Pachytene

Meiosis has a stage called pachytene, but it is also known as pachynema because the homologous chromosomes fully synapse here, forming tetrads. It is an important stage for genetic recombination where crossing over actually occurs between maternal and paternal chromosomes. Hybrid sterility is a condition of reproductive failure, where the pachytene checkpoint acts as a surveillance mechanism which detects the aberrant products of meiosis. It plays a greater role to be one of the factors that prevent the formation of faulty gametes. The pachytene barrier has been proposed as a structure that is not only related to gene-based but also to chromosomal-based hybrid sterility because it impacts the non-haploid or aneuploid gametes.

1.0Meiosis

• Meiosis consists of two consecutive cycles of nuclear and cell division called meiosis I and meiosis II but contains only one cycle of DNA replication.
• Meiosis I begins after the parental chromosomes have replicated to form identical sister chromatids at the S phase.
• Meiosis includes the pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes. After meiosis II, four haploid cells are obtained. Meiosis is a special type of cell division which produces haploid cells by reduction in the number of chromosomes.
• Meiosis is divided into meiosis I and Meiosis II.
• Meiosis I is divided into- Prophase-I, Metaphase-I Anaphase-I and Telophase-I.
• Prophase-I is further divided into five sub stages i.e. leptotene, zygotene pachytene, diplotene and diakinesis.

2.0Introduction of Pachytene

• In the crucial phase of cell division called prophase I, This stage sets the stage for genetic diversity through crossing over, where chromatid segments exchange. In the pachytene stage the bivalent chromosomes undergo thickening, shortening, and condensation. Each chromosome in a bivalent has two chromatids, referred to as sister chromatids.
• The two sister chromatids of one chromosome, in relation to the other chromosome in the pair, are known as non-sister chromatids. A tetrad is formed by the four chromatids in a pair of homologous chromosomes.

• This stage is marked by the emergence of recombination nodules, which are sites where crossing over occurs between the non-sister chromatids of homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes, facilitated by recombinase enzyme. It's a crucial enzyme mediated process in the formation of genetic variation.
• During the tetrad stage, involving four strands, crossing over occurs, involving the exchange of segments between non-sister chromatids of different chromosomes. Endonuclease enzymes facilitate the breakage of chromatids, which then unite with non-sister chromatids through R proteins. This genetic exchange, known as crossing over, happens during the four-strand stage in a bivalent at the pachytene stage.

• Crossing over isn't just a genetic exchange; it's a crucial source of continuous genetic variation in sexually reproducing organisms. Pachytene, the longest stage in prophase I, provides ample time for these genetic maneuvers. The synaptonemal complex stabilizes chromosome pairing, aiding in a smooth crossing-over process.
• In summary, the events in prophase I, including crossing over, showcase the complexity of cellular processes and emphasize the vital role of crossing over in maintaining genetic diversity in sexual reproduction.

3.0Diagram of Pachytene Stage