International A Level Biology / Mitosis, meiosis and reproduction

The stages of mitosis revision notes

Edexcel International A Level Biology (YBI11), chapter 8: Mitosis, meiosis and reproduction, specification point 3.14. Free lesson.

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Mitosis separates sister chromatids so that each new nucleus receives one copy of every chromosome. Knowing the stages helps you explain how this happens and identify mitosis in root tip cells.

What happens in each stage of mitosis

After S phase, each chromosome consists of two joined sister chromatids. Mitosis separates these copies to opposite ends of the cell. The four stages are prophase, metaphase, anaphase and telophase. Prophase: Chromosomes condense, becoming short, thick and visible. In an animal cell, the centrioles move towards opposite poles and a spindle starts to form between them. The nucleolus disappears and the nuclear envelope breaks down. Metaphase: Chromosomes line up in a single line along the equator, the middle of the cell. Spindle fibres attach to each chromosome at its…

How to draw a cell in metaphase

In metaphase, the sister chromatids are still joined. Your drawing should show the spindle reaching the centromeres, with the chromosomes lined up across the middle. Animal cells have centrioles at their poles, but plant cells do not have centrioles and still form a spindle. Show a pair of centrioles at each pole in an animal cell. Draw spindle fibres running from pole to pole. They are made of protein tubes called microtubules. Draw every chromosome in a single line on the equator. Show each centromere as the narrow middle of a chromosome, with spindle fibres reaching it. Keep the…

How to identify stages in root tip cells

Stained root tip cells can be examined in light micrographs. The chromosomes appear magenta. Look at whether the chromosomes are separate, lined up, moving apart or gathered into new nuclei. Interphase: A round nucleus is visible, often with a darker nucleolus, but there are no separate chromosomes. Most cells in a photograph are in interphase. Prophase: The roughly round nucleus contains separate threads as the chromosomes condense. Metaphase: A band of chromosomes lies straight across the middle of the cell. Anaphase: Two groups move towards opposite ends. The V-shaped…

Why chromosome numbers do not halve

Mitosis separates sister chromatids, not whole chromosomes. Each chromatid becomes a chromosome at a pole, so both new nuclei keep the full chromosome number. A cell with 6 chromosomes has 6 pairs of sister chromatids before anaphase. Each chromosome splits at its centromere, so 6 chromatids go to each pole. Halving the chromosome number happens in meiosis, not mitosis.

What happens if spindle fibres do not form

Spindle fibres attach to centromeres and pull sister chromatids apart. Without them, the cell cannot complete anaphase and form new nuclei. If a herbicide stops spindle fibres forming in root tip cells, mitosis stops at metaphase. Nothing attaches to the centromeres, so the chromatids cannot be pulled to opposite poles. The cell does not reach anaphase.