Question 1

How is chromosome number counted?

Accepted Answer

Chromosome number is counted by centromeres, not by the visible X-shape. A replicated chromosome with two sister chromatids has one centromeric region before division and therefore counts as one chromosome.

Question 2

Why does DNA content double during S phase without changing chromosome number?

Accepted Answer

During S phase, each chromosome is replicated to form two sister chromatids, increasing DNA content from 2C to 4C in the example cell. The centromere has not divided, so all copied chromosomes still belong to the same 2n = 4 set.

Question 3

What happens to chromosome number and DNA content after meiosis I?

Accepted Answer

Homologous chromosomes move to opposite poles during anaphase I. After cytokinesis, each daughter cell contains one member of each homologous pair, giving n = 2 for this example, while its chromosomes still have sister chromatids and therefore contain 2C DNA.

Question 4

Why does meiosis II not change the haploid chromosome number?

Accepted Answer

Meiosis II separates sister chromatids within each haploid cell. The chromatids become daughter chromosomes, but the cell was already haploid, so each final product remains n = 2 in this example and has 1C DNA.

Question 5

Are all four products of meiosis always functional gametes?

Accepted Answer

No. One germ cell undergoing meiosis produces four haploid products, but their fate differs. Spermatogenesis normally produces four spermatids, while oogenesis produces one large ovum and polar bodies.

DNA vs Chromosome Number: Count Centromeres, Track DNA at Every Phase

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