Chromosomes
So-called hereditary bodies; visible bearers of genetic information, thread- or loop-shaped components of the cell nucleus. Genes (hereditary units) are arranged in series on the chromosomes. Each chromosome is present in two copies ("diploid chromosomes") in the fertilised egg cell and in other body cells. The exceptions are the sex chromosomes. Male human beings have one X and one Y chromosome and human females two X chromosomes. In the germ cells (egg cells and sperm) there is only one copy of each chromosome ("haploid chromosomes"). Chromosomes consist mostly of deoxyribonucleic acid (DNA) and the proteins of the cell nucleus (histones). They have the ability to duplicate themselves in an identical fashion, so that after cell division (mitosis) both daughter cells have the identical number of chromosomes. So-called reductive division (meiosis) only occurs in the formation of the germ cells and the number of chromosomes is reduced from diploid to haploid. The number of chromosomes depends on the animal species. Human beings have 23 pairs of chromosomes, making 46 chromosomes in diploid cells.
|
|
