An animal has 2n = 8 chromosomes. The figure shows the DNA content of one nucleus against time, for…
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An animal has chromosomes. The figure shows the DNA content of one nucleus against time, for a cell of its testis that passes through interphase and then meiosis. Segment P is G1 phase.
- In segment R, before the first division is complete, how many chromosomes and how many chromatids does the nucleus have? How many bivalents form in this segment?
- In segment T, how many chromosomes and how many chromatids does each nucleus have?
- In segment U, how many chromosomes does each nucleus have? How many nuclei in segment U come from one nucleus of segment P?
The worked solution
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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, , but each chromosome has two chromatids: chromatids. In zygotene the homologous chromosomes pair into bivalents.
(b) Segment T. In anaphase I the homologous chromosomes separate, while the sister chromatids stay together. Each nucleus has chromosomes, still with two chromatids each: 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 chromosomes of one chromatid each (1C). One nucleus of P gives nuclei.
Answer: (a) chromosomes, chromatids, bivalents. (b) chromosomes and chromatids. (c) chromosomes; nuclei.
Check. The DNA content follows the number of chromatids in a nucleus: chromatids at 4C, at 2C and at 1C. Each halving of the chromatids halves the DNA, as the graph shows.
Watch out. Segment T has the same DNA content as segment P, but not the same nucleus: P has chromosomes of one chromatid each, while T has chromosomes of two chromatids each.
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