Question

How do homologs pair? Include such terms as alignment, pairing, synapsis (SC), polarization and recombination in your answer. What role do the nuclear envelope, the cytoskeleton, heterochromatin domains and recombination play in the process?

Answer 1

During meiosis, homologous chromosome pairs to form bivalents that requires homologous recombination and synapsis.

Synapsis involves the formation of the Synaptonemal Complex, a zipper-like structure that joins two aligned homologous chromosomes along their entire length forming chaismata.

Chromosome synapsis starts during zygotene and by the pachytene stage, homologous chromosome are fully aligned and synapsed. Once recombination is completed, the synaptonemal complex disassembles but the homologs remain connected through sister chromatid cohesion via chaismata til anaphase I.

Leptotene phase - The Homologs search for their pair whereas each chromosomes telomeres are attached to the nuclear envelope.

Zygotene stage - Synapsis (pairing of homologs) is initiated.

pachytene stage - Synapsis of homologous chromosome leads to the formation of bivalent and recombination or crossing over occurs at chiasma

The nuclear membrane breaks down at prometaphase II; sister chromatids align at the metaphase plate during metaphase II and then separate at anaphase II.

Cytoskeleton such as dyenin and kinesin provides cytoskeletal forces for the movement of chromosomes.