Copying DNA doesn't double the chromosomes
- The idea
- Predict
- See it
- Why
- Check
The idea
“Copying DNA doubles the chromosomes, since two chromatids are two chromosomes.”
A copied chromosome looks like two rods stuck together, so it's easy to count two. The copies stay joined, and the pair counts as one.
The short version
The mistake: Copying DNA doubles the chromosomes, since two chromatids are two chromosomes.
What's actually true: Replication doubles the chromatids. Two sister chromatids joined at one centromere are still one chromosome.
- S phase copies the DNA, so each chromosome has two identical sister chromatids joined at one centromere. The DNA doubles, but the chromosome number doesn't.
- Count the same way in meiosis. At metaphase of mitosis or metaphase I of meiosis, a human cell has 46 chromosomes and 92 chromatids.
- The count rises only when sisters separate at anaphase of mitosis or meiosis II. After mitosis, each human daughter cell has 46.
On the exam: Count centromeres, not chromatids. S phase never changes the chromosome number.