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D3.1.6—Fertilization in humans

Human fertilization involves sperm and egg nuclear changes that allow their genetic material to combine into a zygote after gamete interaction.

Syllabus
First assessment 2025
Objective
D3.1.6
Level
HL

Human Fertilization Joins Parental Chromosomes

Human fertilization begins in the oviduct when sperm and egg cell membranes fuse and ends with paternal and maternal chromosomes sharing the first zygotic mitosis.

Sperm membrane fuses with egg membrane → sperm nucleus enters while its tail and mitochondria are destroyed → sperm and egg nuclear membranes dissolve → both condensed chromosome sets attach to one mitotic spindle → chromosomes segregate to form two diploid nuclei.

This sequence brings one haploid paternal and one haploid maternal chromosome set into a diploid zygote genome while preventing paternal sperm mitochondria becoming part of the embryo.

The 23 paternal and 23 maternal chromosomes participate together in the first mitosis, so each of the first two embryonic nuclei receives a diploid set.

Fertilization is not implantation: nuclear union begins in the oviduct, while attachment to the endometrium happens later.

Fertilization in humans

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Fertilization is a cellular process involving sperm and egg nuclei.

Representative question

Question 1

[Maximum number: 6]

Describe the process of fertilization in humans.

Retrieve the Core Reproduction Route

Core D3.1 route: reproduction creates offspring, gametes or pollen move, fertilization or germination follows, and the consequence is variation, embryo formation, seed production, or successful early growth.

  • mitosis makes clones; meiosis and fertilization create variation
  • hormones, anatomy, fertilization, and IVF support gamete fusion and embryo development
  • pollination and pollen-tube growth bring gametes together inside ovules
  • dispersal reduces competition and germination starts with water, enzymes, and reserves

Core Reproduction

Core D3.1 exam questions usually combine reproduction strategy with gamete formation, fertilization, human cycles, IVF, plant pollination, or seed germination. Treat each answer as a route: name the process, say what moves or changes, then give the biological consequence.

  • Compare asexual and sexual reproduction by mechanism and genetic outcome.
  • Link meiosis, fertilization, reproductive anatomy, and hormonal cycles to successful reproduction.
  • Explain plant pollination and seed stages by connecting structures to transfer, fertilization, dispersal, and germination.

Concept essentials

  • Fertilization is a cellular process involving sperm and egg nuclei.
  • The event is distinct from meiosis, which has already produced gametes.
  • The zygote begins development only after fertilization has combined gamete genetic material.