Q.In meiosis crossing over is initiated at

In Neurospora, the ascus zygote undergoes one meiosis (4 nuclei) then one mitosis (8 nuclei), giving 8 ascospores.

In the ascomycete Neurospora, fusion of the two mating nuclei forms a diploid zygote nucleus inside the ascus. This nucleus then:

1. Undergoes one meiotic division → 4 haploid nuclei,
2. Each of these undergoes one mitotic division → 8 haploid nuclei. …

Synapsis is a meiosis-specific event: the point-by-point pairing of the two homologous chromosomes of a bivalent, occurring at zygotene of prophase I.

Meiosis I includes an elaborate, extended prophase I, divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

During zygotene, each chromosome begins to find and align closely with its homologous partner — the chromosome derived from the same parent's counterpart (one maternal homolog pairing with the corresponding paternal homolog). This intimate, precise pairing process is called synapsis, and the paired structure it produces is called a bivalent or tetrad (since it consists of 4 chromatids — 2 from each homologous chromosome). A protein scaffold called the synaptonemal complex holds the paired homologs together. Synapsis is essential because it sets up the homologs for crossing over (exchange of genetic material) in the next sub-stage, pa …

Crossing over begins at the pachytene sub-stage of meiotic prophase I, once the bivalents formed at zygotene are firmly paired.

Prophase I of meiosis is divided into five sub-stages, in order: leptotene, zygotene, pachytene, diplotene, and diakinesis.

• Leptotene: chromosomes begin to condense and become visible as long, thread-like structures.
• Zygotene: homologous chromosomes pair up (synapsis) to form bivalents/tetrads, with the synaptonemal complex forming between them.
• Pachytene: the paired homologous chromosomes are now fully synapsed. Each chromosome is visibly composed of two sister chromatids, so each bivalent has four chromatids. It is at this stage that crossing over takes place — recombination nodules appear on the synaptonemal complex, and non-sister chromatids of the homologous chromosomes exchange segments at points called chiasmata. …

The point-by-point pairing of homologous chromosomes seen in meiosis is called synapsis, occurring during the zygotene sub-stage of prophase I.

Meiosis I begins with an extended prophase I, divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During zygotene, the two homologous chromosomes (one maternal, one paternal) of each pair come together and align precisely, gene-for-gene, along their entire length. This intimate, specific pairing is called synapsis, and the paired structure it forms (each pair now consisting of four chromatids) is called a bivalent or tetrad.

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