CHROMOSOMES AND GAMETE PRODUCTION: AN OUTLINE
Cells that are not reproductive cells, i.e. not gametes, are known as somatic cells. Your skin cells, muscle cells, nerve cells and so on are all somatic cells: the term is an important one. Significantly, the number of chromosomes in the nucleus of a tvpical somatic cell of a given animal or plant is always twice_the number found in the nucleus of the gametes. Thus the nucleus of a horse’s nerve cell or muscle cell, for example, contains 64 -chromosomes, whereas the nucleus of a horse’s egg or sperm contains 32.
The number of chromosomes in the non-reproductive cells of an animal or plant is called the diploid number. The general term for the number of chromosomes in the gametes of an animal or plant is the haploid number. For horse’s the diploid number of chromosomes is therefore 64 and the haploid number is 32.
In animals and plants, haploid gametes are produced from precursor somatic cells. In stallions, the sperm (containing 32 chromosomes) are produced from diploid 'sperm mother cells', also called spermatocytes. In mares, the ova (also containing 32 chromosomes) are produced from diploid egg mother cells, or ooytes. When the two haploid gametes fuse at fertilization, a new diploid cell is produced. This first cell of the new individual is called the zygote.
When a cell with the full (diploid) number of chromosomes gives rise to a gamete, the number of chromosomes must somehow be halved. When fertilization happens, the nuclei from the two gametes, each with its half (haploid) set of chromosomes, come together, restoring the number to the diploid value.
The Figure below shows these changes in chromosome numbers. They occur, not just in humans, but in all animals and plants. The prodution of a gamete requires a very special process. It involves meiosis, a form of cell division in which the number of chromosomes is halved from the diploid to the haploid number. Meiosis, holds the key to
understanding the pattern of inheritance of different forms of a particular character.
The whole process of meiosis can take days, months or even years to complete, depending on the species of animal or plant involved. Indeed, in women, meiosis can last for up to 50 years. In horses, it may last for over 20 years.
Many things happen inside cells undergoing mciosis, but some of the most spectacular and intricate events involve the chromosomes.
How the number of chromosomes in the nucleus changes with gamete production and fertilization. Only two pairs of chromosomes are shown for simplicity.
The next figure below shows another way of looking at these events. Remember that meiosis occurs through 2 cycles of cell division (Meiosis I and II) but that the DNA is only replicated or copied in Meiosis I.
Comparison of meiosis and normal cell division. Only one pair of homologous chromosomes is shown. In meiosis, following DNA replication, 2 cell divisions are required to produce the haploid gametes. Each diploid cell that enters meiosis (theoretically) produces 4 haploid cells. Each diploid cell that divides by mitosis produces 2 diploid cells.