D3.1 Reproduction
SL & HL+ AHL sectionsSL and HL, with AHL content · Organisms · Theme D
Big questions
- When is copying yourself a better strategy than mixing genes with a partner?
- How do hormones coordinate the body changes needed to produce and nurture offspring?
Sections framed in purple are AHL (Higher Level only). SL students can hide them in Display settings.
Sexual and asexual reproduction
All living things reproduce, but there are two fundamentally different ways of doing it.
| Asexual reproduction | Sexual reproduction | |
|---|---|---|
| Parents | one | usually two |
| Gametes and fertilization | none | haploid gametes fuse |
| Offspring | genetically identical (clones) | genetically different from each other and from the parents |
| Main advantage | quick; every individual can reproduce; a combination of alleles that works is copied exactly | produces variation, so some offspring may suit a changed environment |
Asexual reproduction suits an organism that is already well adapted to a stable environment: its offspring inherit exactly the same well-adapted genes. If the environment changes, though, all the clones are equally vulnerable. Sexual reproduction produces offspring with new combinations of alleles. Some will be better suited than their parents if conditions change, for example when a new disease or a change of climate arrives, so natural selection has variation to act on.
Meiosis and fertilization in the life cycle
In a sexual life cycle two processes alternate:
- Meiosis halves the chromosome number, making haploid (n) gametes from diploid (2n) cells. Through crossing over and independent assortment it breaks up the combinations of alleles that each parent inherited.
- Fertilization (fusion of gametes) restores the diploid number and brings together alleles from two different parents, creating new combinations.
Differences between the sexes
In sexual reproduction the biological difference between the male and the female role is defined by the gametes. The key fact is that the male gamete moves, while the female gamete stays put and is reached by it. Everything else follows from this.
- Male gametes must move, so they are small, often with a tail, and carry few food reserves. Because each one is cheap to make, males produce them in enormous numbers: millions of sperm per day in human testes.
- Female gametes stay where they are, so they can be large, with a big store of food for the early embryo. Each is costly to make, so far fewer are produced: in humans usually one egg per menstrual cycle.
The same pattern is found in flowering plants: pollen grains carrying male gametes are tiny and produced in huge numbers, while each ovule holds one egg cell and develops into a seed with a food store.
Different reproductive strategies
Because of the difference in investment per gamete, the sexes often follow different strategies:
- Males can, in principle, fertilize many females, so male reproductive success is often limited by the number of matings. In many species males compete for mates, or display to attract them.
- Females invest much more in each offspring (a large egg, and in mammals pregnancy and lactation), so their reproductive success is limited by resources rather than by matings. Females are often more selective about mates.
The human reproductive systems
You need to be able to draw and annotate diagrams of the male-typical and female-typical reproductive systems. Annotations should give the name and the function of each structure.
How the parts work together
- Semen is sperm plus the fluids from the seminal vesicles and the prostate gland. Seminal vesicle fluid is alkaline and provides fructose for respiration; its alkalinity helps neutralise the acidic vagina. The prostate adds a slightly acidic fluid containing ions and enzymes that help the sperm move freely.
- During intercourse, erectile tissue in the penis fills with blood. At ejaculation, muscle contractions in the sperm ducts and urethra propel semen into the vagina.
- Sperm swim through the cervix and uterus into the oviducts. Fertilization normally happens in an oviduct, close to the ovary.
Hormones of the menstrual cycle
The menstrual cycle lasts about 28 days and is made of two linked cycles:
- the ovarian cycle: a follicle grows, releases an egg at ovulation, then becomes the corpus luteum
- the uterine cycle: the endometrium (lining of the uterus) is shed, rebuilt and then kept thick, ready for an embryo.
Day 1 is the first day of menstruation. Four hormones control the cycle:
| Hormone | Secreted by | Main effects |
|---|---|---|
| FSH (follicle-stimulating hormone) | anterior pituitary | stimulates follicles to develop and to secrete oestradiol |
| LH (luteinizing hormone) | anterior pituitary | the LH surge triggers ovulation; then LH stimulates formation of the corpus luteum |
| Oestradiol | follicle wall (and corpus luteum) | repair and thickening of the endometrium; regulates FSH and LH (see below) |
| Progesterone | corpus luteum | maintains the thickened endometrium; inhibits FSH and LH |
Ovary
Endometrium (uterus lining)
Menstruation (early follicular phase) (days 1–5)
- Ovary
- A batch of follicles, each containing a primary oocyte, starts to develop in response to FSH.
- Uterus
- Progesterone has fallen, so the thickened endometrium breaks down and is shed with some blood (menstruation).
