Chromosomes, mitosis and the cell cycle
Chromosomes
The nucleus holds chromosomes: long molecules of DNA, each carrying many genes.
In body cells they come in pairs (humans have 46 chromosomes, 23 pairs).
When a cell divides by mitosis, the two new cells are genetically identical to each other and to the parent, with the same number of chromosomes. Mitosis is for growth, repair, replacing cells and asexual reproduction.
The cell cycle
- Stage 1 (often called interphase, the longest stage): the cell grows, makes more ribosomes and mitochondria, and copies its DNA, so each chromosome has two copies.
- Stage 2 (mitosis): the chromosomes line up across the middle, then one set is pulled to each end; a nucleus forms at each end.
- Stage 3: the cytoplasm and cell membrane divide: two identical cells.
The chromosomes are copied first so that each new cell gets a full set. A body cell with 20 chromosomes has 40 chromosome copies just before mitosis, and each new cell gets 20. Body-cell chromosomes come in pairs, so 20 chromosomes is 10 pairs.
Every round doubles the number of cells.
number of cells = starting number × 2ⁿ (n = number of divisions)
- One cell, 6 divisions
- 1 × 2⁶ = 64 cells
To find how many divisions give at least a target number, keep doubling: 2⁹ = 512 is less than 600 but 2¹⁰ = 1024 is more, so a single cell needs 10 divisions to pass 600 cells. If each division takes 15 hours, that is 10 × 15 = 150 hours.
How long is each stage? (Higher)
In a root tip slide, cells are frozen at random points of the cycle.
The fraction of cells in a stage equals the fraction of the cycle spent in it.
- 200 cells counted, 18 in mitosis
- 18 ÷ 200 × 100 = 9.0% of cells
- Cycle 20 hours = 1200 minutes
- 0.09 × 1200 = 108 minutes in mitosis
Do not halve the chromosome number in mitosis: halving is meiosis, the cell division that makes gametes (sex cells: sperm and eggs).
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