial cells occur as intact circles. The processes involved in this division in modern bacterial cells are very sophisticated. We will consider most of the process in just the barest details.
In the cytoplasm, the double-stranded DNA circle is associated with specific positively charged proteins that shield the negative charges of the DNA. The molecule is condensed considerably by associating with specialized bacterial proteins called H-NS. These proteins, about 20,000 per cell, shrink the DNA loop down to a small fraction of its unbound length. Then the entire circle is supercoiled, much as the rubber band on a child’s model airplane propeller supercoils when it is overwound. Specialized proteins in the cell membrane attach to the daughter chromosomes, assuring that one copy ends up in each of the resultant cells. DNA replication itself requires nearly all of the 45 minute interval for completion. The synthesis of the new DNA strand is the work of an enzyme complex called DNA Polymerase. The system works as follows.
The two strands of the DNA molecule run in opposite directions. Each strand is synthesized beginning with a ribose with its 5' hydroxide free and ending with a ribose with its 3' hydroxide free.The directionality of each strand is determined by which hydroxide group on the ribose sugar at an end of the strand is unlinked. Since the complementary strands of each DNA molecule run in opposite directions, one is referred to as the 5' - 3' strand and the complementary strand as 3' - 5'.
DNA Polymerase III, one of three DNA Polymerases in E. coli, and the enzyme responsible for replication, is only capable of reading and copying in one direction, 5'-3'. This enzyme is a sophisticated protein complex consisting of 10 different polypeptides, and is a dimer in its active form. The various subunits function either to anchor the enzyme to the DNA strand (beta subunits), to replicate the strand, or to keep the enzyme from falling off the strand prematurely.
All of these functions work in a highly coordinated manner to enable the complex to move along the strand copying both strands simultaneously at the rate of 500-1000 bases per second!
While this ends the replication phase, it does not finish the business of duplication. We are left with two interlocking circles of DNA that must now be separated.
What we have described so far is only the process of DNA replication, in the simplest kinds of cells, prokaryotes, where no nucleus is involved, where there is only one chromosome to worry about, and where only the simplest kinds of processes are carried out. I would not argue that information replication could not be done in a less efficient manner in a primitive cell. I would be tempted to ask what pathway one might take to get from the primitive cell to the condition of complexity represented in E.coli.
Bacterial cells are among the earliest reported fossils. One would presume that bacterial cells in the earliest Precambrian faced the same kinds of problems as modern bacterial cells, and that they had therefore similar mechanisms for coping. But where did the information come from for developing the cellular complexity of even the simplest bacterium? All of the history of this earth is not sufficient time to generate even a single molecule with the information content of a single protein. The enzymes we are discussing in cellular processes are so complex one is tempted to anthropomorphize them! And we have just considered DNA replication!

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Cell Division Defies Evolutionary 'Just-so' Stories

Cell division seems comparatively straightforward when viewed under a microscope. One cell replicates itself and splits into two daughter cells, enabling an organism to develop, grow, and replace cells in its body. But there is no easy way to describe the details of how cells actually achieve this orderly division.
Processes at work inside the cell somehow ensure that enough of every required part makes it into both daughter cells, whether it is a complete set of chromosomes, at least one each of every organelle (in eukaryotic cells), and thousands of required proteins.
How do cells keep all of this straight, and how do they continually repeat the process with such precision? If any division phase occurs too soon or out of order, the cell will fail to survive. The success of the process is crucial for any dividing cell, which includes all cells and therefore all living systems.
Cell division occurs in discrete phases, with specific objectives obtained in each phase. One key question that cell biologists have tackled is: How does the cell "know" when to stop one phase and start the next?
Reporting in the journal Cell, researchers found that a particular enzyme called Cdk operates as a master oscillator, undergoing rhythmic periods of activity.1Cdk activates a host of subsidiary oscillators, each one in charge of activating a separate but necessary process at a distinct phase of cell division. Moreover, Cdk is influenced by feedback information sent from its subsidiary oscillators.
And this is just one mechanism, tightly linked with many other major cell processes, that ensures cell division is properly regulated. Without Cdk and its associate enzymes comprising what the authors call a "phase-locking model," cell division would not work. And without that, there would be no life on earth.
How did this interactive, micro-miniaturized network of oscillators come about? In a summarized version of the technical report on Cdk, University of California physiologist David Morgan wrote, "The phase-locking model, like everything in biology, makes particularly good sense in the light of evolution."2
He then summarized speculations regarding what supposedly happened early in the evolutionary history of cells:

Early eukaryotes depended on multiple autonomous oscillators, each driving a different event with similar frequency. Cdk arrived later in evolution…eventually assuming control of multiple oscillators to provide more robust centralized control….More effective coordination and timing of events would have become possible with the duplication and specialization of cyclins, together with the evolution of checkpoint controls. Cdks also acquired the ability to directly control hundreds of proteins involved in every aspect of cell division.2

So, with no actual evidence that any of this took place, what is the difference between this "report" and pure fiction?
In an extreme case of shortsightedness, the quote above immediately followed a discussion of how necessary Cdk and its host of oscillators are to the very life of the cell! Morgan described the oscillators as depending "so completely on the Cdk oscillator that they can no longer be uncoupled from it."2 Reports of biochemical systems that cannot evolve backward, but somehow must have evolved forward to reach their present state of existence highlight the blind faith that evolution requires.3
Logically speaking, if a necessary component of any system is broken, then the whole system breaks. But this also means that the required piece—which in this case is the phase-locking oscillator setup—must also have appeared in its entirety and fully integrated at the very start.
Further, the coupling between Cdk and its associate enzymes is so strong that it "underlies the regulatory circuit driving DNA replication."2
By the author's own admission, without these coordinated oscillators to regulate it, DNA replication would not occur. And without DNA replication, cell division would not occur. Without cell division, there would be no reproduction. And without reproduction there can be no evolution, because evolution supposedly operates by survival and reproduction of the fittest.
Thus, this oscillator makes no "sense in the light of evolution." The real light that this proposed evolutionary "just-so" story sheds is on the human heart, revealing that even when confronted with blatant evidence for creation (in the form of ingeniously designed cellular regulatory machinery), some people choose to remain "willingly...ignorant."4


[1]Lu, Y. and F. R. Cross. 2010. Periodic Cyclin-Cdk Activity Entrains an Autonomous Cdc14 Release Oscillator. Cell. 141 (2): 268-279.

[2]Morgan, D. O. 2010. The Hidden Rhythms of the Dividing Cell. Cell. 141 (2): 224- 226.

[3]Thomas, B. Irreversible Complexity―Evolution Loses Another Round. ICR News. Posted on December 16, 2009, accessed September 23, 2010.

[4]2 Peter 3:5.



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