- Feedback
- Progesterone and oestradiol are low, so they no longer inhibit the pituitary (negative feedback is lifted) and FSH secretion rises.
Day 3: FSH 36, LH 15, oestradiol 10, progesterone 4 (relative units). Menstruation (early follicular phase).
Show data table
| Day | FSH | LH | Oestradiol | Progesterone |
|---|---|---|---|---|
| 1 | 32 | 15 | 10 | 4 |
| 4 | 35 | 15 | 11 | 4 |
| 7 | 24 | 14 | 18 | 4 |
| 10 | 16 | 13 | 54 | 4 |
| 12 | 16 | 15 | 79 | 5 |
| 13 | 27 | 45 | 81 | 5 |
| 14 | 40 | 92 | 46 | 8 |
| 15 | 27 | 45 | 19 | 12 |
| 18 | 14 | 12 | 26 | 46 |
| 21 | 14 | 12 | 42 | 85 |
| 24 | 16 | 13 | 26 | 59 |
| 26 | 20 | 13 | 15 | 27 |
| 28 | 28 | 14 | 11 | 10 |
Qualitative model: the curve shapes and timings follow a typical 28-day cycle, but the units are relative: the four hormones are not drawn on a common scale, so compare the timing of their rises and falls, not their heights. Real cycles vary in length, mostly in the follicular phase.
Text description of this simulation
A line graph of the relative levels of four hormones across a 28-day menstrual cycle, with day 1 as the first day of menstruation. FSH is raised early in the cycle and then falls; oestradiol rises as a follicle grows and peaks just before day 14; LH rises in a sharp surge that peaks at ovulation on about day 14, with a smaller FSH peak; progesterone stays low until ovulation, is high in the luteal phase while the corpus luteum is active and falls before day 28. Bands under the graph show the uterine cycle (shedding, thickening, thick lining maintained) and the ovarian cycle (follicle growing, corpus luteum). A slider moves a marker through the cycle; drawings of the ovary and the endometrium change with the day, and a panel explains what is happening in the ovary and uterus and which positive or negative feedback is acting. A checkbox shows what changes if an embryo implants: hCG keeps the corpus luteum alive, so progesterone stays high and the lining is not shed. The model is qualitative.
Feedback
- Negative feedback: moderate oestradiol inhibits FSH secretion in the follicular phase; high progesterone (with oestradiol) inhibits FSH and LH in the luteal phase, so no new follicles develop.
- Positive feedback: oestradiol increases the number of FSH receptors on follicle cells, boosting oestradiol output further; and when oestradiol reaches its peak it raises GnRH secretion from the hypothalamus and makes the anterior pituitary more sensitive to GnRH, causing a surge of LH that triggers ovulation (see C2.1 Chemical signalling).
If no embryo implants, the corpus luteum degenerates after about 10–14 days. Progesterone and oestradiol fall, the endometrium breaks down (menstruation) and, with the negative feedback removed, FSH rises to start the next cycle.
Fertilization in humans
Fertilization normally happens in an oviduct, within about a day of ovulation. The cell released at ovulation is a secondary oocyte, surrounded by a glycoprotein layer, the zona pellucida, and by follicle cells. Many sperm reach it, but only one fertilizes it.
The key events are:
- Membrane fusion. The plasma membrane of one sperm fuses with the plasma membrane of the egg.
- Entry of the sperm nucleus. The sperm nucleus enters the egg cytoplasm. The sperm’s tail and mitochondria are destroyed, so the zygote’s mitochondria all come from the mother.
- Two separate nuclei. The egg completes its second meiotic division, and the haploid chromosomes from the egg and from the sperm each become enclosed in a nucleus. The two nuclei do not fuse directly.
- Joint mitosis. The membranes of both nuclei break down. All the condensed chromosomes, 23 from each parent, line up on one shared spindle and take part in a single mitosis. This produces two diploid nuclei (2n = 46) in the first two cells of the embryo.
Hormones in IVF treatment
In vitro fertilization (IVF) means fertilization “in glass”: eggs and sperm are brought together in a dish outside the body. It is used, for example, when the oviducts are blocked or the sperm count is low. Hormones are used to collect several eggs at once.
| Stage | What is given | Why |
|---|---|---|
| 1. Down-regulation | a drug (often a GnRH analogue, by injection or nasal spray) for about 2–3 weeks | suspends the woman’s normal secretion of FSH and LH by the pituitary, so the clinic, not her own cycle, controls the timing |
| 2. Superovulation | daily injections of FSH (sometimes with LH) for about 8–14 days | far more FSH than in a normal cycle, so many follicles mature at once rather than one |
| 3. Final maturation | one injection of hCG | acts like the LH surge: the eggs complete meiosis I and mature; they are collected about 36 hours later, just before they would be released |
The eggs are then collected from the follicles with a fine needle, mixed with sperm (or a sperm is injected directly into each egg), and after 2–5 days one or two embryos are placed in the uterus. Progesterone is often given to support the endometrium.
