A cell in the root tip of an onion has 2n = 16 chromosomes, and its nucleus holds 2C of DNA in G1…
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A cell in the root tip of an onion has chromosomes, and its nucleus holds 2C of DNA in G1 phase. The cell goes through one cell cycle and divides by mitosis.
- In G2 phase, how many chromosomes and how many chromatids does the cell have, and what is its DNA content in units of C?
- At metaphase, each chromatid is attached to spindle fibres by its own kinetochore. How many kinetochores are attached in all?
- During anaphase, before cytokinesis, how many chromosomes are there in the whole cell?
- Just after cytokinesis, how many chromosomes does each daughter cell have, and what is its DNA content?
The worked solution
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(a) G2 phase. During S phase the amount of DNA per cell doubles, from 2C to 4C, but there is no increase in the chromosome number. So in G2 the cell has chromosomes, each made of two sister chromatids: chromatids and 4C of DNA.
(b) Metaphase. One chromatid of each chromosome is connected by its kinetochore to spindle fibres from one pole, and its sister chromatid by its own kinetochore to spindle fibres from the opposite pole. That is kinetochores attached.
(c) Anaphase. The chromosomes split, and the two daughter chromatids are now called the chromosomes of the future daughter nuclei. Until cytokinesis they are all in one cell: chromosomes.
(d) Daughter cells. Each daughter nucleus gets one chromatid of every chromosome: chromosomes and 2C of DNA, the same as the parent cell had in G1. This is why mitosis is called an equational division.
Answer: (a) chromosomes, chromatids, 4C. (b) kinetochores. (c) chromosomes. (d) chromosomes and 2C.
Check. The two daughter cells together have chromosomes and of DNA, exactly what the cell had at anaphase.
Watch out. The chromosome number of the cell doubles only for the short time between the splitting of the centromeres and cytokinesis. In G2 and at metaphase the cell has chromosomes, not .
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