When a eukaryotic cell divides into two, each daughter or progeny cell must receive: (1) a complete set of genes (for diploid cells, this means 2 complete genomes, 2n), (2) a pair of centrioles (in animal cells), (3) some mitochondria and, in plant cells, chloroplasts as well, (4) 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. However, ensuring that each daughter cell gets two (if diploid) of every gene in the cell requires the greatest precision.
Figure 7.3.1: DNA released from human chromosome courtesy J. R. Paulson and U. C. Laemmli
This image 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 solution to this problem is:
- Duplicate each chromosome during the S phase of the cell cycle.
- This produces dyads, each made up of 2 identical sister chromatids. These are held together by a ring of proteins called cohesin.
- Condense the chromosomes into a compact form. This requires ATP and protein complexes called condensins.
- 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. When a cell is not engaged in mitosis (which is most of the time), it is said to be in interphase.
Phases of Mitosis: 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 spindle fibers, each containing ~20 microtubules. Microtubules are synthesized from tubulin monomers in the cytoplasm and grow out from each centrosome. The chromosomes become shorter and more compact.
Phases of Mitosis: Prometaphase
The nuclear envelope disintegrates because of the dissolution of the lamins that stabilize its inner membrane. A protein structure, the kinetochore, forms 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. 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 fiber interrupts the process.
Figure 7.3.2: Kinetochore
Phases of Mitosis: 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.
Phases of Mitosis: Anaphase
The sister kinetochores suddenly separate and each moves to its respective pole dragging its attached chromatid (chromosome) behind it.
Phases of Mitosis: Telophase
The endoplasmic reticulum forms a new nuclear envelope around each cluster of chromosomes, and these return to their more extended form.
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.