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This is from SAT subject biology test practice question. The answer it gave me was both chiasmata and centromeres. I can understand they are held by chiasmata but why also centromeres? I looked it up, I didn't find anywhere mentioned that they are held by centromeres also, only synaptonemal complex and chiasmata.
tetrads are truly held by spindle fibers on centromeres too, but isn't the question asking what hold them together? Even without spindle fibers, they can still be held by chiasmata I guess.
A recombinase only active during meiosis - Dmc1 and a general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments. These filaments hold the homologs together at the chiasma, the point of attachment during crossing over.
It may help to think of the question in this way. Within a tetrad (that is 2 bivalents) there are 4 strands of DNA (really chromatin). Only the 2 inside strands are held together by the chiasmata. The outside strands are attached to the tetrad by it's centromere. Each pair of sister chromosomes are held in place at the centromeres during meiosis I. The sister chromosomes segregate together because the attached microtubules are oriented towards the same pole. They separate at the centromere at anaphase II to form the haploid gamete.