Sexual reproduction in flowering plants
Flowers are the reproductive organs of flowering plants. The stages are the same as in animals: meiosis, gamete production and fertilization.
- Gametes. In the anthers, cells divide by meiosis to form haploid pollen grains; each pollen grain contains the male gametes. Each ovule inside the ovary contains a haploid female gamete (the egg cell).
- Pollination. Pollen is transferred from an anther to a stigma, by insects, other animals or wind.
- Pollen development. The pollen grain germinates on the stigma and grows a pollen tube down the style to an ovule, carrying the male gametes.
- Fertilization. A male gamete fuses with the egg cell to form a zygote, which develops into an embryo. The ovule becomes a seed and the ovary becomes a fruit.
Many flowers are hermaphrodite: they have both stamens and carpels, so one plant can be a male and a female parent. Even if a plant fertilizes itself, the reproduction is still sexual, because gametes are made by meiosis and fuse at fertilization.
Reduced motion is on: use the arrows to step through. Change this in Display settings.
Text description of this animation
Seven steps in the sexual reproduction of a flowering plant. Two flowers on different plants, A and B; a bee collects pollen from the anthers of A and carries it to the stigma of B. A section through the carpel of B shows a pollen tube growing from the pollen grain down the style to an ovule. A pollen grain from plant B itself lands on the same stigma, but its tube is stopped (self-incompatibility). Male gametes travel down the tube and one fuses with the egg cell to form a zygote; the ovule develops into a seed and the ovary into a fruit. Winged fruits are carried away from the parent plant by the wind. Finally a seed in the soil absorbs water, its stored food is mobilised and the embryo root and shoot grow out.
- Gametes in flowers. Anthers contain cells that divide by meiosis to form haploid pollen grains; each pollen grain contains male gametes. Ovules inside the ovary each contain a haploid female gamete, the egg cell. A bee arrives at a flower on plant A to feed on nectar, and pollen sticks to its body.
- Cross-pollination. Pollination is the transfer of pollen from an anther to a stigma. The bee flies to a flower on a different plant, B, and pollen rubs off onto the sticky stigma. Because the pollen comes from another plant, this is cross-pollination.
- The pollen tube grows. The pollen grain germinates on the stigma. A pollen tube grows down through the style to the ovary and enters an ovule through a small opening (the micropyle), carrying the male gametes.
- Self-pollen is rejected. A pollen grain from plant B itself lands on the stigma. It carries the same self-incompatibility (S) allele as the stigma, so the plant recognises it as self and its pollen tube stops growing. Self-fertilization, and so inbreeding, is prevented.
- Fertilization. The male gametes move down the pollen tube into the ovule. One fuses with the egg cell, forming a diploid zygote that grows by mitosis into an embryo. (The other fuses with the central cell and forms a food store.) The ovule develops into a seed and the ovary wall into a fruit.
- Seed dispersal. Seeds are carried away from the parent plant, here by wind acting on winged fruits; animals disperse other fruits. Dispersal reduces competition with the parent and between seedlings, and lets the species colonise new areas. It moves seeds after fertilization, whereas pollination moves pollen before it.
- Germination. The seed absorbs water and the embryo becomes active. Enzymes such as amylase digest stored starch into sugars, which are carried with other food reserves to the growing tips and used in aerobic respiration and growth. The embryo root (radicle) grows down first, then the embryo shoot grows up towards the light.
Promoting cross-pollination
Self-pollination (pollen reaching a stigma on the same plant) leads to inbreeding. Inbred offspring are more likely to inherit two copies of a harmful recessive allele, and the population loses genetic diversity and vigour. Cross-pollination between different plants keeps variation high. Plants have several ways of promoting it.
Structural and timing methods
| Method | How it works |
|---|---|
| Pollen and stigma mature at different times | if the anthers release pollen before the stigma is receptive (or the other way round), a flower cannot pollinate itself |
| Separate male and female flowers on one plant | anthers and stigmas are in different flowers (e.g. maize) |
| Separate male and female plants | each plant has only stamens or only carpels (e.g. holly, kiwifruit), so self-pollination is impossible |
| Animals or wind carry pollen | insects visiting flower after flower carry pollen between plants; wind spreads light pollen over long distances |
Self-incompatibility
Many species (for example in the cabbage family, the rose family, which includes apples and cherries, and the potato family) also have a genetic block. A self-incompatibility gene, often called the S-locus, has many alleles. When a pollen grain lands on a stigma, proteins made by the pollen and the stigma are compared:
- If the pollen carries an S-allele that matches one in the stigma, it is recognised as self (or from a close relative). It fails to germinate, or its tube stops growing in the stigma or style.
