Gene Mutations and Phenotype
A gene mutation is a change to the sequence of bases in DNA. This can alter the protein produced, which may in turn affect an organism's observable traits (phenotype), but often these changes have no effect at all.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A gene mutation is a change in the nucleotide base sequence of a section of DNA.
- The genetic code is 'degenerate' or 'redundant', meaning different three-base triplets (codons) can code for the same amino acid.
- Because the code is redundant, many mutations are 'silent' and cause no change to the resulting protein, hence having no effect on the phenotype.
- If a mutation does change the amino acid, it can alter the protein's 3D folded shape. This may change its function (e.g., an enzyme's active site).
- The impact on the phenotype varies: most mutations have no effect, some have a small effect (e.g., polydactyly), and very rarely, a mutation has a large effect that determines a key trait or disease.
Formulae
Number of amino acids = (Number of bases in coding sequence) / 3 To calculate the length of a polypeptide chain produced from a known length of a gene's coding DNA sequence. Remember to exclude stop codons if mentioned.
Definitions
- Gene Mutation
- A permanent alteration in the sequence of nucleotide bases within a gene.
- Phenotype
- The set of observable characteristics of an organism, resulting from its genetic code (genotype) and environmental factors.
- Triplet (Codon)
- A sequence of three consecutive DNA bases that codes for a specific amino acid or a stop signal during protein synthesis.
Worked example
A gene that is 450 nucleotides long codes for a structural protein. A substitution mutation occurs at the 90th nucleotide, but analysis shows the resulting protein is identical to the original. Which of the following statements provides the best explanation?
- 1
Step 1:
Identify the core information.
The gene is 450 nucleotides long.
A mutation occurs, but the protein is unchanged.
- 2
Step 2:
Recall the relationship between DNA and proteins.
The DNA sequence is read in triplets, and each triplet codes for an amino acid.
The amino acid sequence determines the protein's structure and function.
- 3
Step 3:
Consider why a change in DNA might not change the protein.
The key concept here is the redundancy (or degeneracy) of the genetic code.
- 4
Step 4:
Formulate the explanation.
The mutation must have changed one triplet into another triplet that codes for the exact same amino acid.
For example, a change from CCT to CCC might still code for Proline.
- 5
Step 5:
Conclude the effect on the phenotype.
Since the protein is structurally and functionally identical to the original, there will be no change in the organism's phenotype.
Answer: The best explanation is that the genetic code is redundant. The mutation changed the DNA triplet to a different triplet that happens to code for the same amino acid, resulting in an identical protein.
Common mistakes
- ×Assuming every base change leads to a different amino acid. You must remember the genetic code is redundant, so multiple codons can specify the same amino acid. This is the most common reason for silent mutations.
- ×Confusing genotype and phenotype. The mutation is a change to the genotype (the DNA). This may or may not cause a change in the phenotype (the observable trait).
- ×Making simple division errors. A gene with 300 bases codes for 300 / 3 = 100 amino acids. Don't forget this simple division step.
No-calculator tips
- ✓To quickly find the number of amino acids from a gene length, simply divide the number of bases by 3. For example, 630 bases → 63 / 3 = 21 → 210 amino acids.
- ✓Focus on the chain of consequence: DNA change → Amino acid change? → Protein shape/function change? → Phenotype change? A 'no' at any step breaks the chain.