D3.1 ReproductionIB Biology HL: Revision notes

Asexual reproduction produces genetically identical offspring from one parent. This suits individuals already adapted to an existing, stable environment, and every individual can reproduce. Sexual reproduction produces offspring with new combinations of alleles, giving the variation needed for adaptation to a changed environment. In the sexual life cycle, meiosis breaks up parental combinations of alleles and fusion of gametes (fertilisation) creates new combinations.

The prime difference between the sexes is that the male gamete travels to the female gamete, so sperm are small with few food reserves, while the egg is large with much cytoplasm. Males therefore produce vast numbers of gametes; females produce few, and invest more in each.

Male-typical system: testes (sperm and testosterone), epididymis (sperm storage and maturation), sperm duct (transfer), seminal vesicles and prostate (fluid), urethra and penis. Female-typical system: ovaries (eggs, oestradiol, progesterone), oviduct (site of fertilisation), uterus with endometrium (implantation), cervix and vagina.

The ovarian and uterine cycles together make the menstrual cycle.

• FSH (pituitary) stimulates follicle development; the follicle secretes oestradiol.
• Oestradiol thickens the endometrium. At first it inhibits FSH (negative feedback), but high levels trigger an LH surge (positive feedback).
• LH causes ovulation (about day 14) and formation of the corpus luteum.
• The corpus luteum secretes progesterone, which maintains the endometrium and inhibits FSH and LH (negative feedback).
• Without pregnancy, the corpus luteum degenerates, progesterone falls, the endometrium breaks down (menstruation) and FSH rises again.

At fertilisation, the sperm cell membrane fuses with the egg cell membrane; the sperm nucleus enters but the tail and mitochondria are destroyed. The nuclear membranes dissolve and all the condensed chromosomes take part in a joint mitosis, producing two diploid nuclei.

In IVF, the woman's normal hormone secretion is suspended, then large doses of FSH cause superovulation (many follicles). A hormone like LH triggers egg maturation, eggs are collected and fertilised outside the body, and progesterone supports the endometrium for implantation.

Plant reproduction is sexual even in hermaphrodites: male gametes form in pollen grains, female gametes in ovules. Insect-pollinated flowers have coloured petals, scent and nectaries. Cross-pollination is promoted by different maturation times of pollen and stigma, separate male and female flowers or plants, and self-incompatibility, which avoids inbreeding. Seed dispersal (after fertilisation) is distinct from pollination (before). At germination, stored starch is hydrolysed and transported to the embryo, where it is used for growth and respiration — so total dry mass falls until photosynthesis begins.

In childhood, release of GnRH by the hypothalamus is limited. At puberty, increased GnRH triggers increased LH and FSH from the pituitary, which raise sex steroid production (testosterone, oestradiol), causing the changes of puberty.

Spermatogenesis and oogenesis both involve mitosis, cell growth, two meiotic divisions and differentiation. Sperm: continuous mitosis from puberty; four equal sperm per meiosis; most cytoplasm lost. Eggs: mitosis before birth; unequal divisions give one large egg and polar bodies; the egg keeps most cytoplasm.

The acrosome reaction releases enzymes that let a sperm penetrate the zona pellucida. After one sperm fuses with the egg, the cortical reaction releases cortical granule contents that harden the zona pellucida, preventing polyspermy. The zygote divides by mitosis to form a blastocyst, which implants in the endometrium about 7 days after fertilisation.

The embryo, then the developing placenta, secretes hCG, which maintains the corpus luteum. Pregnancy tests use monoclonal antibodies that bind hCG in urine.

The placenta has villi with a large surface area for exchange of oxygen, nutrients, CO₂ and urea between maternal and foetal blood; it lets the foetus stay in the uterus to a later stage of development.

Pregnancy is maintained by progesterone, first from the corpus luteum, then from the placenta. At childbirth, progesterone falls, allowing oxytocin secretion; oxytocin causes contractions, which stimulate more oxytocin — positive feedback.

Early epidemiological (observational) studies found that women on HRT had lower rates of CHD, and this was taken as cause and effect. Later randomised controlled trials showed HRT causes a small increase in CHD risk. The earlier correlation was due to a confounding variable: HRT users had higher socioeconomic status, which itself lowers CHD risk.