Meiosis I: Stages, Synapsis, and Genetic Recombination

First Meiotic Division (Meiosis I or Reduction Division)

It is so named because it reduces by half the number of chromosomes in the initial cell. That is, it starts with a diploid cell, the meiocyte, and produces two haploid cells.

Phases of Meiosis I

The first meiotic division is subdivided into four phases: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

It is the longest stage, which can last for months or even years. At the end of Prophase I, the nuclear envelope disintegrates along with the nucleolus, and the mitotic spindle structure forms, similar to mitotic prophase. However, two events unique to meiosis occur: crossing-over and genetic recombination.

To facilitate study, Prophase I is subdivided, in turn, into five stages:

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene

The chromosomes begin to condense, but the sister chromatids are still not distinguished. The pairs of homologous chromosomes are joined together and hook at their ends to the nuclear lamina by the union called the plaque.

Zygotene

Homologous chromosomes begin to associate in a very intimate and close manner, gene-to-gene throughout their length, which is known as synapsis. Homologues paired at this stage are called bivalents.

Pairing occurs and is maintained by the synaptonemal complex, consisting of two lateral protein filaments (lateral elements) extending parallel to the two homologous chromatids, anchoring the chromatin loops that form these chromatids. The lateral axes of the complex are linked by a central protein filament (central axis), which has a key structure of teeth, like a zipper.

Pachytene

The homologous chromatids intertwine at one or more points and exchange segments of DNA. This process is called crossing over (or crossover). Crossing over involves recombination or gene exchange between non-sister chromatids of homologous chromosome pairs, which constitutes an important source of genetic variability when forming gametes.

Chromosomes are so close together that the actual crossovers are not observed, but synaptonemal protein structures, known as recombination nodules, are appreciated. These nodules are possibly responsible for the crossings.