Fertilisation (HL IB Biology)

• Fertilisation is the fusion of one sperm cell and one ovum (egg cell); this fusion of two haploid nuclei gives rise to a diploid zygote
• During sexual reproduction, many sperm are released, and the sperm cells are attracted towards the secondary oocyte by chemical signals
• When the sperm cells reach the secondary oocyte, the process that takes place at its cell surface prevents more than one sperm from passing through its cell surface membrane
  • The entry of more than one sperm into a single oocyte is known as polyspermy
• When the first sperm cell digests its way through the zona pellucida, it reaches the oocyte cell surface membrane; complementary receptors on the head of the sperm bind with proteins on the oocyte cell surface membrane, enabling the cell surface membranes of the two gametes to fuse together and the sperm nucleus to enter the oocyte
  • At this point vesicles released from the egg destroy the sperm flagellum (tail) and its mitochondria
• Inside the ovum haploid sets of chromosomes from the sperm and egg cell are both within the cytoplasm of the oocyte
• The paternal and maternal chromosomes form a pronucleus within which DNA undergoes replication to prepare for mitosis
  • The two haploid pronuclei come together and the temporary membranes dissolves to create a diploid cell, the zygote, fertilisation is now complete
  • Chromosomes undergo the first mitotic division of the now diploid cell, subsequent mitotic divisions take place to form a blastocyst

Fertilisation diagram