- If the S-alleles are different, the pollen tube grows normally and fertilization can occur.
This ensures that the male and female gametes that fuse at fertilization come from different plants, increasing genetic variation within the species.
Seed dispersal and germination
Dispersal is not pollination
| Pollination | Seed dispersal | |
|---|---|---|
| What moves | pollen (male gametes) | seeds (each containing an embryo) |
| When | before fertilization | after fertilization |
| Purpose | brings gametes together | spreads offspring away from the parent |
Dispersing seeds reduces competition for light, water and minerals between the seedlings and the parent, and lets the species colonise new areas. Seeds are carried by wind (winged or feathery fruits), by animals (fleshy fruits eaten and the seeds passed out in faeces, or hooked fruits caught on fur), by water, or thrown out by exploding fruits.
Germination
A seed contains an embryo (embryo root, embryo shoot and one or two seed leaves, the cotyledons) and a food store. It stays dormant until conditions are suitable. Germination then needs:
- water, absorbed through the seed coat, which rehydrates the tissues and activates enzymes
- oxygen, for aerobic respiration to supply ATP for growth
- a suitable temperature, for enzyme activity.
Once water is absorbed, a growth regulator (gibberellin) stimulates the production of enzymes such as amylase. These mobilise the food reserves: starch is hydrolysed to sugars (and proteins to amino acids), which are transported to the growing regions. The embryo root emerges first, to anchor the seedling and absorb water; then the shoot grows up. Once its leaves reach the light, the seedling can photosynthesise and no longer depends on the food store.
Hormonal control of puberty
Puberty is the period in which the body becomes able to reproduce. It is started by a change in the brain:
- From late childhood, the hypothalamus increases its release of gonadotropin-releasing hormone (GnRH).
- GnRH travels in portal blood vessels to the anterior pituitary, stimulating it to secrete more LH and FSH (the gonadotropins).
- LH and FSH stimulate the gonads. The testes secrete more testosterone and start producing sperm; the ovaries secrete more oestradiol and progesterone, and follicles start to mature.
- The increased steroid sex hormones cause the developmental changes of puberty.
Sex hormones are steroids: they are lipid-soluble, diffuse through the plasma membrane and bind to receptors inside target cells, changing which genes are expressed. This is why their effects (growth of tissues, development of secondary sexual characteristics) take weeks to years.
Spermatogenesis and oogenesis
Gametogenesis is gamete production. In both sexes it has four stages: mitosis, cell growth, meiosis (meiosis I then meiosis II) and differentiation.
Reduced motion is on: use the arrows to step through. Change this in Display settings.
Text description of this animation
Two rows, sperm production in the testis above and egg production in the ovary below, revealed one stage at a time. Mitosis produces diploid spermatogonia and oogonia. They grow into primary spermatocytes and primary oocytes (2n); the primary oocyte sits in a follicle and pauses in meiosis I. Meiosis I gives two equal haploid secondary spermatocytes, but one large secondary oocyte and a tiny first polar body. Meiosis II gives four spermatids, but only one ovum and a second polar body, and only after a sperm has entered. Spermatids differentiate into four sperm with little cytoplasm; the single egg keeps a large food store and the polar bodies degenerate.
- Mitosis. Germ cells divide by mitosis to make many diploid (2n) cells. In the testis, spermatogonia in the wall of the seminiferous tubules divide continuously from puberty onwards. In the ovary, oogonia divide only before birth, in the foetus.
- Cell growth. Cells grow into primary spermatocytes and primary oocytes, still diploid. A primary oocyte grows much larger and is surrounded by follicle cells, forming a primary follicle. It starts meiosis I before birth and then pauses in prophase I. It stays paused until shortly before its own follicle ovulates, which may be any cycle from puberty onwards.
- Meiosis I. Meiosis I halves the chromosome number, so the products are haploid (n), though each chromosome still has two chromatids. A primary spermatocyte divides equally into two secondary spermatocytes. A primary oocyte divides unequally, just before ovulation, into one large secondary oocyte and a small first polar body.
- Meiosis II. Meiosis II separates the chromatids. Two secondary spermatocytes give four haploid spermatids. The secondary oocyte pauses in metaphase II and only completes meiosis II if a sperm enters it, again dividing unequally into one ovum and a second polar body.
