Problem

An organism has 2n = 8 chromosomes and a DNA content of 2C at the start of G1. Track the DNA content and chromosome number through: (a) End of S phase, (b) G2, (c) End of anaphase of mitosis, (d) After cytokinesis, (e) After meiosis I, (f) After meiosis II.

Step-by-Step Solution

Step (a): End of S phase

• Event: DNA replication
• DNA content: 2C × 2 = 4C
• Chromosome number: 8 (2n) — unchanged
• Reason: Sister chromatids formed but joined at centromere → still 1 chromosome per centromere

Step (b): G2

• Event: Tubulin synthesis, preparation
• DNA content: 4C (no replication in G2)
• Chromosome number: 8 (2n) — unchanged
• Note: G2 is a flat line on DNA content graph

Step (c): End of Anaphase of Mitosis

• Event: Centromeres split; sister chromatids move to opposite poles
• DNA content in cell: Still 4C (no DNA is lost or gained — same cell)
• Chromosome number: 16 (4n transiently) — because each chromatid is now an individual chromosome
• DNA at each pole: 2C $\frac{4C total}{2 poles}$
• Chromosomes at each pole: 8 $\frac{16 total}{2 poles}$

Step (d): After Cytokinesis (each daughter cell)

• Event: Cell physically splits
• DNA content: 2C per daughter cell
• Chromosome number: 8 (2n) per daughter cell
• Conclusion: Back to G1 state — cycle can repeat

Step (e): After Meiosis I (each cell)

• Event: Homologous chromosomes separate
• DNA content: 4C / 2 = 2C per cell
• Chromosome number: 8 / 2 = 4 (n) per cell
• Note: Each chromosome still has 2 chromatids (sister chromatids still joined)

Step (f): After Meiosis II (each gamete)

• Event: Sister chromatids separate
• DNA content: 2C / 2 = 1C (C) per gamete
• Chromosome number: 4 (n) — same as after meiosis I
• Note: Chromosome number doesn't change in meiosis II — only DNA content halves

Summary Table