Gametes and fertilisation in mammalsEdexcel A-Level Biology A: Revision notes
Section 1
Gametes
Gametes are the haploid sex cells made by meiosis: sperm from the testes and eggs from the ovaries. In humans the egg released at ovulation is a secondary oocyte.
Each is specialised for its function. The sperm must find and enter the egg and deliver its haploid nucleus. The egg must provide a safe, nourished environment for the embryo and make sure only one sperm fertilises it.
Section 2
The sperm
A sperm has three regions.
- Head: contains the haploid nucleus and, at the front, the acrosome, a vesicle containing hydrolytic (digestive) enzymes.
- Mid-piece: packed with mitochondria that make ATP by aerobic respiration for the tail.
- Tail (flagellum): beats to move the sperm.
Sperm are small and produced in huge numbers because many are lost on the way to the egg.
Section 3
The egg
The egg is a large cell, so it contains a large store of nutrients in its cytoplasm to feed the early embryo.
It is surrounded by the zona pellucida, a thick layer of glycoprotein that protects the egg and which sperm must pass through. Just beneath the egg cell surface membrane are cortical granules, vesicles containing enzymes and other substances that are released after a sperm enters.
The egg cannot move on its own. It is moved along the oviduct by cilia and muscle contraction.
Section 4
Fertilisation step by step
- A sperm reaches the zona pellucida and binds to it.
- Acrosome reaction: the acrosome membrane fuses with the sperm cell surface membrane and releases enzymes, which digest the zona pellucida.
- The sperm moves through the zona pellucida to the egg, and the sperm and egg cell surface membranes fuse. The sperm nucleus enters the egg.
- Cortical reaction: cortical granules release their contents into the zona pellucida, which makes it hard and impenetrable to other sperm.
- The haploid nuclei of the sperm and egg fuse to form a diploid zygote nucleus.
The cortical reaction does not help the first sperm in: it happens after fusion and protects against further sperm.
Section 5
Why only one sperm?
If more than one sperm entered, the zygote would have extra sets of chromosomes. This is polyspermy, and the embryo would not develop properly.
The cortical reaction prevents polyspermy. The fertilised egg then develops by mitosis into an embryo.
Although millions of sperm are released, only one fertilises the egg. This works because the acrosome enzymes from many sperm are needed to digest the zona pellucida, but the cortical reaction then blocks the others.
Link each structure to its function: acrosome enzymes, so the sperm can digest the zona pellucida; mitochondria, so the sperm has ATP for the tail.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gametes and fertilisation in mammals
- A biologist examines human gametes using an electron microscope. The sperm cells have a head, a mid-piece and a tail. The egg is a large cell surrounded by a jelly-like layer called the zona pellucida, with small vesicles lying just beneath its cell surface membrane.Describe two ways in which the egg is specialised for its function.2 marks
- In a fertility laboratory, sperm are added to a dish containing a human egg. The scientists watch as sperm reach the zona pellucida, and they study the events that follow the contact between one sperm and the egg.Explain why it would be harmful if more than one sperm fused with the egg.2 marks
- A fertility clinic investigates two men whose sperm fail to fertilise eggs. In man 1 the sperm have normal tails and mitochondria but have no acrosome. In man 2 the sperm have a normal acrosome but have very few mitochondria in the mid-piece, and their tails barely move.Explain why the sperm from man 1 are unable to fertilise an egg.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).