A flowering plant has 2n = 20 chromosomes.
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A flowering plant has chromosomes. The nucleus of one of its meiocytes holds of DNA in G1 phase ( is ). The meiocyte replicates its DNA and goes through meiosis.
- How many bivalents are seen in pachytene, and how many chromatids does the nucleus have then?
- How much DNA does the nucleus hold in pachytene?
- After telophase I and cytokinesis, how many chromosomes does each cell of the dyad have, and how much DNA does its nucleus hold?
- How much DNA does each nucleus hold at the end of meiosis II?
The worked solution
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(a) Pachytene. The chromosomes pair up as bivalents (tetrads). Each bivalent has four chromatids, so the nucleus has chromatids.
(b) DNA in pachytene. Meiosis I begins after the chromosomes have replicated in S phase, so the DNA has doubled: (4C).
(c) The dyad. In anaphase I the homologous chromosomes separate, while the sister chromatids stay together at their centromeres. Each cell of the dyad gets one homologue of each pair: chromosomes, each still with two chromatids. Its DNA is half of , that is (2C).
(d) After meiosis II. There is no replication of DNA in interkinesis. In anaphase II the centromeres split and the sister chromatids separate, so the DNA halves again: (1C) in each of the four haploid nuclei.
Answer: (a) bivalents and chromatids. (b) . (c) chromosomes and . (d) .
Check. The four nuclei together hold , all the DNA of the pachytene nucleus, and single chromatids, one for each chromatid of pachytene.
Watch out. A dyad cell is haploid in chromosome number () but still has 2C of DNA, the same amount as the G1 meiocyte. The number of chromosomes halves in meiosis I; the DNA reaches 1C only after meiosis II.
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