Cell Division

In human genomes, there are two kinds of cell divisions: mitosis and meiosis.

Mitosis

Mitotic division is the process whereby one cell divides to give rise to two that are genetically identical to the parent.

It is mitosis that allows a single fertilized oocyte to give rise to a complete human being with its estimated 10¹⁴ cells, all (with a few exceptions) genetically identical to the original single cell.

In mitosis, the DNA Opens in new window replicates, so that the cell is temporarily condense, and the two copies of each chromosome Opens in new window are associated at their centromeres — these identical associated copies are known as sister chromatids.

After the nuclear envelope has dissolved, the chromosomes align at the metaphase plate, a region in the center of the cell. The associated centromeres of sister chromatids then separate, and the two chromatids of each chromosome move to opposite poles of the cell.

After this, the nuclear envelope reforms around each set of segregated chromosomes as they decondense, the cytoplasm divides, and cell division is complete, resulting in diploid daughter cells which are genetically identical to the diploid parent.

Mitosis is a fundamental process; each of us starts life as a single cell, the fertilized egg, and develops and survives as a result of the enormous number of mitotic cell divisions (estimated to be about 10) necessary for growth, development and maintenance.

Mitosis has great importance in disease, since errors in mitotic divisions can lead to cancers. However, in evolutionary terms the most important class of cell divisions is that which gives rise to the gametes, and it is these that enable the passage of genetic information to the next generation.

Meiosis

A gamete (egg or sperm) is haploid—in the sense that it contains only one copy of the genome, as opposed to the usual two copies in our somatic, diploid cells.

The production of gametes in the germ-line proceeds first by a series of mitotic cell divisions, leading in females from oogonia to primary oocytes, and in males from spermatogonia to primary spermatocytes. Following this, however, the cells enter a different kind of cell division process, specific to the germ-line, called meiosis.

Meiosis is basically the reduction process that is carried out in gametogenesis to generate sperm and egg cells, each bearing 23 chromosomes. There are mainly two divisions, meiosis I and meiosis II.

In meiosis 1, each chromosome Opens in new window replicates into sister chromatids, just as in mitosis. Unlike mitosis, however, the homologous chromosomes then align in pairs (a process known as synapsis) and separate to opposite poles, with their sister chromatids still together. In meiosis II, the sister chromatids then separate, resulting in 23 chromosomes per gamete.

There are two fundamental distinctions between meiosis and mitosis:

• like mitosis, meiosis involves a single round of DNA replication; however, meiosis involves two, not one, subsequent cell divisions, resulting in a reduction of the amount of genetic material from two copies to one;
• while cells produced by mitosis are genetically identical to each other (and to the parental cell), the haploid cells (gametes) produced by meiosis are all genetically different. The events of meiosis are summarized in Figure X-1.

Figure X-1 | The stages of meiosis. DNA is replicated in interphase, and chromosomes condences and thicken in prophase I. At this stage crossing over also occurs and the nuclear envelope breaks down.

The genetic differences between gamates arise from two distinct processes: clearly, the reduction from diploidy to haploidy necessitates the selection of either the paternal or the maternal copy of each chromosome to pass into the gamete.

This independent assortment of chromosomes alone leads to differences between gametes — provided the choice is random, the possible number of different combinations of haploid chromosome subsets is very large: 2²³ (8 388 608). However, there is a second, important level of modification in the passage of genetic material to the gamete: recombination.

During meiosis paternal and maternal chromosomal homologs align and exchange segments through recombination, also known as crossing-over. This process is reciprocal, and there is no net loss of genetic information. Assortment and recombination, neither of which occurs during a normal mitotic cell division, ensure that any one gamete produced by a man or woman is genetically different from any other.