Continuity of Life: Cell Division, Reproduction and Heredity
This domain joins three things the exam treats together: how cells divide, how organisms reproduce, and how characters are transmitted. Questions commonly ask you to identify a mitotic or meiotic stage, state the significance of each division, name an asexual method with its example, and explain double fertilisation or a simple monohybrid ratio.
Cell division
- Interphase has G1, S and G2; DNA replicates in S phase.
- Mitosis: prophase (chromosomes condense, spindle forms), metaphase (chromosomes at the equator), anaphase (chromatids separate to poles), telophase (nuclei reform). 2n gives 2n; two identical cells for growth and repair.
- Meiosis: two successive divisions, 2n to n; crossing over in prophase I, independent assortment in metaphase I; four haploid cells; produces gametes and variation.
Reproduction
- Asexual: binary fission (Amoeba), budding (yeast, Hydra), fragmentation (Spirogyra), spore formation (Rhizopus), vegetative propagation (potato tuber, ginger rhizome, Bryophyllum leaf).
- Flowering plants: pollination, pollen tube growth, then double fertilisation - one male gamete fuses with the egg to give the diploid zygote, the second fuses with two polar nuclei to give the triploid primary endosperm nucleus.
Heredity
- A gene is a DNA segment; alleles are its alternative forms; the dominant allele shows in the heterozygote.
Worked example
Cross Tt with Tt. Gametes are T and t on both sides, giving TT : Tt : tt in the ratio 1:2:1. Since TT and Tt are both tall, the phenotypic ratio is 3 tall : 1 dwarf, that is 75 per cent tall.
Traps that cost marks
- Crossing over belongs to prophase I, not meiosis II.
- Endosperm is triploid (3n), the zygote diploid.
- Mitosis keeps the chromosome number constant; meiosis halves it.