Revision notes for Edexcel A A Level Biology Gametes, fertilisation and meiosis. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Gametes, fertilisation and meiosis
What you'll learn
- Why gametes are haploid and how fertilisation restores the diploid chromosome number.
- How sperm and egg cells are specialised for their roles in fertilisation.
- How meiosis separates chromosomes and creates genetic variation.
- The key exam distinctions between meiosis I, meiosis II, and mitosis.
The chromosome basics you need first
A chromosome is a long molecule of DNA, associated with proteins, that carries many genes. A gene is a section of DNA that codes for a polypeptide or functional RNA. Different versions of the same gene are called alleles, and the position of a gene on a chromosome is its locus.
In humans, most body cells contain 46 chromosomes arranged as 23 pairs. One chromosome in each pair is inherited from the mother and one from the father.
Definition
Homologous chromosomes
Homologous chromosomes are a matching pair of chromosomes with the same genes at the same loci, although they may carry different alleles.
Before meiosis, DNA is replicated. A replicated chromosome is made of two sister chromatids, which are initially identical copies of the same DNA molecule, joined at a centromere. The centromere is the region where spindle fibres attach during cell division.
Common Mistake
Replicated does not mean more chromosomes
After DNA replication, each chromosome has two sister chromatids, but it is still counted as one chromosome until the centromere divides. DNA content has doubled; chromosome number has not.
Diploid, haploid, gametes and zygotes
Definition
Diploid and haploid
A diploid cell has two sets of chromosomes, shown as 2n2n2n. A haploid cell has one set of chromosomes, shown as nnn.
A gamete is a haploid sex cell, such as a sperm cell or egg cell. A zygote is the first diploid cell formed when two gametes fuse at fertilisation.
Key Idea
Why meiosis is needed
Meiosis halves the chromosome number to make haploid gametes. Fertilisation then restores the diploid number, so chromosome number stays constant from generation to generation.
Example
Tracking chromosome number through the human life cycle
-
A human body cell is diploid, so its chromosome number is 2n=462n = 462n=46. This means one haploid set is n=23n = 23n=23.
-
Meiosis halves the chromosome number, so each human sperm or egg cell contains 23 chromosomes.
-
Fertilisation combines two haploid nuclei: 23+23=4623 + 23 = 4623+23=46, restoring the diploid chromosome number in the zygote.
Gametes are specialised cells
Sperm cells and egg cells are both gametes, but they are specialised in very different ways.
Sperm cells
A sperm cell is specialised to reach and fertilise the egg. It has:
- a haploid nucleus, carrying one set of paternal chromosomes
- an acrosome, a vesicle in the head containing enzymes that help penetrate the egg’s outer layers
- many mitochondria in the midpiece to provide ATP for movement
- a flagellum, or tail, to swim towards the egg
- receptors on its surface that help it bind to the egg
Egg cells
An egg cell is specialised to be fertilised and support the earliest stages of development. It has:
- a haploid nucleus, carrying one set of maternal chromosomes
- a large cytoplasm with molecules needed for early development
- a protective outer layer called the zona pellucida
- cortical granules, small vesicles near the membrane that help prevent more than one sperm entering
Fertilisation
Fertilisation is the fusion of the male and female gamete nuclei to form a diploid zygote. In mammals, this involves carefully controlled stages so that normally only one sperm fertilises the egg.
The diagram below shows the main stages of mammalian fertilisation, including the acrosome reaction and the cortical reaction.
The main stages
- The sperm binds to receptors in the zona pellucida surrounding the egg.
- The acrosome reaction occurs: enzymes from the acrosome are released and digest a path through the zona pellucida.
- The sperm cell surface membrane fuses with the egg cell surface membrane.
- The sperm nucleus enters the egg cytoplasm.
- The cortical reaction occurs: cortical granules release their contents, causing the zona pellucida to harden.
- The male and female haploid nuclei fuse, producing a diploid zygote.
Definition
Polyspermy
Polyspermy is the entry of more than one sperm into an egg. It would produce an abnormal chromosome number, so the cortical reaction helps prevent it.
Tip
Fertilisation wording
Be precise: fertilisation is not just “sperm meets egg”. For full credit, refer to fusion of haploid nuclei and formation of a diploid zygote.
Meiosis: making haploid gametes
Meiosis is a type of nuclear division that produces haploid cells from a diploid cell. It happens in germ-line cells in reproductive organs, such as the testes and ovaries in mammals.
