Mitosis doesn't halve the chromosomes
- The idea
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- Why
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The idea
“When a cell divides, each new cell gets half of the chromosomes.”
You learned that meiosis halves the chromosomes, and splitting one cell into two sounds like halving. The cell copies its chromosomes before mitosis, so nothing is lost.
The short version
The mistake: When a cell divides, each new cell gets half of the chromosomes.
What's actually true: Each chromosome is copied before mitosis, so each new cell gets all of the parent's chromosomes. Only meiosis halves the number.
- Before mitosis, in S phase, the cell copies its DNA. Each chromosome now has two identical sister chromatids joined at a centromere.
- In mitosis, the sister chromatids separate and each new cell gets one copy of every chromosome. A diploid cell makes two diploid cells.
- Only meiosis halves the number. In meiosis I, homologous chromosomes line up in pairs and the pairs separate, so the cells it makes are haploid.
On the exam: Mitosis keeps the chromosome number. Meiosis halves it. Count chromosomes by centromeres, not chromatids.