- Differentiation. Each spermatid differentiates, helped by nurse (Sertoli) cells, into a sperm with an acrosome, mitochondria and a tail, losing most of its cytoplasm. So one primary spermatocyte gives four sperm and millions are made each day. One primary oocyte gives only one egg, which keeps almost all the cytoplasm as a food store; the polar bodies degenerate.
Spermatogenesis (testes)
Sperm are made in the walls of the seminiferous tubules. The outer layer, the germinal epithelium, divides by mitosis continuously from puberty. Cells grow into primary spermatocytes, which undergo meiosis I and II to form four haploid spermatids. These differentiate into sperm, helped by nurse (Sertoli) cells, losing most of their cytoplasm and gaining an acrosome, a mitochondria-packed mid-piece and a tail.
Oogenesis (ovaries)
Oogonia multiply by mitosis only in the foetus. They grow into primary oocytes, start meiosis I and then pause, each inside a primary follicle. From puberty, a few follicles develop each cycle. Just before ovulation, the dominant one completes meiosis I, dividing unequally into a large secondary oocyte and a small polar body. The secondary oocyte starts meiosis II and is released at ovulation, but completes meiosis II only if a sperm enters it, again forming one large cell and a polar body.
Why such different numbers and sizes?
| Spermatogenesis | Oogenesis | |
|---|---|---|
| Gametes per cell entering meiosis | 4 sperm | 1 egg (+ polar bodies) |
| Cytokinesis in meiosis | equal | unequal |
| Production | millions per day, from puberty throughout life | usually one egg per cycle, from puberty to menopause |
| Cytoplasm | almost none (small, streamlined) | very large (food reserves for the embryo) |
Unequal division puts almost all the cytoplasm into a single egg, so it can supply the early embryo. Sperm only need to deliver a nucleus, so they shed cytoplasm and are produced in vast numbers.
Preventing polyspermy
Polyspermy, fertilization of an egg by more than one sperm, would give a zygote with too many chromosome sets, which cannot develop normally. Two mechanisms ensure only one sperm gets through.
The acrosome reaction
The acrosome is a vesicle-like cap on the sperm head, full of digestive enzymes. When a sperm binds to glycoproteins of the zona pellucida, the acrosome releases its enzymes by exocytosis. They digest a narrow route through the zona, allowing that sperm to reach and fuse with the egg membrane.
The cortical reaction
The egg has cortical granules, vesicles just inside its plasma membrane. As soon as the first sperm fuses, a wave of Ca²⁺ release spreads through the egg and the cortical granules release their contents by exocytosis into the space beneath the zona. Their enzymes:
- alter the zona glycoproteins to which sperm bind, so other sperm can no longer attach
- harden the zona pellucida, so no other sperm can digest a way through it.
Reduced motion is on: use the arrows to step through. Change this in Display settings.
Text description of this animation
A human egg (secondary oocyte) surrounded by its plasma membrane, cortical granules just inside the membrane, the zona pellucida and an outer layer of follicle cells, with three sperm approaching. The lead sperm releases enzymes from its acrosome and digests a path through the zona pellucida. Its plasma membrane fuses with the egg membrane and its nucleus enters. Cortical granules release their contents, hardening the zona pellucida so the other sperm cannot get through. The egg completes meiosis II, producing a second polar body; the sperm tail and mitochondria are broken down and two haploid nuclei form. Their nuclear membranes break down, the chromosomes from both join one mitotic spindle, and two diploid nuclei are produced.
- Sperm reach the egg. Sperm swim up the oviduct towards the secondary oocyte released at ovulation. They push between the follicle cells around it and reach the zona pellucida, a layer of glycoproteins. Sperm have receptors that bind to specific zona glycoproteins.
- Acrosome reaction. Binding to the zona triggers the acrosome reaction. The acrosome releases its digestive enzymes by exocytosis, and they digest a narrow path through the zona pellucida. The sperm swims through it to the egg's plasma membrane.
- Membranes fuse. Proteins on the sperm's plasma membrane bind to the egg membrane and the two membranes fuse. The sperm nucleus enters the egg cytoplasm. This is the moment of fertilization.
- Cortical reaction. Fusion triggers the cortical reaction. Cortical granules just inside the egg membrane release enzymes by exocytosis. These alter the zona glycoproteins so that sperm can no longer bind, and harden the zona, so no other sperm can pass through. This prevents polyspermy.
