AQA Biology 4.1.2.1–4.1.2.2 · Process
Chromosomes, the cell cycle and mitosis
You began as one cell. How does one cell become two that carry exactly the same chromosomes, again and again, without losing a single one?
Combined Science · Higher tier
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One fertilised egg. Trillions of cells.
Every cell in your skin, muscles and gut came from one fertilised egg dividing again and again, and nearly every body cell still carries the same 46 chromosomes. Here is a model cell with just 4 chromosomes. It is about to divide into two cells, and each new cell needs all 4.
If you had to guess, does the cell copy its chromosomes before it divides, or share out the 4 it has?
The nucleus of a cell contains chromosomes, each one a long molecule of DNA. Each chromosome carries a large number of genes: a gene is a small section of DNA that codes for a particular protein. In body cells the chromosomes are found in pairs, one of each pair from each parent. A human body cell has 46, in 23 pairs. Sperm and egg cells carry one of each pair, 23 in all, so fertilisation restores 46. In the model, red chromosomes came from one parent and blue from the other.
Run the cell cycle · stage 1 of 3
What does the cell do before it divides?
What happens first?
After copying, how many chromosomes does the model cell hold?
Build the sequence
Describe the cell cycle, in order
This is the order the marks follow in a describe question. Leave out anything that does not happen in the cell cycle.
5 links belong. 2 do not.
The links
Your chain · 0 of 5
Mitosis makes cells identical to the parent, so it is used wherever an organism needs more of the same cells: growth and development from a fertilised egg, repair of damaged tissue, and replacement of worn-out cells. In mature animals, cell division is mainly restricted to repair and replacement. Some organisms reproduce asexually this way: one parent, no gametes, only mitosis, so every offspring is a clone. In animals, mitosis never makes gametes.
Suggest
Mitosis or meiosis?
Decide which division is happening in each situation.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Every stage of the cycle is controlled. Cancer is the result of changes in cells that lead to uncontrolled growth and division, forming a mass of cells called a tumour. A benign tumour stays in one area, usually within a membrane, and does not invade other parts of the body. Malignant tumour cells are cancers: they invade neighbouring tissues and spread in the blood to other parts of the body, where they form secondary tumours.
Think again
“Cancer is caused by mitosis.”
Spot the flaw
ExplainYour cells divide by mitosis every day. So what is actually wrong in a cancer?
Command words in this lesson
- Describe
- Say what happens, stage by stage, in the right order.
- Explain
- Give the reason, and link each step to the next with “so”.
- Use
- Base the answer on the graph or data given. Quote it.
Key fact
The cell grows and its DNA replicates; in mitosis one set of chromosomes is pulled to each end and the nucleus divides; then the cytoplasm and membrane divide into two identical cells.
Key note · AQA 4.1.2.1–4.1.2.2
Chromosomes, the cell cycle and mitosis
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
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Not sure what happens at each stage of the cell cycle?
The model cell has 4 chromosomes in 2 pairs so each one can be followed; a human body cell has 46. Ribosomes and mitochondria are drawn as a few marks, not to scale. The stepper shows AQA's three overall stages; GCSE does not ask for the names of the phases inside mitosis.