Before division, cells copy their DNA so each daughter can receive genetic information. Bacteria generally reproduce by binary fission, involving chromosome copying and separation followed by cell division. Eukaryotic mitosis separates duplicated chromosomes after interphase. Prophase condenses chromosomes; metaphase aligns them; anaphase separates sister chromatids; telophase re-establishes nuclei. Cytokinesis divides the cytoplasm.
A replicated chromosome has two sister chromatids but is still counted as one chromosome until its centromeres separate. Mitosis normally preserves chromosome number. The cell cycle includes growth, DNA synthesis and division, with checkpoints that can delay progression when conditions or DNA are unsuitable. Gene products and external signals such as hormones regulate division. Mutations affecting these controls can contribute to uncontrolled proliferation; carcinogens can increase that risk.
Example 1
A diploid cell has 12 chromosomes before DNA replication. How many chromosomes enter metaphase?
There are 12 duplicated chromosomes, containing 24 chromatids. Replication doubles DNA amount, not the metaphase chromosome count.
Example 2
Order metaphase, telophase, prophase and anaphase.
Prophase → metaphase → anaphase → telophase. Sister chromatids separate during anaphase.
Watch out:It doubles DNA amount and creates sister chromatids; chromosome count remains unchanged until chromatids separate.
Cell cycle, binary fission and mitosis practice | Year 12 Biology | Ascent Arcade