Meiosis

Meiotic division is also known as reduction division. The normal chromosome number of the mother cell is reduced to half in daughter cells. The normal chromosome number in human being is 46 (23 pairs), but as a result of meiosis in ovary and testes this number is halved to 23 in daughter cells (called sperms or the egg).

It occurs in reproductive cells, in the testes of male and in the ovaries of female animals; and in plants, in the pollen mother cell of the anthers (male organs) and in the megaspore mother cells of the ovary (female organ) of the flowers.

Why Does Meiosis Occur

In meiosis, the chromosome number is reduced to half so that when doubled at fertilization (zygote formation) during the reproduction it once again becomes restored to the diploid state. The number of chromosomes remains constant in a species generation after generation.

Cells divide mitotically in the organisms that reproduce vegetatively (asexually). Thus, there is no change in the number of chromosomes, but sexually reproducing organisms form gametes such as sperms in males and ova in females. The male and female gametes fuse to form the zygote which develops into a new individual. If these gametes were produced by mitosis, the offspring developing from zygote would have double the number of chromosomes in the next generation.

Every living organism has a definite number of chromosomes in its body cells. For example, onion cell has 16, potato has 48, horse has 64, humans have 46. Therefore, to keep the chromosome number constant the reproductive cells of the parents divide through meiosis.

How Does Meiosis Occur

Meiosis is characterized by two successive divisions of the nucleus (meiosis I and II) and cytoplasm, whereas the chromosomes divide only once.

The interphasewhich precedes the onset of meiosis is similar to the interphase which precedes mitosis. At S-phase, the DNA molecule of each chromosome duplicates to give rise to two DNA molecules and hence two chromatids are found in one chromosome attached to a single centromere.

Meiosis-I and meiosis-II are continuous and have been divided into sub-stages only only for convenience to study the process of nuclear division.

Significance of Meiosis

It helps to maintain constant number of chromosomes in different generations of a species undergoing sexual reproduction.

Meiosis occurs during gamete formation (gametogenesis) and reduces the number of chromosomes from diploid (2n) to haploid (n) in the gametes. These haploid gametes fuse to form diploid zygote during fertilization. The diploid zygote develops into a normal diploid individual.

Meiosis establishes new combination of characters due to mixing of paternal and maternal chromosomes. As a result the progeny inherits the traits of both the mother and the father in new gene combinations.