During recombination ________________________

Question

(a) sister chromatids undergo crossing-over with each other.
(b) chiasmata hold chromosomes together.
(c) one crossover event occurs for each pair of human chromosomes.
(d) the synaptonemal complex keeps the sister chromatids together until anaphase II.

Answer 1

Choice (b) is correct. Non-sister chromatids undergo crossing-over; because sister chromatids are identical, there would be no exchange of genetic material if sister chromatids underwent recombination [choice (a)]. The number of crossover events can vary for each meiosis; on average, two to three crossover events occur between each pair of human chromosomes [choice (c)]. Although the sister chromatids do not separate until anaphase II, their arms become unglued because the cohesins holding them together are degraded during anaphase I. The synaptonemal complex is important for holding together and aligning the duplicated homologs and is not involved directly in sister-chromatid cohesion [choice (d)].