Meiosis includes a recombination checkpoint that is analogous to the checkpoints in cell-cycle progression. Double-strand breaks in the DNA initiate recombination in meiosis

Question

The broken end of a DNA molecule finds the corresponding sequence on a homologous chromosome and exchanges a chromosomal segment with its homolog, thereby repairing the break. Ongoing recombination sends a negative regulatory signal that prevents cells from entering meiotic division I.
A. Mutations in several genes inactivate the recombination checkpoint. What do you predict will happen if a cell proceeds through meiotic division I before completing recombination?
B. What will happen if a cell fails to initiate recombination and proceeds through meiotic division I? Meiotic division II?

Answer 1

A. If a cell proceeds through meiotic division I when its chromosomes are broken and incompletely repaired, segregation will be disastrous. Some recombination intermediates will be pulled to opposite spindle poles, thus breaking the DNA. Other chromosome fragments lacking centromeres will not be attached to microtubules and thus will segregate randomly, causing some meiotic products to have too little DNA and others to have too much.
B. In the absence of recombination, the homologs will not segregate from each other properly in meiotic division I but the sister chromatids will segregate normally in meiotic division II. In meiotic division I, the unrecombined homologous chromosomes will not be held together by chiasmata. Thus, the homologous chromosomes will line up independently on the metaphase plate and segregate randomly, causing chromosome nondisjunction. Some products will have both homologs of a given chromosome and others will have none. In meiotic division II, the events will proceed normally and sister chromatids will be properly segregated to opposite poles. Nonetheless, because the chromosome sets at the start of meiotic division II were distributed unevenly, the gametes produced after meiotic division II will be aneuploid (that is, they will have an incorrect number of chromosomes).