|Lecture Notes:: 29 October||
As we noted last time, the replication of genetic material, and its subsequent passage to daughter cells is an essential function of any living system.
We saw how mitosis results in the production of cells with identical genetic information - this is how the various cells of our body are produced, such that every cell has the complete and identical information required to produce, in theory, a new individual.
There is one major exception to this picture - the sex cells. These cells, the gametes, are not genetically the same as all of the other cells in the body.
So why sex? Why should we have these special cells? In fact why should we have sex at all? This is one of the major problems of modern biology, particularly evolutionary biology.
But if the generation of diversity is the reason for sex, how is this diversity generated? Why not just use mutation?
So how does sex generate diversity? A second type of cell division is involved Meiosis. Meiosis differs in two fundamental aspects from mitosis:
It turns out sexual organisms exhibit life cycles in which the organism alternates between fertilization (the combination of parental genetic material to produce offspring) to produce diploid cells and meiosis to produce haploid cells. Note that the production of a haploid generation is absolutely essential, otherwise there would be a doubling of genetic material each fertilization, quickly resulting in an infinity of information in a single cell - clearly a problem.
There are many kinds of life cycles.
Meiosis begins, like mitosis, with the replication of the cells chromosomes. Unlike mitosis however, this single replication is followed by two sequential divisions: meiosis I and meiosis II. The result is a single cell giving rise to four haploid daughter cells, each with only half the number of chromosomes as the parent cell.
Meiosis may be divided into 9 stages as noted in the table below. The first 5 stages appear to be very much like those seen in mitosis, however there are some essential differences, while the last four stages are operationally the same as in mitosis.
Followed Figures in text, pp232-3.