Chromosome number is maintained in mitosis despite the duplication of DNA because the chromosome number refers to the number of centromeres, not the amount of DNA. Here's a breakdown:
During mitosis, the chromosome number remains constant due to the careful segregation of duplicated chromosomes (sister chromatids). Although the DNA content doubles during the S phase as each chromosome replicates to form two identical sister chromatids, these chromatids remain attached at the centromere. This ensures that what was originally one chromosome still counts as one chromosome. Only when the sister chromatids are separated later during anaphase, with each chromatid moving to opposite poles, does each chromatid then become an independent, single chromosome.
| Phase | DNA Content per Chromosome | Chromosome Number | Status of Chromosomes |
|---|---|---|---|
| G1 | 1x | n | Single chromatid chromosome |
| S | 1x -> 2x | n | Duplication: One chromatid -> Two |
| G2 | 2x | n | Pair of sister chromatids |
| Prophase | 2x | n | Condensed sister chromatids |
| Metaphase | 2x | n | Sister chromatids aligned |
| Anaphase | 2x -> 1x | n (temporarily 2n) | Sister chromatids separate, becoming chromosomes |
| Telophase | 1x | n | Single chromatid chromosomes in daughter cells |
- S Phase: DNA replicates, resulting in two identical sister chromatids for each chromosome. According to the reference provided, "during a mitotic cell cycle, the DNA content per chromosome doubles during S phase (each chromosome starts as one chromatid, then becomes a pair of identical sister chromatids during S phase)".
- Prophase/Metaphase: The sister chromatids remain joined together. Crucially, even though the amount of DNA has doubled, the two chromatids are still considered one chromosome because they are held together by a single centromere.
- Anaphase: The sister chromatids separate and move to opposite poles of the cell. Each separated chromatid is now considered an individual chromosome.
- Telophase/Cytokinesis: The cell divides, resulting in two daughter cells each with the original chromosome number.
Therefore, the chromosome number stays the same throughout mitosis because the physical separation of the sister chromatids into individual chromosomes only occurs during anaphase, ensuring each daughter cell receives the correct number of chromosomes.