- Two haploid nuclei. The egg now completes meiosis II, and the second polar body is pushed out. The sperm's tail and mitochondria are broken down, so mitochondria are inherited from the mother only. A nuclear membrane forms around each haploid set of chromosomes, giving separate sperm and egg nuclei (23 chromosomes each).
- Joint mitosis. Both nuclear membranes break down. The condensed chromosomes from the sperm (blue) and the egg (red), 23 from each parent, line up together on one shared spindle. (Only a few of the 46 are drawn.)
- Two diploid nuclei. The chromosomes divide in this single, joint mitosis, producing two diploid nuclei (2n = 46), one in each of the first two cells of the embryo. Each nucleus contains one set of chromosomes from each parent.
Blastocyst, implantation and pregnancy tests
From zygote to blastocyst
The zygote divides by mitosis as it is moved along the oviduct by cilia and muscle contractions. After about 5 days it has become a blastocyst: a hollow ball of cells with
- an outer layer, the trophoblast, which will help form the placenta
- a fluid-filled cavity
- an inner cell mass, which will develop into the embryo.
The blastocyst reaches the uterus, escapes from the zona pellucida and, at around day 7, implants: it attaches to the endometrium and the trophoblast grows into it, giving the embryo access to the mother’s blood supply.
hCG and pregnancy testing
Soon after implantation, the trophoblast (and then the developing placenta) secretes human chorionic gonadotropin (hCG). hCG keeps the corpus luteum alive, so it continues to secrete progesterone and the endometrium is not shed.
hCG is excreted in urine, and pregnancy tests detect it using monoclonal antibodies: antibodies made by a clone of identical cells, all binding the same specific site on hCG.
The placenta
The placenta develops from the trophoblast together with the endometrium. It is connected to the foetus by the umbilical cord, which contains two umbilical arteries (carrying blood from the foetus) and one umbilical vein (carrying blood back to it).
Exchange happens across the placental villi: finger-like, branched projections of foetal tissue that contain foetal capillaries and are bathed in maternal blood. The villi give a very large surface area, and the barrier between the bloods is thin, so diffusion is rapid. Maternal and foetal blood do not mix.
| From mother to foetus | From foetus to mother |
|---|---|
| oxygen, glucose, amino acids, fatty acids, vitamins, mineral ions, water | carbon dioxide |
| antibodies (by endocytosis), giving temporary immunity | urea and other wastes |
Some harmful substances can also cross, such as alcohol, nicotine, some drugs and some viruses.
Why the placenta matters
The placenta lets a mammal keep its foetus in the uterus to a much later stage of development than mammals without a placenta. Marsupials, for example, give birth to a tiny, very undeveloped young after a short pregnancy, which completes its development attached to a teat. A placental foetus is supplied continuously with food and oxygen, has its wastes removed and is protected inside the mother, so it can be born well developed. The placenta also secretes hormones, including progesterone (see below).
Hormones in pregnancy and childbirth
Progesterone maintains pregnancy. It keeps the endometrium thick and stops the muscle of the uterus wall (the myometrium) contracting. Its source changes:
- Early pregnancy: the corpus luteum, kept alive by hCG from the embryo.
- From about 8–12 weeks (by the end of the first trimester): the placenta, which secretes enough progesterone (and oestrogen) that the corpus luteum is no longer needed and degenerates.
Childbirth is triggered when progesterone levels fall at the end of pregnancy. With its inhibition removed, oxytocin secretion from the posterior pituitary can increase, and the myometrium starts to contract. Contractions push the baby against the cervix; stretch receptors detect this and send nerve impulses to the hypothalamus, causing more oxytocin to be secreted, which causes stronger and more frequent contractions. This is positive feedback: the response increases the stimulus. It continues until the baby, and then the placenta, are delivered, which removes the stretch stimulus.
HRT and coronary heart disease
Hormone replacement therapy (HRT) gives oestrogen (often with a progestogen) to relieve symptoms after the menopause, when the ovaries stop secreting these hormones. Its story is a classic lesson in the nature of science: correlation does not prove causation.
What happened
- Early epidemiological (observational) studies compared women who chose to take HRT with women who did not. Fewer HRT users developed coronary heart disease (CHD), and researchers concluded that HRT protected the heart.
- Randomised controlled trials later allocated women at random to HRT or a placebo, such as the Women’s Health Initiative (results published 2002). These found that HRT caused a small increase in CHD risk.
Why the observational studies were misleading
Women who took HRT tended to have higher socioeconomic status. Higher socioeconomic status is itself causally linked to lower CHD risk (better diet, more exercise, less smoking, better health care). It was a confounding variable: it made HRT and low CHD occur together without HRT causing the reduced risk. Random allocation in a trial spreads such factors evenly between the groups, so any remaining difference can be attributed to the treatment.
