One homologue carries the alleles A and B of two genes on both of its chromatids…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
One homologue carries the alleles and of two genes on both of its chromatids. The other homologue carries and on both of its chromatids. One crossing over takes place between the two genes, between one chromatid of each homologue.
- In which stage of prophase I does the crossing over take place?
- Between which chromatids does it take place?
- Write the four chromatids of the bivalent after the crossing over.
- How many of the four chromatids are recombinant?
- Which enzyme helps in the exchange?
Need a reminder? The key ideaShow
Meiosis copies the DNA once and divides twice. Homologous chromosomes pair in zygotene, cross over in pachytene, show chiasmata in diplotene, and the chiasmata move to the ends in diakinesis. A bivalent is a pair of homologues and holds four chromatids.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Pachytene.
Step 2. (b) Between a chromatid of one homologue and a non-sister chromatid of the other.
Step 3. (c) The two chromatids not involved stay and . The two that crossed over become and . The four chromatids are , , and .
Step 4. (d) The recombinant chromatids are and , which is 2 of the 4.
Step 5. (e) Recombinase.
Answer: (a) pachytene; (b) non-sister chromatids of the homologues; (c) ; (d) ; (e) recombinase.
Watch out. A crossing over between sister chromatids would change nothing, because they are identical.
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