Question

1. Describe the major events during the 5 stages of prophase I in meiosis I. Note: clear writing please

Answer 1

Answer:

MEIOSIS I

Prophase I

It is the most complicated stage of meiosis. It is of very long duration. The prophase I is divided on the basis of chromosomal behaviour into 5 substages namely Leptotene, zygotene, Pachytene, Diplotene and Diakinesis.

Leptotene:

In leptotene stage nucleus is enlarged. The chromosomes appear as long,fine, uncoiled threads by the condensation of chromatin material. Chromosomes present beaded appearance due to the presence of chromomeres at regular intervals. Chromatids are not distinguishable. The centriole of the cell divides.

Zygotene:

During this stage the homologous chromosomes of each pair are attracted towards one another and undergo pairing. This is called synapsis. Each pair of homologous chromosomes is known as bivalent. The synapsis is equal, exact and occurs from chromomere to chromomere. Centrioles move apart initiating the spinalle formation.

Pachytene:

In each bivalent chromosome four chromatids appear more clearly than earlier. It is called tetrad. The two chromatids of same chromosome are called sister chromatids and those of different chromosomes called non-sister chromatids. An important event that occurs in the pachytene is crossing over, It occurs between the non sister chromotids of two homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. The point of interchange and rejoining appears x-shaped and is known as chiasma. There maybe one , two or more chiasmata in a tetrad. Crossing over leads to recombination of genetic material on the two chromosomes.

Diplotene:

At diplotene, further thickening and shortening of chromosomes take place. Homologous chromosomes start separating from one another. This separation starts at centromeres and travels along towards the ends. This kind of separation from centromere towards the end is known as “ terminalization”.

Diakinesis:

Terminalization is completed, chromosomes undergo complete separation. Each chromosome looks like a short, darkly stained body. By the end of diakinesis, the nucleolus disappears and the nuclear membrane disappears.