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Sexual reproduction requires the formation of specialised sex cells — gametes — and their fusion during fertilisation to produce a new organism. The Edexcel A-Level Biology specification (9BI0) requires you to understand how gametes are formed through meiosis, the structural adaptations of sperm and egg cells, and the process of fertilisation in mammals.
Gametes are the reproductive cells (sex cells) produced by sexually reproducing organisms. They are haploid (n), meaning they contain half the number of chromosomes of the parent organism's body cells.
In mammals:
| Gamete | Produced in | Produced by | Chromosome number (human) |
|---|---|---|---|
| Sperm (spermatozoa) | Testes | Males | 23 (haploid) |
| Egg (ovum; plural: ova) | Ovaries | Females | 23 (haploid) |
When a sperm and egg fuse during fertilisation, the resulting zygote is diploid (2n) — it has the full complement of chromosomes (46 in humans).
Spermatogenesis is the process by which spermatozoa (sperm cells) are produced in the seminiferous tubules of the testes. It begins at puberty and continues throughout adult life.
| Stage | Cell type | Ploidy | Process |
|---|---|---|---|
| 1. Multiplication | Spermatogonia (stem cells) | Diploid (2n) | Divide by mitosis to produce more spermatogonia |
| 2. Growth | Primary spermatocytes | Diploid (2n) | Spermatogonia grow and develop into primary spermatocytes |
| 3. Meiosis I | Primary spermatocyte → secondary spermatocytes | Haploid (n) | First meiotic division produces two haploid secondary spermatocytes |
| 4. Meiosis II | Secondary spermatocytes → spermatids | Haploid (n) | Second meiotic division produces four haploid spermatids |
| 5. Differentiation | Spermatids → spermatozoa | Haploid (n) | Spermatids differentiate into mature sperm cells (spermiogenesis) — this involves developing a tail, acrosome, midpiece and condensing the nucleus |
Exam Tip: Remember that spermatogenesis produces four functional sperm from each primary spermatocyte, while oogenesis produces only one functional egg plus polar bodies. This is a key difference frequently tested.
Oogenesis is the process by which ova (egg cells) are produced in the ovaries. Unlike spermatogenesis, oogenesis begins before birth and involves long pauses.
| Stage | Cell type | Ploidy | Process |
|---|---|---|---|
| 1. Multiplication | Oogonia (stem cells) | Diploid (2n) | Divide by mitosis during fetal development to produce primary oocytes |
| 2. Growth | Primary oocytes | Diploid (2n) | Grow and begin meiosis I, but are arrested in prophase I before birth |
| 3. Meiosis I (at ovulation) | Primary oocyte → secondary oocyte + first polar body | Haploid (n) | Completed just before ovulation; produces one large secondary oocyte and a small first polar body |
| 4. Meiosis II (at fertilisation) | Secondary oocyte → ovum + second polar body | Haploid (n) | Only completed if fertilisation occurs; produces the mature ovum and a second polar body |
Polar bodies are small cells with very little cytoplasm that are produced as a byproduct of meiosis during oogenesis. They serve to discard the extra chromosomes while preserving the maximum amount of cytoplasm in the egg cell. The large cytoplasmic reserve of the egg is essential for nourishing the early embryo after fertilisation, before implantation occurs.
The sperm cell is highly adapted for its function — to reach and fertilise the egg:
| Structure | Adaptation |
|---|---|
| Acrosome | A modified lysosome at the tip of the head; contains hydrolytic enzymes (e.g. acrosin, hyaluronidase) that digest through the protective layers surrounding the egg |
| Nucleus | Highly condensed and streamlined (haploid); contains the paternal genetic material |
| Midpiece | Packed with mitochondria arranged in a spiral; provides ATP for the movement of the tail via aerobic respiration |
| Tail (flagellum) | A long undulipodium containing microtubules in a 9+2 arrangement; propels the sperm through the female reproductive tract by rhythmic beating |
| Cell membrane | Contains specific proteins for recognition and binding to the egg cell |
| Minimal cytoplasm | Streamlined shape reduces drag and allows efficient swimming |
The egg cell is the largest cell in the human body (approximately 120 μm in diameter) and is adapted for fertilisation and early embryonic development:
| Structure | Function |
|---|---|
| Large volume of cytoplasm | Contains nutrient reserves (yolk), organelles (especially mitochondria and ribosomes) and mRNA for early embryonic development |
| Zona pellucida | A glycoprotein layer surrounding the egg; contains receptors for sperm binding (species-specific recognition); hardens after fertilisation to prevent polyspermy |
| Corona radiata | A layer of follicle cells (granulosa cells) surrounding the zona pellucida; provides nutrients and support |
| Cortical granules | Vesicles just beneath the cell membrane containing enzymes; released after fertilisation to modify the zona pellucida and prevent polyspermy (the cortical reaction) |
| Haploid nucleus | Contains the maternal genetic contribution |
| Cell membrane | Contains receptors for sperm recognition |
Exam Tip: You should be able to draw and label both a sperm cell and an egg cell, and explain how each structural feature is an adaptation for its function. This is a common structured question.
