Mutations

This lesson covers mutations — changes in the DNA base sequence — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the different types of mutation, how mutations affect proteins, and whether mutations are beneficial, harmful or neutral.

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What Is a Mutation?

A mutation is a change in the base sequence of DNA. Since the base sequence determines the order of amino acids in a protein, a mutation can change the protein that is produced.

Mutations can occur:

• Spontaneously — random errors during DNA replication (e.g. during cell division)
• Due to mutagens — environmental factors that increase the rate of mutation

Common Mutagens

Mutagen	Example
Radiation	UV light, X-rays, gamma rays
Chemicals	Tar in cigarette smoke, certain dyes
Biological	Some viruses can insert DNA into host cells

Exam Tip: Mutations are random — they are not caused by an organism "needing" to change. This is a common misconception that examiners will test.

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Types of Gene Mutation

Gene mutations affect individual genes (as opposed to chromosome mutations, which affect whole chromosomes). The main types are:

1. Substitution

A substitution mutation occurs when one base is replaced by a different base.

Example:

Original DNA	T A C G G A T T C
Mutated DNA	T A C G C A T T C

• The G at position 5 has been replaced by C.
• This changes one codon, which may code for a different amino acid.
• The rest of the sequence is unaffected.

2. Insertion

An insertion mutation occurs when an extra base is added into the DNA sequence.

Original	T A C G G A T T C
Mutated	T A C A G G A T T C

• An extra A has been inserted after position 3.
• This shifts all subsequent bases along — called a frameshift.
• Every codon after the insertion is changed, potentially producing a completely different protein.

3. Deletion

A deletion mutation occurs when a base is removed from the DNA sequence.

Original	T A C G G A T T C
Mutated	T A C G A T T C

• The G at position 4 has been deleted.
• This also causes a frameshift, altering every codon after the deletion.

Exam Tip: Insertion and deletion mutations are generally more damaging than substitution mutations because they cause a frameshift — every codon downstream is altered.

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Effects of Mutations on Proteins

The effect of a mutation depends on how it changes the protein:

Effect	Explanation	Example
No effect (silent)	The mutated codon still codes for the same amino acid (due to redundancy in the genetic code)	A substitution in the third base of a codon may not change the amino acid
Minor effect	A different amino acid is inserted but the protein still functions	A change in a non-critical region of the protein
Serious effect	A critical amino acid is changed, altering the protein's shape and function	Sickle cell disease — one amino acid change in haemoglobin
No protein produced	A stop codon is introduced prematurely, cutting the protein short	The protein is truncated and non-functional

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Beneficial, Harmful and Neutral Mutations

Most mutations are neutral — they have no significant effect on the organism. However, some mutations can be:

Beneficial Mutations

• Produce a protein that gives the organism a survival advantage.
• Example: A mutation in bacteria that provides antibiotic resistance — the bacterium survives while others are killed.
• Over time, beneficial mutations can spread through a population by natural selection.

Harmful Mutations

• Produce a faulty or non-functional protein.
• Example: Cystic fibrosis is caused by a mutation in the gene coding for a chloride channel protein, leading to thick, sticky mucus in the lungs and digestive system.
• Harmful mutations may cause genetic disorders.

Neutral Mutations

• Do not affect the organism's survival or reproduction.
• Example: A substitution that codes for the same amino acid (silent mutation).
• Mutations in non-coding regions of DNA (sections that do not code for proteins) are often neutral.

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Mutations and Variation

Mutations are the ultimate source of genetic variation. Without mutations, there would be no new alleles and no evolution. Key points:

• Mutations create new alleles (different forms of a gene).
• New alleles can lead to new phenotypes (observable characteristics).
• If a new phenotype increases survival, it may be selected for by natural selection.
• Over many generations, accumulated mutations drive evolution.

graph TD
    A["Mutation occurs in DNA"] --> B["New allele is created"]
    B --> C["New protein may be produced"]
    C --> D{"Effect on organism?"}
    D -->|"Beneficial"| E["Increased survival — selected for"]
    D -->|"Harmful"| F["Decreased survival — selected against"]
    D -->|"Neutral"| G["No effect on survival"]

Exam Tip: Never say a mutation "causes evolution" on its own. Mutations provide the variation; natural selection acts on that variation over time.

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Summary

• A mutation is a change in the base sequence of DNA.
• Mutations can be caused by random errors during DNA replication or by mutagens (UV light, chemicals, radiation).
• The three main types are substitution, insertion and deletion.
• Insertion and deletion cause a frameshift, which is usually more damaging.
• Mutations can be beneficial (e.g. antibiotic resistance), harmful (e.g. cystic fibrosis) or neutral (silent mutations).
• Mutations are the source of genetic variation and drive evolution when acted on by natural selection.

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Worked Example: Frameshift vs Substitution

Imagine a DNA template strand reads:

T A C G G T A C C A A T

Transcribed, the mRNA is:

A U G C C A U G G U U A