(d) Diplotene (Diplonema) : Two important events begin during diplotene; (I) the repulsion of homologous chromosomes and (II) terminalization.

I. Repulsion : In each pair, the homologous chromosomes start repelling each other. As a result they begin to separate and uncoil. However, the non-sister chromatids involved in the cross-overs are held together at the points of crossing over. Each such points is called a chiasma where the separating chromatids form a cross-like #(x) figure. A homologous pair can show one or more chiasmata.

II. Terminalization : The separation and uncoiling of the homologues begins at the centromeres and proceeds towards the ends. This causes progressive shifting of the chiasmata towards the ends of the chromatids. This is called terminalization of chiasma.

(e) Diakinesis : This is the last phase of Prophase-I. Chromosomes are still in the pairs and in contact with each other by terminal chiasma. The chromosomes become shorter, thicker and more prominent.

By the end of the Prophase-I, the nucleolus and nuclear envelope disappear completely and the pairs of chromosomes are seen distributed in the nucleoplasm.

Metaphase - I : There is formation of a bipolar spindle body. It has two types of spindle fibers; (a) Continuous fibers extending from pole to pole and (b) Chromosomal fibers extending from pole to the equator of the spindle body. The homologous chromosomes, still in pairs, move towards the center of the spindle. These are arranged along the equatorial plane in such a way that the maternal homologues of all the pairs are facing one pole while the paternal homologues are facing the opposite pole. The chromosomal fibers connect the centromeres of all the homologues to the pole on their respective side. Each chromosome has only one centromere.

Table of Contents

6.0 - Introduction
6.1 The Cell Cycle
6.2 Mitosis
6.3 Meiosis
6.4 Comparison between Mitosis & Meiosis

Chapter 7