Analyse the events during every stage of cell cycle and notice how the following two parameters change:
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell.
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During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i) Number of chromosomes (N) per cell
During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.
(ii) Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present. During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.
(i) Number of chromosomes (N) per cell
During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.
(ii) Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present. During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.
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In S - phase the DNA content therefore doubles
In meiosis anaphase-I, the number of chromosomes is decreased to zero,
Step by step explanation:
The number of chromosomes and the amount of DNA is modified both in process anaphase and S phase .
(i)
In S- phase, the number of chromosomes remains the same but each chromosome replicates in such a way that two chromatids are present. The DNA content therefore doubles from 1C to 2C or 2C to 4C depending on the haploid / diploid character of the cell.
(ii)
In Anaphase. The number of chromosomes stays equal in mitotic anaphase. Sister chromatids are the only ones that move towards their respective poles. The contents of DNA remain unaltered. Throughout meiosis anaphase-I, the number of chromosomes is decreased to zero, that is, from 2N to 1N. DNA volume of Meiosis-I Anaphase-I drops to one half from 2C to 1C
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