Summary
The fusion of sperm and egg nuclei to form a diploid zygote.
In depth
The sperm binds the zona pellucida, the acrosome reaction releases enzymes, membranes fuse, and the egg's cortical granules harden the coat (block to polyspermy). Calcium waves activate the egg to complete meiosis II. Pronuclei fuse.
Fertilisation restores the diploid number and combines two genomes, and activates the egg metabolically. Only one sperm should enter, since polyspermy produces lethal extra chromosome sets.
Remember: the block to polyspermy has a fast electrical phase in sea urchins and a slow cortical reaction.
Fertilisation begins when a sperm reaches the egg. In mammals the sperm must pass the cumulus cells, bind the zona pellucida, and undergo the acrosome reaction, releasing enzymes that let it penetrate. Membrane fusion triggers a wave of calcium in the egg (egg activation).
Blocks to polyspermy prevent more than one sperm entering: a fast electrical block in some species and a slower cortical reaction in which granules release enzymes that harden the egg coat. The egg completes meiosis II, and the two pronuclei merge to restore the diploid number.
Example: in sea urchins the calcium wave and fertilisation envelope can be watched under a microscope.
Key idea: fertilisation combines two genomes and activates development.
Frequently asked questions
What is Fertilisation?
The fusion of sperm and egg nuclei to form a diploid zygote.
Which area of biology covers Fertilisation, and how advanced is it?
Fertilisation is one of the 54 topics in Developmental Biology. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Zygote, Hedgehog Signalling, Cleavage.
Can you give an example of Fertilisation?
IVF bypasses natural steps by injecting sperm (ICSI).
What should I learn before Fertilisation?
Fertilisation is a Beginner topic, so no earlier topic is required. Other Beginner topics in Developmental Biology that pair well with it include Zygote, Embryo, Cleavage.
What topics are related to Fertilisation?
The closest topics to Fertilisation are Zygote, Hedgehog Signalling, Cleavage, Sex Determination. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Fertilisation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Fertilisation. To practise the whole of Developmental Biology, use the Developmental Biology quiz.
How is Fertilisation different from Zygote?
Fertilisation: The fusion of sperm and egg nuclei to form a diploid zygote. Zygote: The single cell formed at fertilisation that divides to become an embryo. Read both side by side and note what each one explains.
How should I study Fertilisation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Zygote and explain the difference aloud. Finally, take the Developmental Biology quiz and come back to this page for any question you miss.