Fertilisation is the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg) to form a diploid zygote. In mammals, fertilisation occurs in the oviduct (fallopian tube).
| Step | Event | Significance |
|---|---|---|
| Capacitation | Sperm membrane changes | Enables acrosome reaction |
| Acrosome reaction | Release of digestive enzymes | Allows sperm to penetrate egg layers |
| Membrane fusion | Sperm enters egg | Delivers paternal genetic material |
| Cortical reaction | Hardening of zona pellucida | Prevents polyspermy |
| Completion of meiosis II | Secondary oocyte completes division | Produces mature ovum |
| Nuclear fusion | Male and female pronuclei fuse | Restores diploid number; forms zygote |
After fertilisation, the zygote undergoes a series of rapid mitotic divisions called cleavage:
Exam Tip: Understand the link between fertilisation, early cell division and stem cells. The inner cell mass of the blastocyst is the source of embryonic stem cells used in research — this connects Topic 2 content on fertilisation with the content on stem cells.
The Edexcel 9BI0 specification places gametogenesis and fertilisation within Topic 2 (Cells, Viruses and Reproduction) as the synoptic bridge between meiosis (lesson 7) and early embryonic differentiation (lesson 8). Candidates must describe spermatogenesis in the seminiferous tubules (spermatogonium → primary spermatocyte → meiosis I → secondary spermatocytes → meiosis II → 4 spermatids → spermatozoa), supported by Sertoli nurse cells and testosterone-producing Leydig cells; describe oogenesis (oogonium proliferation in the foetus → primary oocyte arrested at prophase I from foetal life → meiosis I completed at ovulation giving a secondary oocyte arrested at metaphase II + first polar body → meiosis II completed only on fertilisation giving the ovum + second polar body); explain the structural adaptations of the sperm (acrosome, condensed haploid nucleus, mitochondrion-packed midpiece, 9+2 flagellum, minimal cytoplasm) and the ovum (large cytoplasmic store, zona pellucida, corona radiata, cortical granules); and describe fertilisation as a cascade — capacitation, acrosome reaction, membrane fusion, fast Na⁺ block, cortical reaction (slow block), meiosis II completion, karyogamy to form the diploid zygote. Synoptic links: lesson 7 (meiosis), lesson 8 (totipotent zygote → pluripotent inner cell mass), Topic 6 (maternal-foetal immune tolerance), Topic 8 (IVF/ICSI and FSH/LH endocrine control) — refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Compare spermatogenesis and oogenesis quantitatively, identifying three differences in cellular output and timing. (3)
(b) Explain how the acrosome reaction and the cortical reaction together prevent polyspermy. Use sequential reasoning. (3)
(c) The secondary oocyte ovulated each cycle is arrested at metaphase II until fertilisation. Explain the biological logic of arresting at this stage rather than completing meiosis II earlier. (2)
Solution with mark scheme:
(a) M1 (AO1.1) — cellular output: spermatogenesis produces four functional sperm from each primary spermatocyte; oogenesis produces one ovum and three polar bodies (the polar bodies degenerate). The polar bodies discard surplus chromosomes while preserving cytoplasm in the ovum.
A1 (AO1.2) — timing: spermatogenesis is continuous from puberty throughout adult life (~64 days per cycle, ~300 million sperm/day); oogenesis is discontinuous — primary oocytes are formed in foetal life (by month 5), arrested at prophase I until each ovulation, and meiosis II completes only on fertilisation. A female is born with her lifetime oocyte pool (~1–2 million, ~400 ovulated).
A1 (AO2.1) — cytoplasmic distribution: spermatogenesis is symmetric (equal cytokinesis at both meiotic divisions); oogenesis is asymmetric — unequal cytokinesis directs almost all cytoplasm into the ovum so that nutrient stores, mitochondria and maternal mRNAs are conserved for the embryo before implantation.
(b) M1 (AO1.2) — first, the acrosome reaction: a capacitated sperm binds ZP3 on the zona pellucida; the acrosomal membrane fuses with the sperm membrane and releases hydrolytic enzymes (acrosin, hyaluronidase) that digest a path through the corona radiata and zona, allowing the sperm to reach the egg membrane.
M1 (AO1.2) — second, the fast block (electrical): sperm-egg membrane fusion triggers a Na⁺ influx that depolarises the egg membrane within seconds, preventing further sperm-membrane fusion.
A1 (AO2.1) — third, the cortical reaction (slow block): the rise in cytosolic Ca²⁺ triggers exocytosis of cortical granules; their enzymes cross-link and harden the zona pellucida and inactivate the sperm-binding ZP3 receptors, making the zona impermeable to further sperm. Together the fast electrical block and the slower cortical reaction prevent polyspermy, which would otherwise produce a triploid (or higher) zygote that fails to develop.
(c) M1 (AO2.1) — completing meiosis II earlier would commit the cell's chromosomes to a haploid configuration and discard maternal cytoplasm into a polar body before fertilisation is guaranteed. Most ovulated oocytes are not fertilised — completing meiosis II in advance would waste the cytoplasmic investment.
A1 (AO3.1a) — arrest at metaphase II couples the completion of meiosis to the calcium-wave signal generated by sperm entry, ensuring that polar-body extrusion and chromosome reduction happen only when a fertilising event has occurred. This is an elegant "wait-for-trigger" coupling of cell-cycle control to extracellular signalling.
Total: 8 marks.
Question (6 marks): Compare the structural and functional adaptations of the human sperm and ovum, and explain how their interaction at fertilisation produces a diploid zygote while preventing polyspermy.
Mark scheme decomposition by AO:
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