Gene & Chromosome Mutations Revision Notes | A Level AQA Biology

1. Why Mutations Matter

Every time a cell copies its DNA, roughly three billion base pairs must be read and reproduced with near-perfect accuracy. When that process goes wrong — even by a single base — the consequences can range from completely silent to life-altering. That is the world of mutations.

Mutations are the ultimate source of genetic diversity. Without them, evolution could not occur. Yet the same changes that drive adaptation can also disrupt protein function and cause disease. Understanding exactly how mutations arise, and why some matter more than others, is central to this topic and appears regularly in AQA exam questions.

2. Gene Mutations: Changes at the Base Level

A gene mutation is a change in the base sequence of DNA. These changes arise spontaneously during DNA replication — errors in base pairing that escape the proofreading mechanisms of DNA polymerase. There are two main types you need to know for AQA: base substitution and base deletion.

Picture the DNA sequence as a sentence written in three-letter words (codons). A base substitution swaps one letter in one word. The words before and after remain unchanged. This is why base substitution has a localised effect — it alters at most one codon, and therefore potentially one amino acid in the resulting polypeptide.

To see why deletion is so disruptive, consider this analogy. Imagine the sentence THE CAT ATE THE RAT. Remove the first letter and you get HEC ATA TET HER AT — every word after the deletion becomes nonsense. The same logic applies to codons: a frameshift scrambles every amino acid from the deletion point to the stop codon, almost always producing a non-functional protein.

3. The Degenerate Genetic Code and Silent Mutations

Here is where it gets genuinely interesting. There are 64 possible codons but only 20 amino acids. That means most amino acids are coded for by more than one codon — the genetic code is said to be degenerate. This redundancy has a crucial consequence for base substitutions.

If a base substitution changes a codon but the new codon still codes for the same amino acid, the primary structure of the protein is unchanged. The mutation is described as silent (or synonymous). For example, both UUA and UUG code for leucine, so a substitution that converts one to the other has no effect on the polypeptide produced.

By contrast, if the substitution produces a codon for a different amino acid, the primary structure changes. Whether this matters depends on where in the protein the change occurs. A substitution at the active site of an enzyme, for instance, could abolish catalytic activity entirely. A substitution in a less critical region might have little or no effect on the protein's tertiary structure or function.

4. Mutagenic Agents

Gene mutations arise spontaneously at a low background rate during DNA replication. However, certain environmental factors — called mutagenic agents (or mutagens) — increase the rate at which mutations occur. The AQA specification requires you to know that mutagenic agents exist and that they elevate mutation rate; you do not need to memorise the precise molecular mechanism of each one.

• High-energy ionising radiation (e.g. X-rays, gamma rays) — can break phosphodiester bonds or cause abnormal base pairing
• Non-ionising radiation (e.g. ultraviolet (UV) light) — can cause mutations by promoting the formation of pyrimidine dimers, which leads to abnormal base pairing and errors during replication
• Chemical mutagens (e.g. certain compounds in tobacco smoke, some industrial chemicals) — can chemically modify bases or intercalate between base pairs, causing errors during replication
• Some viruses — can insert their genetic material into host DNA, disrupting gene sequences

The key point for exam answers is the causal link: mutagenic agents increase the rate of gene mutation by increasing the frequency of errors in base pairing during DNA replication, or by directly damaging the base sequence. Higher mutation rate means greater genetic diversity, but also a greater risk of harmful mutations such as those that can lead to uncontrolled cell division.

Gene mutations, then, are changes at the level of individual bases within a gene. Now we need to zoom out to the whole chromosome and consider a completely different category of mutation — one that changes the number of chromosomes in a cell rather than the sequence of bases within a gene.

5. Chromosome Mutations: Non-disjunction During Meiosis

During meiosis, homologous chromosome pairs must separate precisely so that each gamete receives exactly one chromosome from each homologous pair. When this separation fails, the result is a gamete with either one too many or one too few chromosomes. This failure is called non-disjunction.

Non-disjunction can occur at meiosis I, when homologous chromosomes fail to separate, or at meiosis II, when sister chromatids fail to separate. Either way, the outcome is gametes with an abnormal chromosome number — a condition called aneuploidy.

Worked example: Tracing Non-disjunction Through Meiosis

Explain, step by step, how non-disjunction at meiosis I produces gametes with an abnormal chromosome number, and what happens when such a gamete is fertilised by a normal gamete.

1. A diploid cell (2n) enters meiosis with homologous chromosome pairs lined up at the metaphase I plate. For a human cell, 2n = 46, so there are 23 homologous pairs. — Each pair consists of one chromosome inherited from each parent. Correct separation at anaphase I would send one chromosome from each pair to each pole.
2. One homologous pair fails to separate at anaphase I. Both chromosomes of that pair move to the same pole instead of one going to each pole. — This is the critical error. The spindle fibres either fail to attach correctly or fail to pull the chromosomes apart. The result is that one secondary oocyte/cell receives both chromosomes of the pair, and the other receives neither.
3. Cell at one pole: n + 1 chromosomes (one extra). Cell at other pole: n - 1 chromosomes (one missing). — For a human cell, one cell has 24 chromosomes and the other has 22, rather than the normal 23 each.
4. Each secondary cell divides normally in meiosis II, separating sister chromatids. The n+1 cell produces two gametes each with n+1 chromosomes. The n-1 cell produces two gametes each with n-1 chromosomes. — Meiosis II itself is not faulty here — it is working with the wrong starting number of chromosomes.
5. A normal gamete (n chromosomes) fuses with the n+1 gamete: zygote has 2n+1 chromosomes (trisomy). A normal gamete fuses with the n-1 gamete: zygote has 2n-1 chromosomes (monosomy). — In humans, trisomy 21 (three copies of chromosome 21) results from this process and leads to Down syndrome. The individual has 47 chromosomes rather than the normal 46.

Answer: Non-disjunction at meiosis I produces two types of abnormal gamete: one with n+1 chromosomes and one with n-1 chromosomes. Fertilisation of an n+1 gamete by a normal gamete produces a trisomic zygote (2n+1); fertilisation of an n-1 gamete produces a monosomic zygote (2n-1).

The most studied example in humans is trisomy 21, where three copies of chromosome 21 are present rather than two. This arises because non-disjunction of chromosome 21 during meiosis produces a gamete carrying two copies of chromosome 21. When fertilised by a normal gamete carrying one copy, the resulting zygote has three copies in total.

6. Comparing Gene and Chromosome Mutations

Now that both types of mutation are clear, it is worth placing them side by side. This comparison is exactly the kind of task AQA 'compare' questions demand, and having a precise mental framework saves time in the exam.

7. Exam Technique and Mark-Winning Strategies

Mutations questions on AQA papers follow predictable patterns. The command words 'explain', 'describe', and 'compare' each demand a different response structure, and knowing this distinction is what separates a 4-mark answer from a 6-mark answer.

When asked to explain why a base substitution may not alter the amino acid sequence, you must reference the degenerate nature of the genetic code explicitly — stating that multiple codons can code for the same amino acid. Saying only 'the mutation has no effect' earns no marks without the mechanistic reason.

When asked to describe non-disjunction, sequence matters. Start with the failure of separation, state which stage of meiosis is affected, describe the resulting gametes, and then describe the zygote formed after fertilisation. Examiners follow a mark point list, so covering each stage in order maximises your marks.