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Mitosis

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

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:

  1. Duplicate each chromosome during the S phase of the cell cycle.
    Link to discussion of the cell cycle.
  2. This produces dyads, each made up of 2 identical sister chromatids. These are held together by a ring of proteins called cohesin.
  3. Condense the chromosomes into a compact form. This requires ATP and a protein complex called condensin.
  4. Separate the sister chromatids and
  5. 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.

When a cell is not engaged in mitosis (which is most of the time), it is said to be in interphase.

1. Prophase

2. Prometaphase

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

3. Metaphase

At metaphase all the dyads have reached an equilibrium position midway between the poles called the metaphase plate. The chromosomes are at their most compact at this time.
Link to a discussion of the role of spindle fibers and microtubule motors in the chromosome movements of mitosis.

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 cohesin that has been holding them together. It works like this.
  • Cohesin breakdown is caused by a protease called separase (also known as separin).
  • Separase is kept inactive until late metaphase by an inhibitory chaperone 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 separase and allowing
  • separase to break down cohesin.

5. Telophase

The endoplasmic reticulum forms a new nuclear envelope 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. The interaction of actin and a myosin (not the one found in skeletal muscle) tightens the belt, and the cell is pinched into two daughter cells.

In plant cells, a cell plate forms where the metaphase plate had been. The cell plate is synthesized by the fusion (aided by SNAREs) of multiple membrane-enclosed vesicles. Their fusion supplies new plasma membrane for each of the two daughter cells. Synthesis of a new cell wall between the daughter cells then occurs at the cell plate.

External Links
Mitosis is a dynamic process. Link to John Kyrk's animation of it.
Link to movie (7.6 MB) of mitosis in a plant cell.
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18 September 2011