3.3.2Topic 3 · Voice of the Genome
Fertilisation in Mammals — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 9 parts Lesson 15 of 26 1 quiz
Lesson summary
3.3.2 Fertilisation in Mammals — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the events of fertilisation, including the acrosome reaction and the cortical reaction, and explain why only one sperm normally fertilises an egg.Fertilisation is the fusion of the haploid nucl…
- Exam-style question 1
- Exam-style question 2
- Exam-style question 3
- Capacitation
- Polyspermy
- Blastocyst
Section 1 of 9Where and when
Fertilisation is the fusion of the haploid nucleus of a sperm with the haploid nucleus of an egg to form a diploid zygote.
Where and when
It normally takes place in the upper part of the oviduct (fallopian tube) within about a day of ovulation.
Sperm are deposited in the vagina, swim through the cervix and uterus and into the oviduct.
They are not fully capable of fertilising when they arrive,
- but undergo capacitation in the female tract: changes to the sperm surface membrane that make the acrosome reaction possible and the tail beat more vigorously.
Section 2 of 9Sequence of events
1
Chemical signals from the egg and its surroundings guide capacitated sperm to the egg,
- which is protected by the follicle cells and the zona pellucida.
- (the corona radiata)
2
The sperm head binds to receptor proteins on the zona pellucida.
3Acrosome reaction
Acrosome reaction binding triggers the acrosome membrane to fuse with the sperm surface membrane and release its enzymes by exocytosis.
- These digest a path through the zona pellucida.
4
The sperm cell surface membrane fuses with the egg cell surface membrane, and the head and nucleus enter the egg cytoplasm.
5Cortical reaction
Cortical reaction fusion causes a wave of calcium ions that triggers cortical granules to release their enzymes into the space around the egg.
- These alter the zona pellucida (it hardens and loses its receptors), and the egg membrane depolarises, so no more sperm can bind or enter.
- This prevents polyspermy.
6
The egg completes meiosis II and releases a second polar body.
- The haploid sperm nucleus and egg nucleus become pronuclei, which move together and fuse (syngamy).
- The zygote is now diploid.
Section 3 of 9Why polyspermy must be prevented
If more than one sperm fused with the egg, the resulting cell would have an abnormal number of chromosomes (triploid or more), and the embryo would not develop normally.
Why polyspermy must be prevented
The fast depolarisation of the egg membrane and the slower cortical reaction are two mechanisms that block polyspermy.
Fertilisation also restores the diploid number and produces genetic variation,
- because the new individual receives a combination of alleles from two different parents,
- and the sex of the child is determined by whether the sperm carries an X or a Y chromosome.
Section 4 of 9From zygote to implantation
The zygote begins dividing by mitosis (cleavage) about 24 hours after fertilisation, without growing in overall size, producing a ball of cells called a morula and then a blastocyst.
1
The cilia of the oviduct and muscular contractions move the embryo towards the uterus, which takes several days.
2
The blastocyst consists of an outer layer, the trophoblast,
- which will form the placenta, and an inner cell mass of embryonic stem cells that will form the embryo.
3
It implants in the thick, vascular uterine lining (endometrium) about 6 to 7 days after fertilisation.
4
The trophoblast secretes hCG, which maintains the corpus luteum so that progesterone production continues.
Section 5 of 9Related topics
The pattern of the acrosome and cortical reactions is a good example of exocytosis triggered by cell signalling.
Related topics
In vitro fertilisation (IVF) copies these steps in a dish,
- and in intracytoplasmic sperm injection (ICSI) a single sperm is injected into the egg directly, bypassing the zona pellucida.
The embryo cells can be used as a source of stem cells, which is covered later in this unit.
Fertilisation also links to meiosis,
- since each gamete carries a random mixture of alleles,
- and to the study of inheritance, in which the probability of each genotype in the zygote can be predicted from the parental genotypes.
Section 6 of 9Exam focus
Mark schemes for fertilisation reward the correct order of events: binding, acrosome reaction, fusion, cortical reaction, completion of meiosis II and fusion of nuclei.
Exam focus
Use precise terms: exocytosis for the release of the acrosome enzymes, calcium ions as the trigger, and zona pellucida rather than 'outer layer'.
If asked about IVF, describe the main steps: ovarian stimulation with FSH, egg collection, mixing with prepared sperm in a dish, culture of the embryos for a few days and transfer to the uterus.
Evaluation points include cost, the chance of multiple births, ethical issues around surplus embryos, and the benefits for infertile couples.
Section 7 of 9Exam-style questions
Describe the acrosome reaction and explain its importance. [3]3 marks
Exam-style questions1. When the sperm binds to the zona pellucida, the acrosome membrane fuses with the cell surface membrane and releases its enzymes by exocytosis. [1]2. The enzymes (proteases) digest a pathway through the zona pellucida. [1]3. This allows the sperm to reach and fuse with the egg membrane. [1]Explain how polyspermy is prevented. [3]3 marks
Exam-style questions1. Fusion of the first sperm triggers calcium ions to enter the egg cytoplasm. [1]2. Cortical granules release enzymes that change the zona pellucida so sperm cannot bind. [1]3. The egg membrane depolarises, which also blocks entry of other sperm. [1]A human zygote has 46 chromosomes. State how many chromosomes were in the sperm nucleus and the number in each cell of the blastocyst, and explain. [3]3 marks
Exam-style questionsSperm nucleus: 23 (haploid). [1]Blastocyst cells: 46. [1]Because the zygote divides by mitosis, which makes genetically identical diploid daughter cells. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Fertilisation | Fusion of male and female haploid nuclei to form a diploid zygote. |
| Capacitation | Changes in sperm in the female tract that make them able to fertilise an egg. |
| Acrosome reaction | Release of acrosome enzymes when a sperm binds to the egg coat. |
| Cortical reaction | Release of cortical granule contents that blocks polyspermy. |
| Polyspermy | Entry of more than one sperm into an egg. |
| Zygote | The diploid cell formed by fertilisation. |
| Blastocyst | Hollow ball of cells formed by early cleavage that implants in the uterus. |
Section 9 of 9Exam technique
1
Saying that fertilisation takes place in the uterus. It usually occurs in the oviduct; implantation occurs in the uterus.
2
Confusing fertilisation with implantation, or with ovulation.
3
Believing the whole sperm enters the egg. The tail often stays outside, and only the head contents are needed.
4
Writing that the acrosome reaction prevents polyspermy. That is the role of the cortical reaction.
5
Forgetting that the egg completes meiosis II only after sperm entry.
Marked practiceQuizzes for this lesson
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