Millions of sperm set off and exactly one succeeds. What happens in the moments after it arrives is a tightly ordered sequence, and the exam rewards you for getting the order right — especially the part where the egg deliberately shuts the door behind the winner.
📘 What you need to know
Fertilisation is the fusion of a sperm nucleus and an egg nucleus to produce a diploid zygote.
Sperm are attracted towards the egg by chemical signals; this is not random searching.
The first sperm to reach the egg surface has its membrane fuse with the egg membrane, and only its nucleus enters.
The sperm’s tail and mitochondria are destroyed by vesicles released from the egg — so all of the zygote’s mitochondria come from the mother.
Temporary membranes form around the two haploid sets of chromosomes, making two pronuclei. DNA replicates, the membranes break down, and the now diploid cell undergoes its first mitotic division.
Repeated mitosis produces a blastocyst, the ball of cells that will implant in the uterus lining.
Only one sperm can fertilise the egg. Blocking the rest is an active process, not an accident.
The definition, precisely
Fertilisation
the fusion of a sperm nucleus and an egg nucleus to give rise to a diploid zygote
Read that carefully, because two words in it are doing real work. It is the fusion of nuclei, not of whole cells — almost nothing else from the sperm makes it in. And the product is a zygote, a single diploid cell, not an embryo. The embryo comes later, after mitosis has started.
The sequence
Stage 2 is where the door closes. Everything that happens to the losing sperm happens in the fraction of a second after the winner binds.
What happens in detail
Sperm released during sexual reproduction are drawn towards the egg by chemical signals it releases. When the first sperm reaches the egg cell surface, its membrane fuses with the egg cell membrane, and the sperm nucleus passes into the egg cytoplasm.
Almost immediately, vesicles released from the egg destroy the sperm’s tail and its mitochondria. Temporary membranes then form around each of the two haploid sets of chromosomes, producing structures called pronuclei — one from the sperm, one from the egg. DNA replication takes place at this stage, in preparation for division.
The two pronuclei then move together, their temporary membranes break down, and the chromosomes condense. The now diploid cell carries out its first mitotic division. Repeated mitosis follows, producing the ball of cells called a blastocyst.
Students often write that “the sperm and egg fuse”. Be more precise: the membranes fuse, then the nuclei fuse. Between those two events there is a stage where both sets of chromosomes are in the same cytoplasm but still separate, inside their own pronuclei. Questions about DNA replication are aimed exactly at that gap.
Why only one sperm
If two sperm got in, the cell would have three sets of chromosomes and could not develop. So the egg blocks the others, and it does so within seconds of the first sperm binding.
The mechanism is a release of enzymes from vesicles just beneath the egg’s surface. These enzymes destroy the receptor proteins that sperm bind to, and they harden the coat surrounding the egg. With no receptors to bind to and a hardened barrier to cross, the remaining sperm are locked out. It is worth noticing the logic: the block is triggered by the successful fertilisation, so it can never fire too early.
SL or HL? The fine detail of the acrosome, the zona pellucida and the cortical reaction is HL only. At SL you need the sequence and the outcome: one sperm binds, membranes fuse, nucleus enters, others are blocked. Knowing where the boundary sits saves you revising the wrong things.
What the sperm actually contributes
This is a small point that answers a surprising number of questions. The sperm contributes a haploid nucleus and essentially nothing else. Every mitochondrion in the zygote — and therefore in the whole person that develops from it — came from the egg.
A useful consequence: mitochondrial conditions are passed down the maternal line only. If a question mentions a trait inherited exclusively from mothers, this diagram is the reason.
Worked examples
WORKED EXAMPLE
Explain why it is essential that only one sperm fertilises an egg. [3]
Start from the chromosome arithmeticeach gamete is haploid, so one sperm plus one egg gives the diploid number, 46 in humansSay what a second sperm would doa second sperm would add another 23, giving 69 — three sets, so the zygote could not develop normallyName the blockenzymes released from the egg destroy the receptors and harden the coat, so no further sperm can bind3 marks: correct diploid restoration, consequence of a second sperm, mechanism of the block
WORKED EXAMPLE
A genetic disorder is caused by a mutation in mitochondrial DNA. Explain why an affected father cannot pass this disorder to his children. [2]
Ask what the father actually contributesonly the sperm nucleus enters the eggFollow it throughthe sperm’s mitochondria are destroyed after the membranes fuse, so none are passed onall of the zygote’s mitochondria, and therefore all its mitochondrial DNA, come from the egg1 mark for paternal mitochondria being destroyed, 1 mark for all mitochondria being maternalthis is a classic “apply what you know” question — the disorder itself is irrelevant
💡 Exam tips
Give the stages in order. Marks are often awarded for sequence, so an unordered list of correct facts can still lose them.
Use zygote for the single fertilised cell and embryo or blastocyst for what comes after mitosis. They are not interchangeable.
Remember that DNA replication happens at the pronuclei stage, before the first division — not after fusion of the nuclei.
If asked where fertilisation occurs, the answer is the oviduct, not the uterus.
Check the command term. “Outline” wants the sequence; “explain” wants a reason attached to each step.
⚠️ Common mix-ups
Saying the whole sperm enters the egg. The membranes fuse and the nucleus enters; the tail and mitochondria are broken down.
Calling the zygote an embryo. The zygote is one cell. It becomes an embryo only once mitosis has produced a ball of cells.
Writing that the fastest sperm wins. The first to bind wins, and binding depends on receptors, not just speed.
Revising acrosome and zona pellucida detail at SL. That material is HL only — useful context, but not what you will be assessed on.
Confusing fertilisation with implantation. Fertilisation happens in the oviduct within about a day; implantation happens in the uterus around a week later.
Assuming the zygote is immediately diploid at fusion of membranes. It becomes diploid when the two pronuclei come together and the membranes break down.
Up next: Plant Reproduction — the same logic of meiosis, gametes and fertilisation, but in an organism that cannot move to find a partner.
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