Thinking DNA gets copied right before division
- The idea
- Predict
- See it
- Why
- Check
The idea
“The DNA gets copied in G2 or during mitosis, right before the cell splits.”
Copying right before you split sounds efficient. And chromosomes only show up as clear X shapes in mitosis, so that looks like when they double.
The short version
The mistake: The DNA gets copied in G2 or during mitosis, right before the cell splits.
What's actually true: DNA is copied in S phase, so each chromosome has two sister chromatids from S until anaphase.
- The cycle runs G1, S, G2, then M. S stands for synthesis. It is the phase where the cell copies its DNA.
- After S, each chromosome is two sister chromatids joined at one centromere. You count chromosomes by centromeres, so that number stays the same.
- At anaphase the sisters split, and each one becomes its own chromosome.
On the exam: DNA is copied in S. From S until anaphase, every chromosome has two chromatids.