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Fertilisation (DP IB Biology): Revision Note

Cara Head

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Cara Head

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DP Biology: HLDP Biology: SL

Human fertilisation

  • Fertilisation can be defined as:

the fusion of a sperm and an egg cell nucleus to give rise to a diploid zygote

  • During sexual reproduction many sperm are released, and the sperm cells are attracted towards the egg cell by chemical signals

  • When the first sperm cell reaches the egg cell surface, its membrane fuses with the egg cell membrane

  • The sperm cell nucleus then enters the egg cell

    • At this point vesicles released from the egg destroy the sperm tail and its mitochondria

  • Temporary membranes form around the two haploid sets of chromosomes within the egg cell cytoplasm; the resulting structures are known as pronuclei

    • DNA replication takes place at this stage in preparation for mitosis

  • The two haploid pronuclei come together and the temporary membranes break down

  • Condensed chromosomes undergo the first mitotic division of the now diploid cell

    • Subsequent mitotic divisions take place to form a blastocyst

Diagram of fertilisation showing stages: sperm digesting zona pellucida, binding to oocyte receptors, nuclei merging, and completion of fertilisation.
During fertilisation the sperm cell nucleus enters the egg cell but its tail and mitochondria are broken down

Examiner Tips and Tricks

Note that details in the diagram above relating to the acrosome, cortical reactions and zona pellucida are only required at HL.

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Cara Head

Author: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding