Sexual and asexual reproduction (including runners and cuttings)
The two types of reproduction
Sexual reproduction involves two parents and the fusion of gametes (sex cells). Asexual reproduction involves one parent and no gametes.
| Sexual | Asexual | |
|---|---|---|
| Parents | Two (usually) | One |
| Gametes | Yes — male and female gametes fuse | No gametes |
| Cell division involved | Meiosis to make gametes, then mitosis | Mitosis only |
| Offspring | Genetically different from parents and each other — variation | Genetically identical to the parent — clones |
| Speed / numbers | Slower, needs a mate | Fast, many offspring quickly |
| Advantage | Variation means some offspring may survive a change in the environment | Rapid population growth; no mate needed; good characteristics passed on exactly |
Fertilisation (3.2)
Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
The single cell produced is a zygote.
The zygote then divides by mitosis to form a ball of cells — this is the embryo, which continues to divide and differentiate.
Sequence to learn in order:
male gamete + female gamete → fertilisation → zygote → cell division (mitosis) → embryo
In humans the gametes are sperm and egg cell (ovum); in flowering plants they are the pollen nucleus and the nucleus of the ovule.
Asexual reproduction in plants (3.7)
Natural method — runners (e.g. strawberry, spider plant)
- The parent plant grows a horizontal stem, a runner, along the soil surface.
- At a node on the runner a new plant grows roots and leaves.
- The runner dies back and the new plant is independent.
- The new plant is a clone — genetically identical to the parent.
Artificial method — cuttings
- A short length of stem with a few leaves is cut from a healthy parent plant.
- The cut end may be dipped in rooting hormone (auxin) and planted in damp compost.
- Covering with a clear plastic bag reduces water loss until roots grow.
- Every plant grown this way is a clone of the parent, so a grower can produce many identical plants with a desirable feature quickly and cheaply.
Examiner tip. Two marks are thrown away here every year. (1) Write zygote for the single cell formed at fertilisation and embryo for the ball of cells it divides into — candidates often say 'zygote' when the mark scheme wants 'embryo'. (2) Say that the nuclei of the gametes fuse, not 'the genes fuse' or 'the cells join'. Also, read the question: candidates frequently write about cuttings or cloning when the question was about sexual reproduction — asexual detail scores zero there.