3.1 The cell cycle and cancer
A cell's life consists of interphase and the division phase. During the G1 phase of interphase the cell grows, in S phase the DNA is copied, and in G2 it prepares to divide. After replication, each chromosome consists of two sister chromatids joined at the centromere.
The cycle is guarded by checkpoints in G1, G2 and metaphase: if the DNA is damaged or chromosomes aren't properly attached to spindle fibres, the cycle is halted. Tumour-suppressor proteins such as p53 send damaged cells to repair or to programmed cell death (apoptosis). Mutations that break this control can make cells divide uncontrollably — cancer.
3.2 Mitosis: identical copies
Mitosis turns one cell into two genetically identical cells, allowing growth, repair and asexual reproduction. In prophase the chromosomes condense and spindle fibres form. In metaphase the chromosomes line up across the middle of the cell. In anaphase the sister chromatids separate and are pulled to opposite poles. In telophase two new nuclei form, and the cytoplasm then divides (cytokinesis).
In animal cells, cytokinesis happens by the membrane pinching in. Plant cells can't pinch because of their wall, so a cell plate forms across the middle instead. Cells made by mitosis have the same chromosome number as the parent: a human body cell (2n = 46) produces two cells with 46 chromosomes.
3.3 Meiosis: a variety factory
Meiosis makes reproductive cells (gametes) in sexually reproducing organisms. Through two divisions in a row, one diploid (2n) cell becomes four haploid (n) cells; in humans, a 46-chromosome cell gives gametes with 23 chromosomes. When two gametes join at fertilisation, the number returns to 46.
In prophase I, homologous chromosomes pair up and swap segments (crossing over), making new combinations of the mother's and father's genes. In metaphase I, the way homologous pairs line up and separate is random (independent assortment); with n pairs, 2ⁿ different arrangements are possible. If homologues or chromatids fail to separate (non-disjunction), gametes get an extra or missing chromosome; three copies of chromosome 21 cause Down syndrome.
FormulaGamete types from independent assortment = 2ⁿ (n = number of homologous pairs; in humans 2²³ ≈ 8.4 million)
Key terms
- Homologous chromosomes
- A pair of chromosomes, one from each parent, carrying the same genes.
- Crossing over
- Homologous chromosomes swapping segments during meiosis.
- Apoptosis
- Programmed death of a damaged or unneeded cell.
- Non-disjunction
- Chromosomes failing to separate properly during division.
End-of-unit problems
Solve each one yourself first, then open the solution.
An organism has 2n = 8 chromosomes in its body cells. Assuming no crossing over, how many kinds of gamete can independent assortment produce? How does crossing over affect this number?
Show the solution
2n = 8 means n = 4 homologous pairs, so there are 2⁴ = 16 kinds of gamete. Crossing over mixes the chromosomes themselves, so it raises the number much further.
Let the DNA content of a human cell in G1 be 2c. What is the DNA content in G2, in each cell after mitosis, and in each gamete at the end of meiosis II?
Show the solution
DNA is copied in S phase, so G2 has 4c. In mitosis the sister chromatids separate and each cell gets 2c. After meiosis I each cell has 2c (n chromosomes, each still with two chromatids), and after meiosis II each gamete has c.
While an egg is forming, chromosome pair 21 fails to separate in meiosis I. If the egg is fertilised by a normal sperm, how many chromosomes could the zygote have? Give both possibilities.
Show the solution
Non-disjunction gives an egg with either 24 chromosomes (two copies of 21) or 22 (no chromosome 21). A normal sperm adds 23: the zygote has either 47 chromosomes (three copies of 21 — Down syndrome) or 45 (a single 21, usually not viable).
A drug prevents spindle fibres from forming. At which stage, and why, does it stop the cycle in fast-dividing cancer cells?
Show the solution
Without spindle fibres, chromosomes can't line up at metaphase or be pulled to the poles. The metaphase checkpoint won't allow anaphase until every chromosome is attached to the spindle, so the cell is stuck in metaphase and often undergoes apoptosis. Some cancer drugs work this way.
🦉 An easier explanation: Professor Owl's lesson: Cells: the building blocks of life