Mitosis and Meiosid

As viewed from a human perspective, nature has done some ingenious engineering to overcome some of the obstacles it has faced. Take the evolution of sex, for instance. To make the move from asexual to sexual reproduction, nature took a system by which parent cells reproduced simply by dividing (asexual reproduction) and altered it to allow two parent cells to combine to create offspring (sexual reproduction). It met this challenge by devising (again, speaking from a human perspective) a system by which parent cells incorporate genetic information from both of its parents but contain half the amount of DNA. With only half the DNA, when the parent cell combines with another parent cell, the proper amount of DNA is maintained. This solution is called meiosis.

1. Mitosis and meiosis are the two types of cell division

1.1Mitosis is the normal form of cell division

As a person develops from an embryo, through fetus and infant to an adult, cell divisions are needed to generate the large numbers of cells required. Additionally, many cells have a limited lifespan, so there is a continuous requirement to generate new cells in the adult. All these cell divisions occur by mitosis. Mitosis is the normal process of cell division, from cleavage of the zygote to death of the person. In the lifetime of a human there may be something like 1017 mitotic divisions.

The M phase of the cell cycle consists of the various stages of nuclear division (prophase, prometaphase, metaphase, anaphase and telophase of mitosis), and cell division (cytokinesis), which overlaps the final stages of mitosis. In preparation for cell division, the previously highly extended chromosomes contract and condense so that, by metaphase of mitosis, they are readily visible under the microscope. Even though the DNA was replicated some time previously, it is only at prometaphase that individual chromosomes can be seen to comprise two sister chromatids, attached at the centromere.

The mitotic spindle is formed from tubulin-based microtubules and microtubule-associated proteins. Polar fibers, which extend from the two poles of the spindle towards the equator, develop at prophase while the nuclear membrane is still intact. Kinetochore fibers do not develop until prometaphase. These fibers attach to the kinetochore, a large multiprotein structure attached to the centromere of each chromatid, and extend in the direction of the spindle poles. The interaction between the different spindle fibers pulls the chromosomes towards the center, and by metaphase each chromosome is independently aligned on the equatorial plane (metaphase plate). Paternal and maternal homologs do not associate at all during mitosis. Following centromere division at anaphase, the spindle fibers pull the separated sister chromatids of each chromosome to opposite poles. The DNA of the two sister chromatids is identical, barring any errors in DNA replication. Thus the effect of mitosis is to generate daughter cells that contain precisely the same DNA sequences.

Figure 2.11. Mitosis: homologous chromosomes align independently on the metaphase plate and spindle fibers then pull the separated sister chromatids to opposite poles. (A) At metaphase, paternal (black) and maternal (blue) homologs of each chromosome pair are independently aligned at the metaphase plate, and not associated with each other. Microtubules attached to the kinetochores link chromosomes to each of the poles. For clarity, only chromosomes 1 and 17 and a small fraction of the microtubules are shown. Other spindle microtubules include astral microtubules that radiate from each pole, and polar microtubules that form attachments linking the two poles. (B) At anaphase, the centromere of each of the 46 chromosomes duplicates and the two chromatids separate. Spindle fibers pull on the kinetochores of the centromeres, eventually pulling the two sister chromatids of each chromosome to opposite poles. At this stage (telophase) they become enclosed in a nuclear envelope. Subsequently the cell divides (cytokinesis).

1.2Meiosis is a specialized form of cell division giving rise to sperm and egg cells

The process, during which the germ cells are generated, is called meiosis. It represents nature's solution of the problem of chromosome doubling that would occur, if two diploid cells, i.e. two cells with a double set of chromosomes would fuse.

Primordial germ cells migrate into the embryonic gonad and engage in repeated rounds of mitosis (many more in males than in females, which may be a significant factor in explaining sex differences in mutation rates -to form oogonia in females and spermatogonia in males. Further growth and differentiation produces primary oocytes in the ovary and primary spermatocytes in the testis. These specialized diploid cells can undergo meiosis. Meiosis involves two successive cell divisions but only one round of DNA replication, so the products are haploid. In males, the product is four spermatozoa; in females, however, the cytoplasm divides unequally at each stage: the products of meiosis I (the first meiotic division) are a large secondary oocyte and a small cell (polar body). The secondary oocyte then gives rise to the large mature egg cell and a second polar body.

There are two crucial differences between mitosis and meiosis.

* The products of mitosis are diploid; the products of meiosis are haploid.

* The products of mitosis are genetically identical; the products of meiosis are genetically different.

Mitosis involves a single turn of the cell cycle. The DNA is replicated in S phase and the two copies are divided exactly equally between the daughter cells in M phase. Meiosis is also preceded by one round of DNA synthesis, but then there are two cell divisions without intervening DNA synthesis, so that the products end up haploid. The second division of meiosis is identical to mitosis, but the first division has important differences whose purpose is to generate genetic diversity between the daughter cells. This is done by two mechanisms, independent assortment of paternal and maternal homologs, and recombination.

1.2.1 Independent assortment of paternal and maternal homologs

During meiosis I the maternal and paternal homologs of each chromosome pair form a bivalent by pairing