crossing over occurs in pachytene stage in prophase 1.
crossing over occurs in pachytene stage in prophase 1.
when does crossing over occur in prophase 1?
pachytene stage
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
section::::mitotic prophase.:beginning of nucleoli breakdown.









































the nucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production. this indicates a redirection of cellular energy from general cellular metabolism to cellular division. the nuclear envelope stays intact during this process.









































section::::meiotic prophase.









































meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase i and prophase ii. prophase i the most complex phase in all of meiosis because homologous chromosomes must pair and exchange genetic information. prophase ii is very similar to mitotic prophase.









































section::::meiotic prophase.:prophase i.
prophase