Reproduction

📚The biology

Sexual and asexual reproduction

Meiosis and fertilization in the sexual life cycle

A sexual life cycle alternates two events:

• Meiosis halves the chromosome number to make haploid gametes, and in doing so breaks up the parental combinations of alleles through random orientation and crossing over (D2.1).
• Fusion of gametes — fertilization — restores the diploid number and produces new combinations of alleles, since each gamete comes from a different parent.

Male and female sexes

The prime difference between the sexes is that the male gamete travels to the female gamete. As a result, the male gamete is small and mobile, carrying little more than a nucleus, while the female gamete is large, with food reserves for the early embryo. From this follow other differences:

• Numbers of gametes: small gametes are cheap to make, so males produce them in enormous numbers (hundreds of millions of sperm per ejaculation); females produce few large eggs (usually one per menstrual cycle in humans).
• Reproductive strategies: males can in principle father many offspring and often compete for mates; females invest more in each offspring and are often more selective.

Human reproductive systems

You should be able to draw and annotate both systems, with each structure’s function.

The menstrual cycle

The menstrual cycle, about 28 days long, consists of two linked cycles: the ovarian cycle (development of a follicle, ovulation, and formation of the corpus luteum) and the uterine cycle (build-up and shedding of the endometrium). Four hormones control it:

The sequence, starting with menstruation on day 1:

1. Days 1–5, menstruation. Progesterone is low, so the endometrium breaks down and is shed. Low progesterone no longer inhibits the pituitary, so FSH rises.
2. Follicular phase. FSH stimulates a follicle to develop. The follicle secretes increasing amounts of oestradiol, which causes the endometrium to thicken. At first, oestradiol has a negative feedback effect on FSH.
3. Around day 14. High oestradiol now has a positive feedback effect on the pituitary, causing a surge of LH (and FSH). The LH surge triggers ovulation: the follicle releases its egg.
4. Luteal phase. LH causes the empty follicle to become the corpus luteum, which secretes progesterone (and some oestradiol). Progesterone keeps the endometrium thick and ready for implantation, and inhibits FSH and LH by negative feedback, so no new follicles develop.
5. If no fertilization occurs, the corpus luteum degenerates after about 10–14 days, progesterone falls, the endometrium breaks down (menstruation), FSH is no longer inhibited, and the cycle begins again.

Fertilization in humans

1. A sperm reaches the egg in the oviduct and the sperm’s cell membrane fuses with the egg’s cell membrane.
2. The sperm nucleus enters the egg; the sperm’s tail and mitochondria are destroyed (so all the embryo’s mitochondria come from the egg).
3. The nuclear membranes of the sperm and egg nuclei dissolve, rather than the nuclei fusing directly.
4. The condensed chromosomes from both gametes take part in a joint mitosis, on a single spindle, producing two diploid nuclei in the two cells of the embryo.

Hormones in IVF

In vitro fertilization (IVF) is used when couples cannot conceive naturally. Hormones control the process:

1. Down-regulation: a drug is given that suspends the normal secretion of FSH and LH by the pituitary, so the woman’s own cycle does not interfere with the timing.
2. Superovulation: artificial doses of FSH (and LH) are injected for several days, stimulating many follicles to develop at once rather than just one.
3. An injection of hCG (which acts like LH) matures the eggs, and they are collected from the follicles about 36 hours later.
4. Eggs are fertilized with sperm in a dish; the embryos are grown for a few days and one or two are transferred into the uterus. Progesterone may be given to support the endometrium.

Sexual reproduction in flowering plants

Reproduction in flowering plants is sexual: it involves haploid gametes and fertilization, even when a species is hermaphroditic (with male and female parts in the same flower).

1. Gamete production. Male gametes are produced inside pollen grains, made in the anthers. Female gametes (egg cells) are produced inside ovules, within the ovary.
2. Pollination. Pollen is transferred from an anther to a stigma.
3. Pollen development. A pollen tube grows from the pollen grain, down through the style to an ovule, carrying the male gametes.
4. Fertilization. A male gamete fuses with the egg cell inside the ovule, forming a zygote that develops into an embryo. The ovule becomes a seed, and the ovary develops into a fruit.

An insect-pollinated flower

Promoting cross-pollination

Cross-pollination transfers pollen between different plants, and many plants have features that encourage it:

• Different maturation times: the anthers release pollen before or after the stigma of the same flower is receptive.
• Separate male and female flowers on one plant (as in maize), or separate male and female plants (as in papaya and holly).
• Animals such as insects, birds and bats carry pollen between plants as they visit flowers for nectar; wind carries the light pollen of grasses and many trees over long distances.

Self-incompatibility

Self-pollination leads to inbreeding, which reduces genetic diversity and vigour, because harmful recessive alleles are more likely to be inherited in pairs. Many plant species have genetic self-incompatibility mechanisms: genes that allow the stigma to recognize pollen carrying the same alleles as itself and prevent it from germinating or its pollen tube from growing. This ensures that the gametes that fuse at fertilization come from different plants, increasing genetic variation.

