Crossing over trades pieces both ways
- The idea
- Predict
- See it
- Why
- Check
The idea
“In crossing over, one chromatid passes a piece of its DNA over to the other.”
The name makes it sound like DNA crosses from one chromosome to the other and stays there. It's really a trade between two chromatids.
The short version
The mistake: In crossing over, one chromatid passes a piece of its DNA over to the other.
What's actually true: Crossing over is a reciprocal swap, so one exchange makes two recombinant chromatids at once.
- In prophase I, homologs pair up. One chromatid from each breaks at the same point, and each rejoins to the other's piece. The crossing point is a chiasma.
- The swap is reciprocal. Both chromatids give up a segment and receive the matching one, so no genes are lost or doubled.
- So one crossover makes two recombinant chromatids with mirror-image combinations, A b and a B. The two chromatids that didn't take part stay parental.
On the exam: One crossover, two recombinant chromatids. Each partner gives a piece and gets one back.