Meiosis

Meiosis is the cell division that gives rise to gametes a d haploid spores. In flowering plants and animals, it occurs only in reproductive organs.

   A diploid cell has two sets of chromosomes : one from the male gamete/ parent, and the other from the female gamete / parent. When a diploid cell undergoes meiosis,

• the chromosomes replicate once , and

• the nucleus and cell duplicate (divide equally) twice.

This results in the diploid parent cell giving rise to four haploid gamete cells.

    At the start of meiosis, each member of a homologous pair of chromosomes moves to lie side by side so that all part of two chromosomes match exactly. Each chromosome is also made up of two chromatids. While they are thus paired , genetic material is exchanged between the chromatids. this is known as crossing over and leads to greater variation in the offspring.

   When the  nucleus divides for the first time, the chromosomes in a given pair (not the chromatids!) separate and move to opposite ends of the cell. This results in only half the member of chromosomes going to each daughter cell.

   During the second nuclear division, the chromatids separate and move to opposite ends of each daughter cell. This gives rise to four gamete cells, each with a haploid number of chromosomes.