Ch 2 · 5.2 Chromosome behaviour in mitosis · AS & A Level Biology 9700
Topic 2 of 2 in The mitotic cell cycle — AS & A Level Biology 9700 — 39 lessons.
Introduction
Mitosis is the nuclear division that produces two genetically identical daughter nuclei from one, and Cambridge examiners frequently ask candidates to identify the stage of mitosis from a photomicrograph and to justify the identification from chromosome behaviour. This lesson works through the four stages in order, the mechanism of chromosome movement, how plant and animal cells finish dividing, and how to identify each stage on a slide. (Syllabus section 5.2.)
Key ideas — The stages of mitosis
In prophase, chromosomes supercoil and become visible as two sister chromatids joined at the centromere, the nucleolus disappears, the nuclear envelope breaks down, and the spindle begins to form (in animal cells, centrioles move to opposite poles). In metaphase, chromosomes are moved to, and line up individually along, the equator of the spindle, attached by their centromeres to spindle microtubules from both poles. In anaphase, the centromere of each chromosome divides, and the two sister chromatids — now separate daughter chromosomes — are pulled to opposite poles, centromere first, giving a V-shape. In telophase, the chromosomes decondense into chromatin, nuclear envelopes reform around each set, nucleoli reappear, and the spindle breaks down; cytokinesis usually follows immediately.
🧑🏫 Teacher's explanation
Think of mitosis as a highly organized dance rehearsal where the goal is to ensure every single dancer (chromosome) ends up in the correct position so that when the curtain rises (cell division), both sides of the stage have exactly the same number of performers. If one dancer trips or is missing, the entire performance is ruined.
📖 Definitions
🔍 Worked example — Easy
A cell in prophase contains 46 chromosomes, each consisting of two sister chromatids. How many chromosomes and how many chromatids are present in this cell?
🔍 Worked example — Standard
A cell undergoes mitosis. If the original cell had 20 chromosomes, how many chromosomes will each daughter cell have after telophase is complete?
🔍 Worked example — Exam-style
A student observes a cell under a light microscope. They see chromosomes lined up individually along the equator of the cell. Identify the stage and explain the significance of this arrangement.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The role of the spindle
The spindle is made of protein microtubules radiating from two poles. Some microtubules attach to the centromere of each chromosome at a protein structure called the kinetochore; these microtubules shorten during anaphase, by loss of tubulin subunits, pulling the attached chromatid towards that pole. Other microtubules run from pole to pole and lengthen the cell during anaphase, pushing the poles further apart. The spindle therefore ensures that one chromatid from every chromosome is moved to each pole, so both daughter nuclei receive a complete and identical set of chromosomes.
🧑🏫 Teacher's explanation
Imagine a massive tug-of-war happening inside a cell. To ensure the game ends fairly, every single player (chromatid) must be pulled toward a specific side so that both teams end up with the exact same number of players. If one player is missed, one team loses, and the cell becomes dysfunctional.
📖 Definitions
🔍 Worked example — Easy
Identify the stage of mitosis where chromosomes are aligned at the equator of the cell due to the action of spindle fibres.
🔍 Worked example — Standard
Explain how the spindle ensures that daughter cells are genetically identical.
🔍 Worked example — Exam-style
A cell is observed under a light microscope. The chromosomes are being pulled towards the poles, and the cell is elongating. Identify the stage and describe the role of the non-kinetochore microtubules.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Cytokinesis: animal cells and plant cells
After telophase the cytoplasm divides, and here plant and animal cells differ:
Key ideas — Identifying the stages in photomicrographs
Root tips (e.g. onion or garlic) are used because cells there divide rapidly. Look for these clues:
🧑🏫 Teacher's explanation
Think of the cell cycle as a highly organized construction project. Before any building begins, the blueprints (DNA) must be copied and organized. During mitosis, the goal is to ensure that each new 'building' (daughter cell) receives an exact copy of these blueprints.
📖 Definitions
🔍 Worked example — Easy
A student observes a field of view containing 50 cells. Out of these, 10 cells are clearly in various stages of mitosis. Calculate the mitotic index.
🔍 Worked example — Standard
Describe the appearance of chromosomes and the nuclear envelope during prophase in an animal cell.
🔍 Worked example — Exam-style
A photomicrograph shows chromosomes lined up along the equatorial plane of the cell, with spindle fibres attached to the centromeres. Identify the stage and describe the state of the nuclear envelope.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the appearance of chromosomes during metaphase. [2] (b) The student counts 100 cells in total. 15 cells are in prophase, 5 are in metaphase, 5 are in anaphase, and 5 are in telophase. Calculate the mitotic index. [1] [3 marks]
Worked example
Question. A student examined a stained root tip squash and counted 300 cells in one field of view. Of these, 45 cells showed visible chromosomes undergoing mitosis. (a) Calculate the mitotic index of this sample, giving your answer as a percentage. [2] (b) A biopsy from a tumour gave a mitotic index of 38%. Suggest what this value indicates about the tumour tissue. [2]
🔍 Worked example — Easy
A student observes a slide of onion root tip cells under a light microscope. They identify 50 cells in prophase, 20 in metaphase, 15 in anaphase, and 5 in telophase. Describe the state of the nuclear envelope during prophase.
🔍 Worked example — Standard
Compare the arrangement of chromosomes in metaphase and anaphase.
🔍 Worked example — Exam-style
A photomicrograph shows chromosomes being pulled towards opposite poles of the cell by spindle fibers. Identify the stage and describe the state of the cell surface membrane.
⚠️ Common errors
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Practice
Answers