Seed dispersal and germination

Seed dispersal is the spreading of seeds away from the parent plant, which reduces competition with the parent and between seedlings and allows new areas to be colonized. It must not be confused with pollination, which moves pollen, before fertilization. Seeds and fruits are dispersed by wind (winged or feathery seeds), animals (fleshy fruits eaten, or hooked fruits caught on fur), water (coconuts) or explosive pods.

Germination begins when the seed has water, oxygen and a suitable temperature. The seed absorbs water; enzymes are activated and mobilize the food reserves, hydrolysing stored starch to sugars and proteins to amino acids, which are transported to the embryo and used for respiration and growth. The embryonic root emerges first, then the shoot, which grows up towards light and begins to photosynthesize.

Puberty AHL

During childhood, the hypothalamus secretes little gonadotropin-releasing hormone (GnRH). As puberty approaches, it increases GnRH release. GnRH stimulates the pituitary to increase release of LH and FSH. These stimulate the gonads to produce more sex hormones — testosterone in the testes, oestradiol and progesterone in the ovaries — and it is these steroid hormones that cause the physical changes of puberty, such as growth of the reproductive organs, breast development, deepening of the voice, and growth of body hair.

Spermatogenesis and oogenesis AHL

Both processes involve the same four stages: mitosis, cell growth, two divisions of meiosis and differentiation. They differ in outcome:

So typical male bodies produce vast numbers of sperm with little cytoplasm, while typical female bodies produce few eggs, each with a large amount of cytoplasm.

Preventing polyspermy AHL

An egg fertilized by more than one sperm (polyspermy) would have too many chromosomes and could not develop. Two mechanisms act in sequence:

• Acrosome reaction: when a sperm reaches the zona pellucida (the glycoprotein layer around the egg), its acrosome releases enzymes that digest a path through, allowing that sperm to reach and fuse with the egg membrane.
• Cortical reaction: fusion triggers cortical granules just inside the egg membrane to release their contents by exocytosis. Enzymes from them alter the zona pellucida so that it hardens and no other sperm can pass through.

Blastocyst and implantation AHL

After fertilization, the zygote divides by mitosis as it moves along the oviduct. After about five days it has become a blastocyst: a hollow ball of cells, with an outer layer that will form the placenta and an inner cell mass that will form the embryo. Around day 6–10 it reaches the uterus and implants: it sinks into the thickened endometrium, from which it obtains nutrients and where the placenta begins to form.

Pregnancy testing AHL

From soon after implantation, the embryo, and then the developing placenta, secretes human chorionic gonadotropin (hCG). hCG passes into the mother’s blood and urine. A home pregnancy test detects hCG in urine using monoclonal antibodies — many identical antibodies that all bind specifically to hCG:

1. Urine soaks into the strip and passes antibodies against hCG that are attached to coloured particles. If hCG is present, it binds to these.
2. The urine carries the hCG–antibody complexes along to a test line of fixed antibodies that also bind hCG. The complexes are trapped there and a coloured line appears: a positive result.
3. Further along, a control line of fixed antibodies catches the unbound coloured antibodies, showing that the test has worked.

The placenta AHL

The placenta allows exchange between mother and foetus without their blood mixing. Placental villi, finger-like projections of foetal tissue containing foetal capillaries, give it a large surface area, and they are bathed in maternal blood.

The placenta also secretes hormones. By providing nutrition and gas exchange, it allows the foetus to be retained in the uterus until a much later stage of development than in mammals without a placenta, such as marsupials, whose young are born tiny and complete development in a pouch.

Hormones in pregnancy and childbirth AHL

Continuity of pregnancy depends on progesterone, which maintains the endometrium and keeps the uterine muscle relaxed. For the first few months it is secreted by the corpus luteum, which is kept alive by hCG from the embryo; after that, the placenta takes over progesterone production.

Childbirth is triggered by a decrease in progesterone. With progesterone falling, the pituitary can increase secretion of oxytocin, which stimulates contractions of the uterine muscle. Contractions push the baby’s head against the cervix, stretch receptors send signals to the brain, and more oxytocin is released, causing stronger contractions: positive feedback. The cycle continues, with contractions becoming stronger and more frequent, until the baby is born.

HRT and coronary heart disease AHL

Hormone replacement therapy (HRT) gives oestrogen and progesterone to women after menopause. Early epidemiological studies found that women taking HRT had a lower incidence of coronary heart disease, and this was taken to be cause and effect. Later randomized controlled trials, in which women were assigned to HRT or placebo by chance, found instead a small increase in risk of CHD.

The explanation was a confounding variable: women who chose HRT tended to have higher socioeconomic status, and higher socioeconomic status is causally linked to lower risk of CHD (through diet, exercise, healthcare and so on). The correlation between HRT and less heart disease was real, but it was not a cause-and-effect relationship. It is a standard example of why correlation does not prove causation, and why randomized trials give stronger evidence than observational studies.