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Why Cells Divide: Growth, Repair and Life Itself
Every living thing starts as one cell. A body grows because cells divide, wounds heal because neighbouring cells divide to fill the gap, and a single-celled amoeba reproduces by dividing. Two different kinds of division do two different jobs. Mitosis makes two identical copies of a body (somatic) cell, so the chromosome number stays the same. Meiosis happens only in the reproductive organs to make gametes (sperm and egg, or pollen and ovule cells) with half the usual number of chromosomes, so that fertilisation restores the full number in the next generation. Both are preceded by interphase, the long preparation period in which the cell grows and copies its DNA. Exam questions test three things: the chromosome count after each division, the order of phases, and the one event, crossing over, that makes meiosis a source of variation.
1. In which type of cell division is the chromosome number of the daughter cells half that of the parent cell?
2. At which stage of mitosis do the chromosomes line up on the equator of the cell?
3. A pea plant body cell has 14 chromosomes. How many chromosomes will each cell have after meiosis?
4. Crossing over, the exchange of segments between homologous chromosomes, occurs during:
5. A single cell undergoes mitosis three times in succession. How many cells result?
6. How many functional ova are produced from one primary oocyte after meiosis?
7. During cytokinesis a plant cell divides its cytoplasm by forming:
8. In which division do homologous chromosomes (not chromatids) separate from each other?