Meiosis

The cells of a particular species have a constant number of chromosomes. Meiosis consists of two divisions which takes place one after the other, during which the number of chromosomes is halved. The two divisions are known as the first meiotic division and the second meiotic division. The first meiotic division has the same stagtes as mitosis. These stages are known as prophase I, prometaphase I, metaphase I, anaphase I and telophase I.

Types of Meiosis. There are three types of meiosis, zygotic or initial meiosis, gametic or terminal meiosis and sporic and indeterminate meiosis.

(1) Zygotic or initial meiosis (haplotonic pattern) occurs in lower plants. Fertilization is immediately followed by meiosis, giving rise to cells with the haploid chromosome number. The zygote is the only diploid stage in the life cycle.

(2) Gametic or terminal meiosis (diplotonic pattern) occurs in animals. The cells resulting from the two meiotic divisions are transformed into sperms or eggs without further cell division. The process is as follows. Germs cells divide repeatedly to give rise to many generations of gonocytes. The gonocytes become primary gonial cells which are transformed into spermatogia (in males) or oogonia (in females) The spermatogonia and oogonia are transformed into primary spermatocytes and primary oocytes, respectively. Each primary sperm atocyte undergoes the first maturation (metiotic) division and forms two secondary spermatocytes. Each secondary spermatocyte undergoes the second maturation (meiotic) division to give rise to two spermatids, which are then transformed into spermatozoa. In the female, during the first meiotic division the primary oocyte divides into a large cell. the secondary oocyte, and a smaller cell, the first polar body. The second oocyte undergoes the second melotic division and divides into an ovum and a second polar body. The first polar body may or may not divide into two.

(3) Sporic or indeterminate meiosis is characteristic of higher plants and some thallophytes, but is not found in animals. In the diploid sporophyte, meiosis takes place at some indeterminate point between the zygote and gamete formation. As a result of meiosis haploid megaspores and microspores are formed. The megaspores and microspores undergo repeated mitotic divisions and form the male and female gametophytes, respectively. Certain cells of the gametophyte differentiate and form the gametes. In plants the cells that undergo meiosis are the microspore mother cell and the megaspore mother cell.