Reproduction

In short: Reproduction makes new organisms. Asexual reproduction uses one parent and gives identical offspring, while sexual reproduction joins gametes from two parents and gives varied offspring. In flowering plants, pollination is followed by fertilisation, and ovules become seeds.

Starting point: what you already know

Offspring inherit DNA from their parents.
Sperm and egg join at fertilisation (Year 9 and 10 science).
Flowers are the reproductive organs of flowering plants.

Sexual and asexual reproduction

Organism	Asexual methods	Sexual reproduction?
Bacteria	Binary fission: the cell copies its DNA and splits in two, as often as every 20 minutes	No, but they can swap DNA (conjugation)
Protists (Amoeba, Paramecium)	Binary fission	Some do
Fungi (yeast, moulds)	Yeast budding. Moulds make huge numbers of spores	Yes, many fungi also make sexual spores
Plants	Vegetative propagation: runners (strawberries), tubers (potatoes), bulbs (onions), cuttings	Yes, through flowers
Animals	Budding (Hydra), fragmentation (sea stars), parthenogenesis (some sharks, lizards, bees)	Yes, mostly

Fertilisation in animals

Fertilisation is the joining of a male gamete (sperm) and a female gamete (egg) to form a zygote.

	External fertilisation	Internal fertilisation
Where	Outside the body, usually in water	Inside the female's body
Gametes released	Huge numbers, because most are lost	Fewer
Examples	Most fish, frogs, corals	Mammals, birds, reptiles, sharks
Advantage	Little energy spent finding a mate	Higher chance of fertilisation, embryo is protected

The annual mass spawning of corals on the Great Barrier Reef, a few nights after a full moon in spring, is external fertilisation on a huge scale.

Reproduction in flowering plants

Pollination: pollen is carried from the anther (male) to the stigma (female) by wind, insects, birds or mammals. Banksias and grevilleas are pollinated by honeyeaters and even small possums.
A pollen tube grows down the style to the ovary.
Fertilisation: a male gamete from the pollen joins the egg in the ovule.
The ovule becomes a seed and the ovary becomes a fruit.
Seeds are dispersed by wind, water, animals or explosion.

Hormones in human pregnancy

Hormones are chemical messengers carried in the blood.

Hormone	Made by	Job in pregnancy
hCG (human chorionic gonadotropin)	The early embryo	Keeps the corpus luteum making progesterone. Pregnancy tests detect it in urine
Progesterone	Corpus luteum, then placenta	Keeps the uterus lining thick. Stops new eggs being released and the uterus contracting
Oestrogen	Placenta	Helps the uterus and breasts grow
Oxytocin	Pituitary gland	Causes contractions in birth and milk release
Prolactin	Pituitary gland	Makes milk

Near birth, progesterone falls and oxytocin rises, so the uterus begins to contract.

Reproduction

Asexual

One parent, clones
Binary fission: bacteria
Budding: yeast, Hydra
Spores: fungi
Runners, tubers: plants

Sexual

Two parents, variation
Gametes form a zygote

Animals

External: fish, frogs, coral
Internal: mammals, birds

Plants

Pollination
Fertilisation in ovule
Seed and fruit

Pregnancy hormones

hCG
Progesterone
Oxytocin

Year 11Weighing up the two strategies

	Asexual	Sexual
Speed	Fast, can colonise quickly	Slow
Energy	Low	High (finding mates, making gametes)
Variation	None, unless a mutation	High
In a stable environment	Very successful	Less efficient
When the environment changes	Whole population may be wiped out	Some offspring likely to survive

Many organisms switch between the two. Aphids reproduce asexually all summer, then sexually before winter. Strawberries spread by runners but also make seeds.

Many eggs or few? Animals with external fertilisation release millions of eggs because most are never fertilised or are eaten. Animals with internal fertilisation make far fewer eggs, because each has a much better chance of being fertilised and protected.

Asexual parasites. A parasite such as the trypanosome copies itself asexually inside its host, so it can build up huge numbers very quickly without needing a mate.

Year 12Manipulating reproduction in agriculture

Farmers and scientists change how plants and animals reproduce:

Artificial insemination (AI): semen from a chosen bull is used to fertilise many cows, spreading wanted traits fast.
IVF (in vitro fertilisation) and embryo transfer in cattle and in people.
Cross-pollination by hand to breed new crop varieties.
Cloning by cuttings, tissue culture or somatic cell nuclear transfer (Dolly the sheep, 1996).

Benefits: faster improvement of crops and livestock. Risks: lower genetic diversity, so a whole crop or herd may be hit by one disease. These are developed further in Module 6.

The menstrual cycle. The pituitary gland releases FSH, which makes a follicle grow in the ovary. The growing follicle makes oestrogen, which thickens the uterus lining. When oestrogen gets high, it triggers a surge of LH from the pituitary, and the LH surge causes ovulation (the egg is released).

The empty follicle becomes the corpus luteum, which makes progesterone to keep the lining thick and stop new follicles developing. If no embryo arrives, the corpus luteum breaks down, progesterone falls, and the lining is shed (menstruation). If an embryo implants, its hCG keeps the corpus luteum going until the placenta takes over making progesterone.

Reasoning with hormones.

No FSH or LH means no follicles grow and no ovulation happens, so there is no cycle.
Low levels of oestrogen normally hold back FSH and LH (negative feedback). Clomiphene blocks the brain's oestrogen receptors, so the brain "sees" too little oestrogen and releases more FSH and LH, which brings on ovulation.
An older woman's eggs are more likely to have chromosome errors, so IVF with eggs donated by younger women succeeds more often.

Quick summary

Asexual reproduction: one parent, identical offspring. Sexual: two parents, varied offspring.
Bacteria use binary fission. Yeast buds. Fungi make spores. Plants use runners, tubers and bulbs.
Fertilisation can be external (fish, frogs, coral) or internal (mammals, birds, reptiles).
In flowering plants, pollination is followed by fertilisation in the ovule. Ovules become seeds, ovaries become fruit.
hCG and progesterone keep a pregnancy going. Oxytocin causes contractions at birth.

Quiz

13 questions with instant marking. It is free, and your scores are saved to your account so you can carry on from any device.

Turn on JavaScript to sign in and take the quiz.

Try 3 questions now

How many parents are needed for asexual reproduction?
1. A One
2. B Two
3. C Three
4. D None
Show answer
A: One
Asexual reproduction needs one parent.
Bacteria reproduce asexually by
1. A budding
2. B binary fission
3. C spores only
4. D pollination
Show answer
B: binary fission
The cell copies its DNA and splits in two.
Which is the main advantage of sexual reproduction?
1. A It is faster
2. B It needs no mate
3. C It produces variation
4. D It uses less energy
Show answer
C: It produces variation
Variation helps species survive change.