homologous chromosomes exchange segments during pachytene stage of prophase i.
homologous chromosomes exchange segments during pachytene stage of prophase i.
during which phase of meiosis do homologous chromosomes exchange segments?
pachytene stage of prophase i
in prophase i of meiosis i, each chromosome is aligned with its homologous partner and pairs completely. in prophase i, the dna has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere. during the zygotene stage of prophase i, the homologous chromosomes pair up with each other. this pairing occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the homologous chromosomes along their lengths. cohesin crosslinking occurs between the homologous chromosomes and helps them resist being pulled apart until anaphase. genetic crossing-over, a type of recombination, occurs during the pachytene stage of prophase i. in addition, another type of recombination referred to as synthesis-dependent strand annealing (sdsa) frequently occurs. sdsa recombination involves information exchange between paired homologous chromatids, but not physical exchange. sdsa recombination does not cause crossing-over.
homologous chromosome
in meiosis, dna replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. the two meiotic divisions are known as meiosis i and meiosis ii. before meiosis begins, during s phase of the cell cycle, the dna of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. this s-phase can be referred to as "premeiotic s-phase" or "meiotic s-phase". immediately following dna replication, meiotic cells enter a prolonged g2-like stage known as meiotic prophase. during this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which dna is cut and then repaired, which allows them to exchange some of their genetic information. a subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the greek letter chi (x)) between the homologous chromosomes. in most organisms, these links are essential to direct each pair of homologous chromosomes to segregate away from each other during meiosis i, resulting in two haploid cells that have half the number of chromosomes as the parent cell. during meiosis ii, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. in some cases all four of the meiotic products form gametes such as sperm, spores, or pollen. in female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to
meiosis