Reproduction
⏱ 9 min✏️ Quiz at the end
Types of Reproduction
All organisms reproduce to pass their genetic information to the next generation. There are two fundamental types of reproduction.
Asexual Reproduction
- Requires one parent only
- Offspring are genetically identical to the parent (clones)
- No need to find a mate — fast and energy-efficient
- No genetic variation — all offspring are identical
| Method | Example organisms |
|---|---|
| Binary fission (splitting in two) | Bacteria, amoeba |
| Budding | Hydra, yeast |
| Runners / stolons | Strawberry plants, spider plants |
| Spores | Ferns, mosses, fungi |
| Vegetative propagation | Potatoes (tubers), daffodils (bulbs) |
| Fragmentation | Starfish, flatworms |
Advantages: Fast; no need for a mate; all offspring can reproduce; useful in stable environments.
Disadvantage: No genetic variation — a single disease or environmental change could wipe out the entire population.
Sexual Reproduction
- Requires two parents each contributing a gamete
- Gametes fuse during fertilisation to form a zygote
- Offspring are genetically different from both parents and from each other
- Slower and requires more energy than asexual reproduction
Advantage: Genetic variation — offspring may be better adapted to changing environments; variation is the raw material for evolution by natural selection.
Gametes
Gametes are specialised sex cells with half the normal chromosome number (haploid):
| Gamete | Produced in | Characteristics |
|---|---|---|
| Sperm (male) | Testes | Small, with tail for swimming; millions produced |
| Egg / Ovum (female) | Ovaries | Large, contains food reserves; one released per month |
When a sperm (23 chromosomes) fuses with an egg (23 chromosomes), the resulting zygote has 46 chromosomes — the full diploid number.
The Human Reproductive System
Female reproductive system:
- Ovaries — produce eggs (one released roughly every 28 days — ovulation) and hormones (oestrogen, progesterone)
- Fallopian tubes (oviducts) — carry the egg from ovary to uterus; site of fertilisation
- Uterus (womb) — where the embryo implants and develops; lining thickens each month
- Cervix — lower part of uterus; dilates during birth
- Vagina — birth canal; receives sperm during intercourse
Male reproductive system:
- Testes — produce sperm and testosterone; kept outside the body (in scrotum) at slightly below body temperature
- Epididymis — where sperm mature and are stored
- Vas deferens — tube that carries sperm toward the urethra
- Seminal vesicles and prostate gland — produce seminal fluid that nourishes and carries sperm
- Urethra — carries sperm (semen) out of the body; also carries urine (but not simultaneously)
- Penis — delivers sperm into the female reproductive system
Fertilisation and Development
- Sperm are deposited in the vagina and swim through the cervix and uterus
- A sperm meets the egg in the fallopian tube
- One sperm penetrates the egg — fertilisation occurs; a zygote forms
- The zygote undergoes repeated mitosis as it travels down the fallopian tube
- By the time it reaches the uterus, it is a ball of cells (blastocyst)
- The blastocyst implants into the thickened uterine lining
- It develops into an embryo (first 8 weeks), then a foetus (week 8 to birth)
- The placenta forms — supplies nutrients and oxygen from mother's blood; removes waste
Meiosis
Meiosis is the type of cell division used to produce gametes. It differs from mitosis in important ways:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of daughter cells | 2 | 4 |
| Chromosome number | 46 (diploid) | 23 (haploid) |
| Genetic identity | Identical to parent cell | Genetically different |
| Purpose | Growth and repair | Gamete production |
How meiosis creates genetic variation:
- Crossing over — during meiosis I, homologous chromosomes exchange segments, creating new combinations of alleles
- Independent assortment — chromosomes from each parent are randomly distributed to daughter cells
- Random fertilisation — any sperm can fertilise any egg
The Menstrual Cycle
The menstrual cycle prepares the uterus for a potential pregnancy approximately every 28 days:
- Days 1–5: Menstruation — uterine lining sheds if no implantation occurred
- Days 6–13: Uterine lining rebuilds; egg matures in ovary
- Day 14: Ovulation — egg released from ovary (triggered by LH surge)
- Days 15–28: Uterine lining maintained by progesterone; if no fertilisation, progesterone drops and cycle restarts
Key Terms
- Gamete — a sex cell (sperm or egg) with half the normal chromosome number (haploid)
- Fertilisation — fusion of sperm and egg nuclei to form a zygote
- Zygote — fertilised egg cell with full diploid chromosome number
- Meiosis — cell division producing haploid gametes with genetic variation
- Diploid — containing two sets of chromosomes (46 in humans)
- Haploid — containing one set of chromosomes (23 in humans)
- Implantation — attachment of the blastocyst to the uterine wall
- Placenta — organ connecting embryo to mother's blood supply
Common Mistakes
- Confusing meiosis (produces gametes, 4 haploid cells) with mitosis (produces body cells, 2 diploid cells)
- Thinking fertilisation occurs in the uterus — it occurs in the fallopian tube
- Saying asexual reproduction produces variation — it produces identical clones
- Forgetting that the placenta does not allow mother's blood to mix directly with the embryo's blood — exchange occurs by diffusion
Tips and Tricks
- Meiosis = gametes = half the chromosomes = variation; Mitosis = growth = same number = identical
- Remember the fallopian tube as the "meeting place" for sperm and egg
- Asexual = one parent = clones; Sexual = two parents = variation
- Sperm are small and numerous; eggs are large and rare — these differences reflect their different roles