Meiosis I - Prophase I

Description:

Prophase I is the first phase of Meiosis I. This phase is more complicated and prolonged as compared to the similar stage of mitosis.

Prophase I is divided into five sub-phases: leptotene, zygotene, pachytene, diplotene and diakinesis. Another sub-phase called preleptonema is sometimes recognized prior to leptonema.

Chromosomes are not distinguishable because of their thinness. Sex chromosomes are often seen in heterochromatic bodies.

The changes a cell undergoes during the following stages are −

Leptotene

• Also called thin-threaded stage.
• Volume of nucleus increases.
• Formation of aster.
• Condensation of nuclear chromatin by dehydration.
• Chromosome appears long thread like and made of two sister chromatids.

Zygotene

• Also called yoked-threaded stage.
• Pairing of homologous chromosomes occurs. This is called synapsis or syndesis.
• Formation of bivalents.
• Pairing of homologous chromosomes in a zipper-fashion.

Pachytene

• Also called thick-threaded stage.
• Further condensation of chromosomes.
• Sister chromatids are visible which are joined at the centromere and is known as dyad.
• Crossing over occurs between two non-sister chromatids of homologous chromosomes.
• Crossing over is regulated by recombinase enzyme.

Diplotene

• Called double threaded stage.
• Longest duration.
• Nuclear membrane and nucleoli start disappearing.
• The homologous chromosomes start separating.
• The point of attachment or crossing over is known as chiasmata.
• The chiasmata starts moving towards the ends of chromosomes called terminalisation.

Diakinesis

• Terminalisation is completed.
• The non-sister chromatids remain in contact with each other at the telomeres.
• Complete disappearance of nuclear membrane and nucleoli.