Meiosis I

In eukaryotic organisms, diploid cells in reproductive organs such as the ovaries and testes produce haploid gametes through meiosis. This process includes two successive cell divisions called meiosis I and meiosis II.

Meiosis I, also called reductional division, is the first division of meiosis that produces two haploid cells from one diploid cell. Meiosis I has four stages: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I has five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

During leptotene, chromosomes begin to condense and appear as thin, thread-like, beaded structures. Each chromosome contains two sister chromatids held together by cohesin complexes.

During zygotene, homologous chromosomes begin to pair through a process called synapsis, forming tetrads composed of four chromatids. In this process, matching gene regions on the paternal and maternal chromosomes align.

A protein-based, ladder-like structure called the synaptonemal complex forms between homologous chromosomes. This complex helps the chromosomes pair correctly and supports genetic recombination.

In pachytene, crossing over happens between non-sister chromatids of homologous chro