Meiosis and Prophase – I
Meiosis – I consists of four stages; prophase I, metaphase I, Anaphase I and Telophase.
Prophase – I
It is the first stage of meiosis I. This is the longest phase of the meiotic division. It includes five sub-stages;
Leptotene: The word Leptotene means thin thread. The chromosomes uncoil and become large and thinner. Each chromosome consists of two chromatids held together by a centromere but these are not easily visible.
Zygotene: Homologous chromosomes come together and lie side by side throughout their length. This is called pairing or synapsis. The paired chromosomes are now called bivalents. The adjacent non-sister chromatids are joined together at certain points called chiasmata.
Pachytene: The chromosomes condense further and become very shorter and thicker. They are very distinct now. The two sister chromatids of each homologous chromosome become clearly visible. The bivalent thus becomes a tetrad with four chromatids. In the region of chiasmata, segments of non-sister chromatids of the homologous chromosomes are exchanged and this process is called crossing over.
Diplotene: The homologous chromosomes condense further. They begin to separate from each other except at the chiasmata. Due to this separation the dual nature of a bivalent becomes apparent and hence the name Diplotene. Exchange of segments of chromatids between homologous chromosomes has taken place at the chiasmata. The process of gene exchange is known as genetic recombination.
Diakinesis: The nuclear membrane starts disintegrating. The nucleolus also disintegrates and disappears. The homologous chromosomes of a bivalent move apart from each other and spindle formation completed.
Metaphase – I
The homologous pairs of chromosomes are lined up double file along the metaphase plate. Spindle fibers from the poles of the cell are attached to the centromeres of each pair of homologues.
Anaphase – I
Homologous chromosomes are separated as they are pulled by spindle fibers and migrate to opposite poles. At each pole only half the number of chromosomes is received. These chromosomes are not the same as existed at the beginning of prophase. Each chromosome consists of one if its original chromatids and the other has a mixture of segments of its own and a segment of chromatid from its homologue (due to crossing over).
Telophase – I
Homologous pairs continue to separate until they reach the poles of the cell. Each pole has the monoploid number of chromosome. The nucleolus reappears and nuclear membrane forms. The daughter nuclei begin the second meiotic division.