Must-know definitions
- Sexual vs asexual reproduction
- Asexual reproduction produces genetically identical offspring from one parent, which suits a stable environment the parent is already adapted to. Sexual reproduction combines gametes from two parents, producing offspring with new combinations of alleles: the variation needed to adapt if the environment changes.
- Fertilization
- Fusion of a male and a female gamete (each haploid) to form a diploid zygote. Meiosis breaks up the parents' combinations of alleles; fertilization creates new ones.
- Menstrual cycle
- The ovarian cycle (follicle growth, ovulation, corpus luteum) and the uterine cycle (shedding, rebuilding and maintenance of the endometrium) together, regulated by FSH, LH, oestradiol and progesterone through positive and negative feedback. Day 1 is the first day of menstruation.
- Corpus luteum
- The remains of the follicle after ovulation, which develops under the influence of LH and secretes progesterone (and some oestradiol). It degenerates after about 10–14 days unless hCG from an embryo keeps it alive.
- IVF hormone treatment
- The woman's own FSH and LH secretion is first suspended (down-regulation, e.g. with a GnRH analogue); then injections of FSH cause superovulation (many follicles mature) and an hCG injection about 36 hours before collection makes the eggs complete their maturation.
- Pollination
- Transfer of pollen from an anther to a stigma. Cross-pollination is between flowers on different plants, usually by animals or wind; it is different from seed dispersal, which moves seeds away from the parent after fertilization.
- Self-incompatibility
- A genetic mechanism (in many species controlled by S-alleles) by which a plant recognises its own pollen and stops it from fertilizing its ovules, so only gametes from different plants fuse. It prevents inbreeding and so maintains genetic diversity and vigour.
- Germination
- Growth of the embryo in a seed into a seedling. The seed absorbs water, enzymes digest stored food (for example starch to sugars) and the food is moved to the growing root and shoot tips.
- Cortical reactionAHL
- After the first sperm fuses with the egg, cortical granules release enzymes by exocytosis that alter and harden the zona pellucida so no more sperm can bind or pass through, preventing polyspermy.
- Human chorionic gonadotropin (hCG)AHL
- A hormone secreted by the embryo and then the developing placenta (trophoblast). It keeps the corpus luteum secreting progesterone in early pregnancy, and it appears in urine, where pregnancy tests detect it with monoclonal antibodies.
- Oxytocin positive feedbackAHL
- In childbirth, stretching of the cervix causes more oxytocin secretion from the posterior pituitary, which causes stronger uterine contractions, which stretch the cervix further. The loop continues until the baby is born.
Common exam mistakes
Loses marks Reproduction in a hermaphrodite plant that pollinates itself is asexual.
Write instead It is still sexual: gametes are made by meiosis and fuse at fertilization. Asexual reproduction involves no gametes and no fertilization, so the offspring are clones.
Loses marks LH peaks at the same time as oestradiol.
Write instead Oestradiol peaks just before ovulation; its high level stimulates the LH surge by positive feedback, so LH peaks slightly later, at ovulation (about day 14).
Loses marks Progesterone is made by the follicle before ovulation and causes ovulation.
Write instead Progesterone is secreted mainly by the corpus luteum after ovulation. The LH surge causes ovulation; progesterone maintains the endometrium and inhibits FSH and LH.
Loses marks Menstruation is caused by a rise in progesterone.
Write instead Menstruation is caused by a fall in progesterone (and oestradiol) when the corpus luteum degenerates, so the endometrium can no longer be maintained.
Loses marks In IVF, FSH injections are given to stop the woman's natural cycle.
Write instead The natural cycle is stopped first by down-regulation (a drug that suspends the pituitary's own FSH and LH secretion). FSH injections are then given to stimulate many follicles (superovulation).
Loses marks Pollination and seed dispersal both mean moving seeds to new places.
Write instead Pollination moves pollen (male gametes) to a stigma before fertilization; seed dispersal moves seeds (embryos) away from the parent after fertilization.
Loses marks Drawing the sperm duct going straight from the testis to the penis, or joining the bladder.
Write instead The sperm duct starts at the epididymis, loops up over the bladder and joins the urethra inside the prostate gland, after the seminal vesicle duct has joined it.
Loses marks In oogenesis meiosis produces four eggs, just as spermatogenesis produces four sperm.
Write instead Cytokinesis in oogenesis is unequal: each meiotic division gives one large cell and a small polar body, so one primary oocyte gives only one egg (plus polar bodies that degenerate).
AHLLoses marks Progesterone rises at the end of pregnancy to start contractions.
