AP BiologyCommon mistakesMeiosis copies DNA once, then divides twice

Meiosis copies DNA once, then divides twice

  1. The idea
  2. Predict
  3. See it
  4. Why
  5. Check

The idea

“Each division in meiosis needs its own round of DNA replication first.”

In mitosis, every division comes after an S phase. So it's easy to expect one before each of the two divisions in meiosis.

The short version

The mistake: Each division in meiosis needs its own round of DNA replication first.

What's actually true: DNA is copied once, before meiosis I. Then meiosis I splits homologous pairs and meiosis II splits sister chromatids.

  1. DNA is copied once, in S phase, before meiosis. Each chromosome is then two identical sister chromatids.
  2. Meiosis I separates homologous chromosomes, the matching pairs with one from each parent. That halves the chromosome number.
  3. There's no S phase between the divisions. Meiosis II just pulls apart chromatids that already exist, much like mitosis.
  4. So each gamete has half the DNA of a G1 body cell, and fertilization restores the full amount instead of doubling it.

On the exam: One copy, then two divisions. Meiosis II splits sister chromatids that S phase already made.

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