The Mitotic Cell Cycle: Question 3
Syllabus 5.1
The broad bean, Vicia faba, has a diploid chromosome number of 12. A root tip cell from a broad bean seedling completes interphase and then enters mitosis, passing through prophase, metaphase, anaphase and telophase before the cytoplasm divides.
(a) Describe the behaviour of the chromosomes during prophase. [2]
(b) Describe the behaviour of the chromosomes during metaphase. [2]
(c) Describe what happens to the centromeres and to the sister chromatids of each chromosome during anaphase. [2]
(d) State two events, other than the movement of chromosomes, that occur during telophase. [2]
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Part (a): Prophase
At the start of prophase, the chromosomes are already present in the nucleus (each one having been replicated into two sister chromatids during S phase), but they exist as long, thin, loosely coiled chromatin and cannot be seen individually. During prophase, each chromosome condenses, supercoiling into a shorter, thicker structure that becomes visible under a light microscope as two sister chromatids joined at a centromere. At the same time, the nuclear envelope breaks down and the nucleolus disappears, and spindle fibres begin to form, extending from the two poles of the cell (organised by centrioles in animal cells).
Part (b): Metaphase
By metaphase, the nuclear envelope has completely broken down and the spindle fibres span the cell. Each chromosome, still made of two sister chromatids joined at a centromere, is moved by the spindle fibres until it lies individually along the equator of the cell, forming the metaphase plate. A spindle fibre from each pole attaches to the centromere of every chromosome, one fibre pulling towards each pole, so that every chromosome is held under tension at the centre of the cell before separation begins.
Part (c): Anaphase
Anaphase begins when the centromere of each chromosome divides, breaking the attachment between its two sister chromatids. The spindle fibres attached to each former sister chromatid then shorten, pulling the two chromatids apart and towards opposite poles of the cell, centromere first. Once separated, each chromatid is a complete, independent DNA molecule with its own centromere, so from this point onward each one is correctly referred to as a chromosome rather than a chromatid.
Part (d): Telophase
During telophase, the chromosomes (one full, identical set at each pole) begin to decondense, uncoiling back into their less visible, more loosely arranged chromatin form. A nuclear envelope reforms around each of the two groups of chromosomes, creating two new nuclei, and the nucleolus reappears inside each one. The spindle fibres also break down. (Cytokinesis, the physical division of the cytoplasm into two separate daughter cells, follows telophase but is a separate stage from mitosis itself.)
Final answers
- (a) Chromosomes condense and become visible (as two sister chromatids); nuclear envelope breaks down; nucleolus disappears; spindle begins to form.
- (b) Chromosomes (each still two sister chromatids) line up individually along the equator, attached to spindle fibres from both poles by their centromeres.
- (c) Centromeres divide, separating sister chromatids, which are pulled by shortening spindle fibres to opposite poles, becoming individual chromosomes.
- (d) Nuclear envelope reforms around each set of chromosomes; chromosomes decondense; nucleolus reappears.