Write instead Progesterone, which inhibits contractions, falls at the end of pregnancy; this allows oxytocin secretion to increase, and contractions build up by positive feedback.
AHLLoses marks Because HRT users had less heart disease, HRT must protect the heart.
Write instead That was a correlation caused by a confounding factor: HRT users tended to have higher socioeconomic status, which itself lowers CHD risk. Randomised controlled trials found HRT slightly increases CHD risk.
AHL
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Asexual vs sexual reproduction — the advantage of each
Asexual reproduction gives genetically identical offspring, suited to a stable environment the parent is already adapted to. Sexual reproduction gives new combinations of alleles, providing variation for adaptation when the environment changes.
Roles of meiosis and fertilization in a sexual life cycle
Meiosis halves the chromosome number and breaks up the parents' combinations of alleles. Fertilization (fusion of gametes) restores the diploid number and creates new combinations.
Why are male gametes small and numerous?
Sperm must swim to reach the egg, so each one is small and carries few food reserves. Each costs little to make, so huge numbers are produced. The stationary egg is large, with food for the embryo, and far fewer are made.
Path of sperm from the testis out of the body
Testis → epididymis (storage and maturation) → sperm duct → joins the seminal vesicle duct → through the prostate gland into the urethra → out through the penis.
Functions of the oviduct
Its fimbriae collect the egg at ovulation; it is the usual site of fertilization; cilia and muscle contractions move the embryo to the uterus.
Source and role of each menstrual-cycle hormone
FSH (pituitary) — follicle development. LH (pituitary) — surge triggers ovulation, then corpus luteum formation. Oestradiol (follicle) — rebuilds the endometrium; peak triggers the LH surge. Progesterone (corpus luteum) — maintains the endometrium, inhibits FSH and LH.
Give one example each of positive and negative feedback in the menstrual cycle.
Positive — peak oestradiol stimulates the LH surge (also oestradiol raises FSH receptors on follicle cells). Negative — progesterone (with oestradiol) inhibits FSH and LH secretion in the luteal phase.
What happens to the sperm and egg nuclei at fertilization?
The sperm nucleus enters the egg (tail and mitochondria are destroyed). Both nuclear membranes break down and all the condensed chromosomes take part in one joint mitosis, producing two diploid nuclei.
The three hormone steps in IVF
1 Down-regulation suspends the woman's own FSH/LH secretion. 2 FSH injections cause superovulation (many follicles). 3 hCG injection matures the eggs about 36 h before collection.
Name and give the function of the parts of a carpel.
Stigma — sticky, traps pollen. Style — holds up the stigma; the pollen tube grows through it. Ovary — contains ovules (each with an egg cell); becomes the fruit.
Three ways plants promote cross-pollination (other than self-incompatibility)
Pollen and stigma mature at different times; separate male and female flowers on a plant; separate male and female plants. Animals or wind carry pollen between plants.
What happens in germination?
The seed absorbs water; enzymes (e.g. amylase) digest stored starch to sugars, which are moved to the growing tips for respiration and growth; the embryo root emerges, then the shoot.
Sequence of hormones that starts puberty
Increased GnRH from the hypothalamus → more LH and FSH from the anterior pituitary → more steroid sex hormones (testosterone, oestradiol, progesterone) from the gonads → changes of puberty.
Spermatogenesis vs oogenesis — products of one cell entering meiosis
Spermatogenesis — equal divisions, 4 sperm with little cytoplasm. Oogenesis — unequal divisions, 1 egg with lots of cytoplasm plus polar bodies (meiosis II completed only after a sperm enters).
Acrosome reaction vs cortical reaction
Acrosome reaction (sperm) — enzymes digest a path through the zona pellucida. Cortical reaction (egg) — cortical granules release enzymes that harden the zona and stop more sperm binding, preventing polyspermy.
Structure of a blastocyst
A hollow ball of cells formed by about day 5 — outer trophoblast (helps form the placenta, invades the endometrium, secretes hCG), fluid-filled cavity, and inner cell mass (becomes the embryo). Implants at about day 7.
Why does a placenta need villi, and what crosses it?
Villi give a very large surface area with a thin barrier for exchange. To the foetus — O₂, glucose, amino acids, water, ions, antibodies. To the mother — CO₂, urea. The bloods do not mix.
Hormonal control of pregnancy and childbirth
Progesterone maintains pregnancy — from the corpus luteum (kept by hCG), then the placenta. At term progesterone falls, allowing oxytocin to rise; contractions stretch the cervix, causing more oxytocin (positive feedback).
Last reviewed 19 October 2026. Spotted a mistake? Report it.