Crossing over happens in prophase I
- The idea
- Predict
- See it
- Why
- Check
The idea
“Chromosomes swap pieces while the DNA is copied, or when they line up in the middle at metaphase.”
The lined-up chromosomes at metaphase are the picture everyone remembers, so it seems like the moment they'd trade pieces. The trading finished earlier.
The short version
The mistake: Chromosomes swap pieces while the DNA is copied, or when they line up in the middle at metaphase.
What's actually true: Crossing over happens in prophase I of meiosis, while homologous chromosomes are paired up side by side.
- S phase copies each chromosome into two identical sister chromatids. Swapping between matching copies would make nothing new.
- In prophase I, homologous chromosomes pair up tightly, which is called synapsis. Non-sister chromatids then trade matching segments.
- By metaphase I the swaps are done. Meiosis II and mitosis don't pair homologs, so crossing over isn't a normal part of either.
On the exam: Crossing over needs synapsed homologs, and synapsis happens only in prophase I.