Revise Reproduction for IB Biology (D3.1)

Asexual reproduction involves a single parent and no fusion of gametes. The offspring are produced by mitosis and are therefore genetically identical to the parent and to each other — they are clones. It is fast, energetically cheap and needs no mate, which makes it a powerful strategy when an organism is already well suited to a stable environment. Its weakness is the flip side of its strength: because every individual shares one genotype, the population carries almost no genetic variation, so a single new threat can affect all of them equally. Sexual reproduction, by contrast, involves the fusion of two haploid gametes, usually from two parents. It generates genetic variation, so offspring differ from one another and from their parents. That variation is the raw material for natural selection and lets a population adapt when conditions change. The cost is that it is slower and requires finding a mate.

A gamete is a specialised sex cell that carries just one set of chromosomes — it is haploid (n). Gametes are produced by meiosis, a form of cell division that halves the chromosome number. This halving is essential: at fertilisation two gametes fuse to form a zygote, and if the gametes were diploid the chromosome number would double every generation. Instead, meiosis (2n to n) followed by fertilisation (n to 2n) keeps the chromosome number constant from one generation to the next. In a human, for example, a body cell has 46 chromosomes (2n = 46), each gamete has 23 (n = 23), and the zygote formed when sperm meets egg has 46 again.

Fertilisation does more than restore the diploid number — it is one of the two engines of genetic variation in sexual reproduction. Meiosis is the first engine: through independent assortment of chromosomes and crossing over, every gamete produced carries a different combination of alleles, so no two gametes are genetically identical. Fertilisation is the second engine: which particular sperm fuses with which particular egg is a matter of chance, so a huge number of genetically distinct zygotes is possible from the same two parents. It is the combination of meiosis AND fertilisation that makes each sexually produced offspring genetically unique — naming only one of the two is an incomplete explanation.

Flowering plants reproduce sexually, and their life cycle involves the same haploid-gamete, diploid-zygote logic, packaged differently. The male gametes are carried inside pollen grains made in the anthers; the female gametes (egg cells) sit inside ovules within the ovary. Two distinct events move the process forward. First is pollination — the transfer of pollen from an anther to a stigma, carried by wind or by animals such as insects. Pollination simply delivers the male gamete to the right place; nothing has fused yet. Second is fertilisation: after landing on a compatible stigma, the pollen grain grows a pollen tube down through the style to an ovule, and the male gamete fuses with the female gamete to form a diploid zygote. Keeping these two events separate is a common exam requirement.

After fertilisation the flower is transformed. The fertilised ovule develops into a seed, which contains the embryo plant and a food store. The wall of the ovary develops into the fruit, which encloses and protects the seeds and often aids their dispersal — a fleshy fruit tempts an animal to eat it and carry the seeds away, while a dry winged fruit catches the wind. Because two parents contribute gametes, the seeds are genetically variable, exactly as in animal sexual reproduction.

Human reproduction is sexual, so it depends on two very different gametes whose forms match their jobs. The male gamete, the sperm, is small and motile, with a flagellum for swimming and little more than a nucleus, mitochondria and an enzyme-filled acrosome. Its role is delivery: to carry paternal DNA to the egg, and sperm are produced in enormous numbers to raise the chance that one reaches its target. The female gamete, the egg (ovum), is large and non-motile. As well as carrying maternal DNA, it stockpiles cytoplasm, organelles and nutrients that will sustain the zygote through its first divisions before it can obtain resources any other way. When a sperm fertilises the egg, the two haploid nuclei fuse, restoring the diploid number and combining alleles from both parents.

In humans the release of the female gamete is governed by a roughly 28-day menstrual cycle controlled by four hormones: FSH and LH from the pituitary gland, and oestrogen and progesterone from the ovary. The cycle is a beautiful example of feedback control, using both negative and positive feedback. It begins as FSH (Follicle-Stimulating Hormone) stimulates a follicle in the ovary to grow. The growing follicle secretes oestrogen, which repairs and thickens the lining of the uterus in preparation for a possible pregnancy. At moderate levels this rising oestrogen inhibits further FSH release — negative feedback that stops too many follicles maturing at once.

As the dominant follicle matures, oestrogen climbs to a high peak, and above a threshold its effect reverses: high oestrogen now stimulates a sharp surge of LH (Luteinising Hormone) — this is positive feedback. The LH surge, around day 14, triggers ovulation: the release of the egg from the follicle. The empty follicle then becomes the corpus luteum, which secretes progesterone (and some oestrogen). Progesterone maintains the thickened uterus lining, keeping it ready for implantation, and it inhibits FSH and LH by negative feedback, which prevents any further ovulation. If the egg is not fertilised, the corpus luteum breaks down after about two weeks, progesterone and oestrogen levels fall, the uterus lining is shed (menstruation), and the fall in these hormones releases FSH from inhibition so that a new cycle begins.

Compare-and-contrast questions in D3.1 are marked analytically: each distinct valid comparison is worth one mark. The engine is looking for linked comparative points — a statement about sexual reproduction set against the matching statement about asexual reproduction — not two disconnected lists. Answer marks (A) credit a correct comparative point, method reasoning credits the underlying idea, and error-carried-forward means a slip early on does not sink the points that follow. Study how each mark below attaches to a specific, named comparison rather than to loose phrasing.

The central idea of D3.1 — that sexual reproduction manufactures genetic variation while asexual reproduction preserves a genotype — is the hinge that connects the whole of biology's continuity-and-change theme. Meiosis and fertilisation reappear in inheritance and in the mechanisms of evolution; the variation they generate is precisely what natural selection sorts. Master the habit of asking 'what does this mode of reproduction do to variation, and what does that variation buy the population', and you have the reasoning template that unlocks genetics, evolution and biodiversity alike.