Chromosome behaviour in mitosis

Biology AS · The mitotic cell cycle. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Mitosis is one continuous process, but it is described in four stages: prophase, metaphase, anaphase, telophase. In each stage follow three things: the chromosomes, the nuclear envelope and the spindle.

The spindle is made of microtubules. Its fibres attach to the centromeres and pull the sister chromatids apart, so each new nucleus receives one copy of every chromosome.

To identify a stage in a photomicrograph, look at where the chromosomes are: scattered and thickening, lined up across the middle, in two groups moving apart, or in two groups at the ends with new nuclei forming.

Rules to remember

Prophase: chromosomes condense and become visible as two chromatids; the nuclear envelope breaks down; the spindle starts to form
Metaphase: chromosomes line up along the equator of the cell, with spindle fibres attached to their centromeres
Anaphase: centromeres divide; sister chromatids are pulled to opposite poles, centromere first, giving V shapes
Telophase: chromatids reach the poles and uncoil; nuclear envelopes re-form; the spindle breaks down
Cytokinesis in animal cells: the cell surface membrane pinches inwards. In plant cells: a cell plate forms across the middle and a new cell wall is built
Animal cells have centrioles at the poles of the spindle; plant cells form a spindle without centrioles
Count chromosomes by counting centromeres: once the centromere divides, each chromatid counts as a chromosome

Common mistake

Halving the chromosome number after mitosis. Daughter cells have the same number of chromosomes as the parent cell; only the number of chromatids per chromosome changes from two to one.

Worked example

A body cell of a fruit fly has 8 chromosomes. How many chromatids are present at prophase, how many chromosomes move to each pole in anaphase, and how many are in each daughter nucleus?

DNA was replicated in S phase, so at prophase each chromosome has two chromatids: 8 × 2 = 16 chromatids.
In anaphase the centromeres divide and the 16 chromatids separate equally: 16 ÷ 2 = 8 to each pole.
Each daughter nucleus forms around one group, so it has 8 chromosomes.

Answer: 16 chromatids at prophase, 8 chromosomes moving to each pole, 8 chromosomes in each daughter nucleus.

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