Sister chromatids are copies. Homologs are partners.
- The idea
- Predict
- See it
- Why
- Check
The idea
“A chromosome's two sister chromatids are the copy from my mom and the copy from my dad.”
Both come in twos and get drawn side by side. It's easy to picture the left and right halves of an X as mom's and dad's.
The short version
The mistake: A chromosome's two sister chromatids are the copy from my mom and the copy from my dad.
What's actually true: Sister chromatids are two copies of one chromosome, made in S phase. Homologs are partner chromosomes, one from each parent.
- Homologs are a pair of chromosomes, one from each parent. They have the same genes in the same order, but their alleles can differ.
- In S phase each chromosome copies its DNA. The two copies stay joined at the centromere as sister chromatids. They start out identical.
- Meiosis I separates homologs. Meiosis II separates sister chromatids, much like mitosis.
- Crossing over in prophase I swaps matching pieces between homologs. That's how two sisters can end up with different alleles.
On the exam: Meiosis I splits the homologs from mom and dad. Meiosis II splits the copied sisters.