Section 1 of 6Types of Mutation
A mutation is a change in the base sequence of DNA, and the bases are read in groups of three that each code for an amino acid.
Insertion
- An extra base is added to the sequence, which changes the group of three where it lands and every group after it.
Deletion
- A base is lost from the sequence, which also changes the group where it occurs and all of the groups after it.
Substitution
- One base is swapped for another, which can change only one group of three and so at most one amino acid.
| Mutation | Bases after the change | Likely effect on protein |
|---|---|---|
| Insertion | Read in shifted groups | Many amino acids may change |
| Deletion | Read in shifted groups | Many amino acids may change |
| Substitution | Unchanged grouping | At most one amino acid changes |
Section 2 of 6From DNA to Phenotype
A change in DNA can only affect the organism if it alters a protein.
1
A mutation changes the sequence of bases in a gene.
2
The changed gene may code for a different sequence of amino acids in the protein.
3
A different amino acid sequence can make the protein fold into a different shape.
4
If the shape is altered, the protein may no longer work, for example an enzyme whose active site no longer fits its substrate.
5
A change in protein function can alter the phenotype of the organism.
Section 3 of 6Effects of Mutations
Effects of Mutations
Usually no effect
- Most mutations do not change the protein or change it so slightly that its function is not affected.
Harmful effects
- Sickle cell anaemia is caused by a mutation in the gene for haemoglobin.
Beneficial effects
- Occasionally a mutation makes a new allele that gives an advantage and can be passed on to later generations.
Cancer
- A higher rate of mutation can make cells divide out of control and become cancerous.
Section 4 of 6Causes of Mutations
Mutations happen spontaneously and all the time, but some factors raise the rate.
Causes of Mutations
Ionising radiation
- X-rays and gamma rays can damage DNA and change its base sequence.
Ultraviolet light
- UV rays from the Sun can damage the DNA of skin cells and increase the chance of skin cancer.
Chemical mutagens
- Substances such as the tar in tobacco smoke can change DNA and raise the mutation rate.
Spontaneous errors
- Mistakes made when DNA is copied before cell division cause mutations at a low steady rate.
Section 5 of 6Worked Example
Question
- A section of a gene has the base sequence ATG CCA TTG GAC GGT, read in groups of three.
- Describe the effect of (a) deleting the fourth base and (b) substituting the final C of GAC with T.
- The fourth base is the first C in CCA, so removing it gives ATGCATTGGACGGT.
- Regrouping in threes gives ATG CAT TGG ACG GT, so every group after the first has changed.
- In (b) the sequence becomes ATG CCA TTG GAT GGT, and only the fourth group differs from the original.
Answer
- (a) The deletion changes all of the groups from the second onwards, so many amino acids may change.
- (b) The substitution changes only one group, so at most one amino acid is altered.
Section 6 of 6Exam Checklist & Tips
2.2.18 – Key Points for the Exam
- Mutation A change in the base sequence of DNA.
- Insertion A mutation in which an extra base is added to the DNA sequence.
- Deletion A mutation in which a base is removed from the DNA sequence.
- Substitution A mutation in which one base is replaced by a different base.
- Mutagen A factor, such as ionising radiation or a chemical, that increases the rate of mutation.
Common Mistakes
- Saying that all mutations are harmful, when most have no effect on the phenotype at all.
- Claiming that a substitution changes every amino acid after it;
- it affects only the group of three where it occurs.
- Forgetting that bases are read in threes when explaining why insertions and deletions have a larger effect.
- Believing that mutations only occur after exposure to radiation, when they also occur spontaneously.