Meiosis involves one round of DNA replication followed by two divisions:
- Meiosis I: separates homologous chromosomes.
- Meiosis II: separates sister chromatids.
The overall pattern is shown below. Notice that each final gamete has one chromosome from each homologous pair.
Meiosis I: the reduction division
Meiosis I is called a reduction division because it reduces the chromosome number from diploid to haploid.
Prophase I: pairing and crossing over
In prophase I, homologous chromosomes pair up to form a bivalent. Each bivalent contains one maternal homologous chromosome and one paternal homologous chromosome, each made of two sister chromatids.
A chiasma is the point where non-sister chromatids overlap and exchange sections of DNA. This exchange is called crossing over.
Definition
Crossing over
Crossing over is the exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I, producing new combinations of alleles on the chromatids.
Metaphase I: independent assortment
In metaphase I, bivalents line up at the equator of the cell. The orientation of each homologous pair is random. This is called independent assortment, because the way one homologous pair lines up does not affect how another pair lines up.
Anaphase I: homologous chromosomes separate
Spindle fibres pull homologous chromosomes to opposite poles of the cell. The centromeres do not split in meiosis I, so sister chromatids stay together.
Telophase I and cytokinesis
The cell divides into two haploid cells. Each cell has one chromosome from each homologous pair, but each chromosome is still made of two sister chromatids.
Meiosis II: the equational division
Meiosis II is similar to mitosis. There is no DNA replication between meiosis I and meiosis II.
In meiosis II, chromosomes line up singly at the equator. The centromeres divide, and sister chromatids are pulled to opposite poles. Cytokinesis then produces four haploid gametes.
Key Idea
The one-line meiosis summary
Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
Common Mistake
Confusing meiosis I with mitosis
In mitosis, homologous chromosomes do not pair up as bivalents and crossing over does not normally occur. In meiosis I, homologous chromosomes pair, cross over, and then separate.
Why meiosis creates genetic variation
Meiosis generates variation in two main ways.
Crossing over
Crossing over produces chromatids with new combinations of alleles. This means a gamete can contain chromosomes that are not identical to either original parental chromosome.
Independent assortment
Independent assortment produces different combinations of maternal and paternal chromosomes in gametes. For a cell with nnn homologous pairs, the number of possible chromosome combinations from independent assortment alone is:
2n2^n2nExample
Counting gamete combinations from independent assortment
-
If a species has three homologous pairs, use 2n2^n2n because each pair has two possible orientations at metaphase I.
-
Substitute n=3n = 3n=3: 23=82^3 = 823=8 possible chromosome combinations in gametes, ignoring crossing over.
-
For humans, n=23n = 23n=23, so independent assortment alone can produce 223=8,388,6082^{23} = 8{,}388{,}608223=8,388,608 possible chromosome combinations per parent.
-
Random fertilisation then combines one sperm with one egg, so the minimum number of possible zygote combinations is 8,388,6082≈7.0×10138{,}388{,}608^2 \approx 7.0 \times 10^{13}8,388,6082≈7.0×1013, before crossing over is even included.
Common Mistake
Variation is not the same as mutation
Meiosis usually reshuffles existing alleles into new combinations. Mutation is what creates new alleles.
Random fertilisation adds even more variation
Because any sperm can fertilise any egg, fertilisation is random. This means the genetic combination in a zygote depends on which particular sperm and egg happen to fuse.
This is why siblings can be genetically different even though they have the same biological parents.
Common Mistake
When separation fails
If chromosomes or chromatids fail to separate properly, this is called non-disjunction. It can produce gametes with too many or too few chromosomes, which may lead to an abnormal chromosome number after fertilisation.
Exam technique
In the exam
-
When describing meiosis, always say what separates: homologous chromosomes in meiosis I, sister chromatids in meiosis II.
-
Link meiosis to both chromosome number and variation: it halves chromosome number and produces genetically different gametes.
-
For fertilisation, include fusion of haploid nuclei, formation of a diploid zygote, and prevention of polyspermy if the question mentions the egg membrane or zona pellucida.
Self review
Check yourself
- Why does DNA replication not double the chromosome number before meiosis begins?
- What is the difference between crossing over and independent assortment?
- How does the cortical reaction help ensure the zygote has the correct chromosome number?
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?