Reproduction

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

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):

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

1. Sperm are deposited in the vagina and swim through the cervix and uterus
2. A sperm meets the egg in the fallopian tube
3. One sperm penetrates the egg — fertilisation occurs; a zygote forms
4. The zygote undergoes repeated mitosis as it travels down the fallopian tube
5. By the time it reaches the uterus, it is a ball of cells (blastocyst)
6. The blastocyst implants into the thickened uterine lining
7. It develops into an embryo (first 8 weeks), then a foetus (week 8 to birth)
8. 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:

How meiosis creates genetic variation:

1. Crossing over — during meiosis I, homologous chromosomes exchange segments, creating new combinations of alleles
2. Independent assortment — chromosomes from each parent are randomly distributed to daughter cells
3. 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