Chapter 3 in one view: comparing the ways of reproducing
Chapter 3 described many ways of reproducing, and they fall into two groups. In asexual reproduction one parent is enough: a spore, a bud or a piece of the body grows into a new organism. It rests on mitosis, so every new cell gets a full 2N set and the offspring is a copy of the parent. This is quick and needs no partner, and it works well while conditions stay the same. But when conditions change, all the copies share the same weakness, and many may be lost. In sexual reproduction two gametes, usually from two parents, fuse in fertilization. It costs more time and energy, but the offspring have new combinations of features, which helps them when surroundings change. It rests on meiosis, which halves the chromosome number, so gametes are haploid (N) and fertilization restores the diploid (2N) number. In humans the body cells have 46 chromosomes, each gamete has 23, and the zygote has 23 + 23 = 46. The chain is: zygote (2N), then mitosis makes the body cells (2N), then meiosis in the gonads makes gametes (N), then fertilization makes a new zygote (2N). If meiosis did not halve the number, it would double in every generation: 46, 92, 184 and so on. The same chain appears in plants and in the malaria parasite, with different organs.
| Asexual | Sexual | |
|---|---|---|
| Parents | one | usually two |
| Cell division behind it | mitosis | meiosis (gametes) and fertilization |
| Offspring | a copy of the parent | new combinations of features |
| Chromosomes | full 2N set in every new cell | gametes N; zygote 2N |
| Examples in Chapter 3 | fission, budding, regeneration, spores, tissue culture | conjugation, gametes and fertilization, flowers, humans |
| Example | Kind |
|---|---|
| Amoeba dividing in two | asexual |
| Yeast forming a bud | asexual |
| A piece of planaria growing into a whole animal | asexual |
| Carrot cells grown into a plant in tissue culture | asexual |
| Two Spirogyra filaments in conjugation | sexual |
| Pollen tube fertilizing an egg in a flower | sexual |
Chromosome model with beads, with a grown-up. Take 3 red and 3 blue beads, and label each pair a, b, c, with one red and one blue in each pair. This is a cell with 2N = 6. Make a gamete by taking exactly one bead of each letter, and write its number. Make a second gamete the same way, in a different colour mix, and join them to make a zygote. Write the number of beads in the gamete and in the zygote, and say what meiosis and fertilization did to the number.