Homologous: Chromosomes that have the same loci. Members of an homologous pair of chromosomes pair up during meiosis. Diploid organisms, produced by sexual reproduction have homologous pairs of chromosomes i.e. one member from male & one from female parent
Chromosomes are double structured. Each chromosome consists of two molecules of DNA that are packaged tightly to make two sister chromatids. As replication is very precise, the two sister chromatids are genetically identical.
Matching chromosomes into pairs to give an arrangement is called a karyotype.
Pairs of chromosomes are homologous. The chromosomes in each pair are the same size, and they have the same genes
Meiosis: Nuclear division that results in the formation of cells containing half the number of chromosomes of the adult cell.
The number of body cells of species that reproduce sexually remains constant. The number of chromosomes in the body cells is the diploid number. The human diploid number is 46.
When gametes (egg & sperm) are produced, a different type of division occurs - meiosis. this occurs only in special parts of the body such as the testes and ovaries in animals, and anthers and ovary in flowering plants.
During meiosis, the chromosome number is halved so human gametes have 23 chromosomes - one of each type. The number of chromosomes in gametes is the haploid number. The human haploid number is 23.
The reduction in chromosome number ensures that at fertilisation, when the gametes fuse to form a zygote, the diploid number of chromosomes is restored.
Meiosis involves two divisions of the nucleus, but it is during the first division most of the variation is generated. Homologous chromosomes pair together during meiosis and then separate. This ensures that each new nucleus has one full complement of chromosomes - one from each pair.
In a human cell with a diploid number of 46, there are 2 to the power of 23 = 8388608 different ways in which homologous chromosomes can be arranged in pairs. This causes variation because, although a pair of homologous chromosomes carries the same genes, one or more of these genes may have variant forms, so they control the characteristics in different ways.
For example, the gene that controls production of a protein carrier in cells lining the airways may not function correctly. Some people may inherit two copies of this faulty gene and develop cystic fibrosis.
Haploid gametes in animals
In animals, the haploid gametes are either spermatozoa (sperm) or ova (eggs). human sperm are adapted for the role of delivering a haploid nucleus to an egg in following ways:
- The undulipodium beats to move the sperm cell through the female reproductive tract.
- Mitochondria provide energy in the form of ATP for movement of the undulipodium.
- The acrosome at the tip of the sperm contains enzymes to digest a pathway through the cells and jelly coat that surrounds the egg.
- Sperm are compact and streamlined as they have little cytoplasm and a small nucleus with very highly condensed chromosomes.