Only the nucleus of the sperm survives the journey. The tail that got it there and the mitochondria that powered it are both destroyed within minutes of arrival — which is why you inherit mitochondrial DNA from your mother alone.
📚 What you need to know
Fertilisation is the fusion of a sperm nucleus and an egg cell nucleus to give rise to a diploid zygote.
Many sperm are released, and they are attracted towards the egg cell by chemical signals.
When the first sperm reaches the egg cell surface, its membrane fuses with the egg cell membrane.
The sperm nucleus enters the egg. Vesicles released from the egg then destroy the sperm tail and its mitochondria.
Temporary membranes form around the two haploid sets of chromosomes, forming pronuclei. DNA replication takes place at this stage.
The two pronuclei come together, the temporary membranes break down, and the condensed chromosomes undergo the first mitotic division of the now diploid cell.
Repeated mitotic divisions then form a blastocyst.
The definition, and why the wording matters
Fertilisation is not “when the sperm meets the egg”. It is the fusion of the two nuclei. Everything before that — the swimming, the binding, the membranes fusing — is the approach. The event itself is the joining of two haploid nuclei to give one diploid nucleus.
Definition to learn word for word
the fusion of a sperm and an egg cell nucleus to give rise to a diploid zygote
What happens, step by step
Many sperm are released during sexual reproduction, and chemical signals from the egg attract them towards it. Only the first one to reach the egg cell surface matters. Its membrane fuses with the egg cell membrane, and the sperm cell nucleus passes into the egg.
At that point the egg releases vesicles that destroy the sperm tail and its mitochondria. The sperm has done its job of delivery; nothing else it carried is needed, and none of it becomes part of the embryo.
The sperm contributes a nucleus and essentially nothing else, which is why mitochondrial DNA is inherited only from the mother.
Pronuclei and the first division
Once inside, the two haploid sets of chromosomes are each surrounded by a temporary membrane. These structures are called pronuclei: one from the sperm, one from the egg, sitting in the same cytoplasm but not yet joined.
DNA replication happens at this stage, in preparation for mitosis. The two pronuclei then come together, their temporary membranes break down, and the condensed chromosomes undergo the first mitotic division of the now diploid cell. Further mitotic divisions follow, and the ball of cells becomes a blastocyst.
Why replication comes first. Mitosis needs two copies of every chromosome to pull apart. Replicating the DNA while the pronuclei are still separate means the very first division can happen as soon as they fuse, with no delay.
The finer detail — the acrosome reaction, the cortical reaction and the hardening of the zona pellucida — is HL material and is covered fully in Fertilisation & Implantation later in this topic. Get the basic sequence solid first.
Worked examples
WORKED EXAMPLE
Outline the events that take place from the moment a sperm reaches the egg cell surface to the formation of a zygote. [4]
Point 1 — membranes
The sperm cell membrane fuses with the egg cell membrane.
Point 2 — entry
The sperm nucleus enters the egg cell, and vesicles released from the egg destroy the sperm tail and mitochondria.
Point 3 — pronuclei
Temporary membranes form around the two haploid sets of chromosomes, creating pronuclei, and DNA replication occurs.
Point 4 — fusion
The pronuclei come together, the temporary membranes break down, and a diploid zygote is formed.
4 marksKeep the events in order. Marks are usually awarded for sequence as well as content.
WORKED EXAMPLE
Suggest why the mitochondria of a human embryo are all inherited from the mother. [2]
Point 1 — what happens to the sperm
After the sperm nucleus enters the egg, vesicles released from the egg destroy the sperm’s tail and its mitochondria.
Point 2 — the consequence
No paternal mitochondria survive in the zygote, so every mitochondrion in the embryo came from the egg cell cytoplasm.
2 marksA tidy link to mitochondrial DNA being used to trace maternal ancestry.
💡 Exam tip
Define fertilisation as the fusion of nuclei, not of cells.
Always include diploid when describing the zygote — it is usually a marking point.
Pronuclei are temporary and haploid. Say both if you name them.
DNA replication happens before the first mitotic division, not after.
The first division of the zygote is mitosis, not meiosis. Meiosis is finished by this stage.
If the question is about the sperm’s structure, link the tail to movement and the mitochondria to ATP for that movement.
⚠ Common mix-up
The whole sperm does not enter and stay. The nucleus enters; the tail and mitochondria are destroyed.
Pronuclei are not the same as the zygote nucleus. They fuse to produce it.
Fertilisation happens in the oviduct, not in the uterus.
Many sperm are released, but only one fertilises the egg. Say “the first to arrive”.
A zygote is a single cell. Once it has divided it is an embryo.
Do not describe the chemical signals as the sperm “choosing” a direction. They move up a chemical gradient.
Up next: Plant Reproduction — the same principles in flowering plants, plus one thing animals never do: fertilising twice at once.
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