An animal has 2n = 8 chromosomes. The figure shows the DNA content of one nucleus against time, for…

Name the segments. P (2C) is G1. Q, the steady rise from 2C to 4C, is the S phase, when the DNA is replicated. R (4C) covers G2 and meiosis I up to the end of telophase I. The first drop, to 2C, comes when the homologues have gone to two nuclei. T (2C) covers interkinesis and meiosis II. The second drop, to 1C, comes when the sister chromatids have gone to four nuclei, so U (1C) belongs to the haploid products.

(a) Segment R. After S phase the chromosome number is unchanged, $8$, but each chromosome has two chromatids: $16$ chromatids. In zygotene the homologous chromosomes pair into $4$ bivalents.

(b) Segment T. In anaphase I the homologous chromosomes separate, while the sister chromatids stay together. Each nucleus has $4$ chromosomes, still with two chromatids each: $8$ chromatids, which is why its DNA is 2C.

(c) Segment U. In anaphase II the centromeres split and the sister chromatids go to opposite poles. Each nucleus has $4$ chromosomes of one chromatid each (1C). One nucleus of P gives $2 \times 2 = 4$ nuclei.