IMAT Biology · Cell Division and Reproduction
Asexual and Sexual Reproduction and Life Cycles
Asexual reproduction copies one parent by mitosis or fission; sexual reproduction mixes two parents' genes through meiosis and fertilisation.
Why this matters
The 2014 paper compared asexual and sexual reproduction in a which-statement-is-correct question, and a 2021 question asked which structure in a set of diagrams is formed by mitosis. Life cycles have not been asked yet, but they test whether you know where meiosis and mitosis sit in plants.
Concept 1 of 2: Asexual and sexual reproduction: methods, advantages and costs
Asexual reproduction is fast and needs no partner, but every offspring is a copy, so one new disease or change in climate can hit them all. Sexual reproduction is slower and costs energy, but it mixes genes, so some offspring may cope with a change. Many organisms keep both options.
Definition
Asexual reproduction: one parent; offspring are clones, genetically identical to the parent apart from mutations.
- In eukaryotes it uses mitosis; bacteria use binary fission.
Sexual reproduction: two haploid gametes fuse at fertilisation; meiosis and fertilisation create variation.
- Mutations happen in both kinds; only sexual reproduction adds recombination.
- Many organisms can do both: strawberries (seeds and runners), aphids (sexual eggs and parthenogenesis), yeast, Hydra.
| Method | How it works | Example |
|---|---|---|
| Binary fission | One cell copies its DNA and splits in two | Bacteria, Amoeba |
| Budding | An outgrowth grows and breaks off | Yeast, Hydra |
| Fragmentation | A piece of the body regrows into a whole organism | Flatworms (planarians), starfish |
| Vegetative propagation | A new plant grows from a stem, root or leaf | Potato tubers, strawberry runners, onion bulbs |
| Parthenogenesis | An egg develops without being fertilised | Aphids, male honeybees, some lizards |
Sexual reproduction is favoured when the environment changes; asexual reproduction when it is stable.
Practice this conceptself-check · 4 quick reps
Variation is not only for sexual reproducers
Asexual populations also vary, through mutation, and natural selection acts on them (bacterial antibiotic resistance is the famous case). Sexual reproduction adds much more variation by recombining genes, but it is not the only source.
Concept 2 of 2: Life cycles: where meiosis, mitosis and fertilisation fall
Every sexual life cycle alternates two events: meiosis halves the chromosome number and fertilisation doubles it. Organisms differ in which stage grows into the main body. In animals the body is diploid; in most fungi it is haploid; in plants both stages grow into a body, one after the other.
Definition
Three patterns of sexual life cycle:
- Diplontic (animals): the adult is diploid; meiosis makes gametes directly, so gametes are the only haploid cells.
- Haplontic (most fungi, many algae such as Chlamydomonas): the adult is haploid; the zygote is the only diploid cell and undergoes meiosis at once.
- Alternation of generations (all land plants): a diploid sporophyte makes haploid spores by meiosis; a spore grows by mitosis into a haploid gametophyte, which makes gametes by mitosis.
- In mosses the gametophyte is the main plant; in ferns and flowering plants the sporophyte is.
- Flowering plants have double fertilisation: one sperm nucleus joins the egg (zygote, 2n), the other joins two polar nuclei (endosperm, 3n).
| Life cycle | Main body | Meiosis makes | Example |
|---|---|---|---|
| Diplontic | Diploid | Gametes | Humans and other animals |
| Haplontic | Haploid | Haploid cells from the zygote | Most fungi, Chlamydomonas |
| Alternation, gametophyte dominant | Haploid gametophyte | Spores | Mosses |
| Alternation, sporophyte dominant | Diploid sporophyte | Spores | Ferns, flowering plants |
Practice this conceptself-check · 4 quick reps
Gametes are not always made by meiosis
In animals, meiosis makes gametes. In plants, meiosis makes spores, and the haploid gametophyte then makes gametes by mitosis. An option saying gametes always come straight from meiosis is wrong for plants.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Asexual and sexual reproduction: methods, advantages and costs5 rows
| Method | How it works | Example |
|---|---|---|
| Binary fission | One cell copies its DNA and splits in two | Bacteria, Amoeba |
| Budding | An outgrowth grows and breaks off | Yeast, Hydra |
| Fragmentation | A piece of the body regrows into a whole organism | Flatworms (planarians), starfish |
| Vegetative propagation | A new plant grows from a stem, root or leaf | Potato tubers, strawberry runners, onion bulbs |
| Parthenogenesis | An egg develops without being fertilised | Aphids, male honeybees, some lizards |
Sexual reproduction is favoured when the environment changes; asexual reproduction when it is stable.
Life cycles: where meiosis, mitosis and fertilisation fall4 rows
| Life cycle | Main body | Meiosis makes | Example |
|---|---|---|---|
| Diplontic | Diploid | Gametes | Humans and other animals |
| Haplontic | Haploid | Haploid cells from the zygote | Most fungi, Chlamydomonas |
| Alternation, gametophyte dominant | Haploid gametophyte | Spores | Mosses |
| Alternation, sporophyte dominant | Diploid sporophyte | Spores | Ferns, flowering plants |
Watch out for (2)
- Variation is not only for sexual reproducers→ Asexual and sexual reproduction: methods, advantages and costs
- Gametes are not always made by meiosis→ Life cycles: where meiosis, mitosis and fertilisation fall
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