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Fertilisation in mammals

Cell Structure, Reproduction and Development · Chromosomes, meiosis and gametes · note 4 of 5

Spec 3.12Download all Chromosomes, meiosis and gametes notes (PDF)Download all notes (PDF)7 pages

Fertilisation in mammalsSpec 3.12

In short

In mammalian fertilisation, a sperm binds to the zona pellucida and undergoes the acrosome reaction, releasing enzymes that digest a path through it. The sperm and egg membranes fuse and the sperm nucleus enters. The cortical reaction then hardens the zona pellucida to block other sperm, and the two haploid nuclei fuse to form a diploid zygote.

  1. Sperm swim towards the egg and pass through the layer of follicle cells.
  2. The head of a sperm binds to receptors in the zona pellucida.
  3. The acrosome reaction takes place: the acrosome membrane fuses with the sperm cell surface membrane and releases its enzymes by exocytosis. The enzymes digest a path through the zona pellucida.
  4. The sperm cell surface membrane fuses with the egg cell surface membrane, and the sperm nucleus enters the egg cytoplasm.
  5. The cortical reaction takes place: the cortical granules fuse with the egg cell surface membrane and release their enzymes. These change the zona pellucida so that it hardens and sperm can no longer bind to it.
  6. The haploid sperm nucleus and haploid egg nucleus fuse, which forms a diploid zygote. This fusion of nuclei restores the diploid number.

The cortical reaction prevents polyspermy (more than one sperm fertilising the egg). If a second sperm entered, the zygote would have three sets of chromosomes and could not develop normally.

Four-stage diagram of fertilisation in mammals: a sperm binds to receptors in the zona pellucida; the acrosome releases enzymes that digest a path through the zona pellucida; the sperm and egg cell surface membranes fuse, the sperm nucleus enters and cortical granules release enzymes so the zona pellucida hardens; the haploid egg and sperm nuclei fuse to give a diploid zygote nucleus. (opens full size in a new tab)
The acrosome reaction lets the sperm reach the egg; the cortical reaction stops other sperm entering.
Exam tip:

Link each reaction to its purpose: the acrosome reaction lets the sperm reach the egg membrane, and the cortical reaction stops further sperm entering.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

How does meiosis produce genetic variation?

Meiosis produces genetically different gametes in two ways. Crossing over in prophase I swaps alleles between non-sister chromatids of homologous chromosomes. Independent assortment in metaphase I means the maternal and paternal chromosomes of each pair separate into gametes in random combinations.

What is the acrosome reaction?

The acrosome reaction happens when a sperm head binds to the zona pellucida. The acrosome membrane fuses with the sperm cell surface membrane and releases digestive enzymes by exocytosis. These enzymes digest a path through the zona pellucida so that the sperm can reach the egg cell surface membrane.

What is the cortical reaction and why does it happen?

The cortical reaction happens after a sperm fuses with the egg. Cortical granules release enzymes that harden the zona pellucida, so that no further sperm can bind. This prevents polyspermy, which would give a zygote with too many chromosomes that could not develop normally.

All 5 questions on Chromosomes, meiosis and gametes