Cell-cycle & cell division

What you will understand

Keep chromosome number, ploidy and DNA amount separate. Use a simple 2n = 4 model to predict counts at defined stages. Counts here concern nuclear chromosomes and DNA, not mitochondrial DNA.

Quick answer: DNA replication does not double ploidy

During S phase each chromosome is copied into joined sister chromatids. Chromosome number remains the same until sisters separate; nuclear DNA amount doubles. C denotes DNA amount relative to one unreplicated haploid genome. A diploid G1 cell has 2C DNA, rising to 4C after S phase. Ploidy describes sets of homologous chromosomes, not the number of DNA copies.

Track one mitotic cycle

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Meiosis: identify what separates

Meiosis has one preceding S phase followed by two divisions. Homologous chromosomes separate in meiosis I; sister chromatids remain joined. Sister chromatids separate in meiosis II without an intervening DNA replication. The model below omits crossing over to make counting visible; it does not imply that meiotic products are generally genetically identical.

Follow the two meiotic divisions

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Common mistakes to catch

Do not double chromosome number merely because an X has two arms. During mitotic anaphase, separated sisters become daughter chromosomes: in the still-undivided 2n = 4 model, the whole cell temporarily contains eight chromosomes, with four moving to each pole. Always specify whole cell, pole or daughter nucleus. Interphase is metabolically active; calling it an inactive “rest” phase hides growth and DNA synthesis.

Try it yourself 1: after S phase

A diploid cell has 2n = 10 and 2C DNA in G1. After S phase, before mitosis, which pair is correct?

A. 20 chromosomes, 4C

B. 10 chromosomes, 4C

C. 10 chromosomes, 2C

Try it yourself 2: after meiosis I

For 2n = 10, what does each cell contain after meiosis I and cytokinesis?

A. Five duplicated chromosomes, 2C DNA

B. Five single-chromatid chromosomes, 1C

C. Ten duplicated chromosomes, 4C

A useful check on your next visit

Reconstruct the worked example from memory before looking at the diagrams. If a self-check exposed a mix-up, explain why your original answer failed, then try chapter practice.

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Sources and review

Technical review: Codex (automated). Sources checked on 23 October 2026. Diagrams, numerical models, worked examples and self-checks are original teaching material. Human subject review is not claimed.