1. To review the structure of a chromosome.
2. To study the events associated with meiosis.
3. To apply this knowledge to human genetics by analyzing a karyotype.
Meiosis is the second important kind of nuclear division. It resembles mitosis in many ways but the consequences of meiotic divisions are very different from those of mitotic divisions. While mitotic division may occur in almost any living cell of an organism, meiosis occurs only in special cells. In animals, meiosis is restricted to cells that form gametes (eggs and sperm). Each species has a characteristic number of chromosomes per somatic cell. Fruit flies have 8; normal humans have 46. They exist as homologous pairs (partners) that are similar in size and shape and carry the same kinds of genes. Thus humans have 23 homologous pairs. The full complement of 46 chromosomes is referred to as the diploid number (referring to the fact that each kind of chromosome is represented twice). In higher organisms when an egg is fertilized the egg and sperm fuse to form a single cell called a zygote which develops into a new organism. If the egg and sperm were both diploid (46 chromosomes each in the case of humans) then the resulting zygote would be tetraploid. This would be an intolerable situation, so a mechanism has evolved to insure that each gamete (egg or sperm) contains only one representative of each homologous pair (or half the diploid number). This is referred to as the haploid number.
Haploid Egg + Haploid Sperm = Diploid Zygote
The mechanism that makes this possible is meiosis. Meiosis consists of two divisions, Meiosis I and Meiosis II, and can potentially result in the production of four cells. However the DNA is only synthesized once (prior to Meiosis I). The subdivisions of meiosis are named like the subdivisions of mitosis (prophase, metaphase, anaphase, telophase) but as we shall see the events are somewhat different.