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Mitosis

When a eukaryotic cell divides into two, each daughter or progeny cell must receive

a complete set of genes (for diploid cells, this means 2 complete genomes, 2n)
a pair of centrioles (in animal cells)
some mitochondria and, in plant cells, chloroplasts as well
some ribosomes, a portion of the endoplasmic reticulum, and perhaps other organelles

There are so many mitochondria and ribosomes in the cell that each daughter cell is usually assured of getting some. But ensuring that each daughter cell gets two (if diploid) of every gene in the cell requires the greatest precision.

This image (provided by J. R. Paulson and U. C. Laemmli) provides a graphic illustration of the problem. It shows a bit (no more than 3%) of the single molecule of DNA released from a single human chromosome. (The chromosome was treated to remove its histones). Remembering that this is 3% of the DNA of only one of the 46 chromosomes in the human diploid cell, you can appreciate the problem faced by the cell of how to separate without error these great lengths of DNA without creating horrible tangles.

The answer:

Duplicate each chromosome during the S phase of the cell cycle.
Link to discussion of the cell cycle.
This produces dyads, each made up of 2 identical sister chromatids. These are held together by proteins called cohesins.
Condense the chromosomes into a compact form. This requires ATP and a protein complex called condensin.
Separate the sister chromatids and
distribute these equally between the two daughter cells.

Steps 3 - 5 are accomplished by mitosis. It distributes one of each duplicated chromosome (as well as one centriole) to each daughter cell. It is convenient to consider mitosis in 5 phases.

1. Prophase

The two centrosomes of the cell, each with its pair of centrioles, move to opposite "poles" of the cell.
The mitotic spindle forms. This is an array of microtubules, synthesized from tubulin monomers in the cytoplasm, that develops from each centrosome.
The chromosomes become shorter and more compact.

2. Prometaphase

The nuclear envelope disintegrates.
A protein structure, the kinetochore, appears at the centromere of each chromatid.
With the breakdown of the nuclear envelope, spindle fibers attach to the kinetochores as well as to the arms of the chromosomes.

Link to a discussion of the role of spindle fibers
and microtubule motors in the
chromosome movements of mitosis.

The microtubules attached to a kinetochore exert tension on its chromatid. For each dyad, one of the kinetochores is attached to one pole, the second (or sister) chromatid to the opposite pole. Failure of a kinetochore to become attached to a spindle fibers interrupts the process.

Link to a discussion of the spindle checkpoint in the cell cycle.

3. Metaphase

The tension is proportional to length; thus if a dyad approaches one pole, the tension in the opposite direction increases and the dyad is pulled back to an equilibrium position midway between the poles. In due course, all the dyads reach this position, the equatorial plane or metaphase plate. The chromosomes are at their most compact at this time.

4. Anaphase

The sister kinetochores suddenly separate and each moves to its respective pole dragging its attached chromatid (chromosome) behind it.

Separation of the sister chromatids depends on the breakdown of the cohesins that have been holding them together. It works like this.

Cohesin breakdown is caused by a protease called separin (also known as separase).
Separin is kept inactive until late metaphase by another protein called securin.
Anaphase begins when the anaphase promoting complex (APC) destroys securin (by tagging it for deposit in a proteasome) thus ending its inhibition of separin and allowing
separin to break down the cohesins.

5. Telophase

A nuclear envelope reforms around each cluster of chromosomes and these return to their more extended form.

Cytokinesis

Mitosis is the process of separating the duplicates of each of the cell's chromosomes. It is usually followed by division of the cell. However, there are cases (cleavage in the insect embryo is an example) where the chromosomes undergo the mitotic process without division of the cell. Thus a special term, cytokinesis, for the separation of a cell into two.

In animal cells, a belt of actin filaments forms around the perimeter of the cell, midway between the poles. As the belt tightens, the cell is pinched into two daughter cells.

In plant cells, a membrane-bounded cell plate forms where the metaphase plate had been. The cell plate, which is synthesized by the Golgi apparatus, supplies the plasma membrane that will separate the two daughter cells. Synthesis of a new cell wall between the daughter cells also occurs at the cell plate.

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2 August 2001