4.3.3Additional lessons
Mutations and Genetic Variation — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 39 of 101
Section 1 of 6Types of Gene Mutation
A gene mutation is a change in the base sequence of a gene, and it creates a new allele.
Substitution
- One base is swapped for another, so at most one codon changes and only one amino acid in the polypeptide can be affected.
Insertion
- Extra bases are added to the sequence;
- if the number is not a multiple of three, every codon downstream is read differently.
Deletion
- Bases are lost from the sequence;
- again a loss that is not a multiple of three shifts the reading frame of the whole remaining gene.
Frameshift
- A change in the reading frame caused by inserting or deleting one or two bases, usually producing a non-functional protein.
SNP
- A single nucleotide polymorphism is a substitution that is common in a population, and the human genome holds millions of them.
| Mutation | Effect on codons | Typical severity |
|---|---|---|
| Substitution | One codon altered | Often mild or silent |
| Insertion/deletion of 1 or 2 bases | All later codons altered | Usually severe |
| Insertion/deletion of 3 bases | Amino acids added or lost, frame kept | Variable |
Section 2 of 6Causes and Randomness
Mutations arise spontaneously at a low rate, but mutagens raise that rate.
Radiation
- Ionising radiation and UV light damage DNA chemically, for example by fusing neighbouring thymine bases into dimers.
Chemical mutagens
- Substances such as benzo[a]pyrene and nitrosamines in tobacco smoke, or mustard gas, alter or damage bases in DNA.
Replication errors
- DNA polymerase occasionally inserts a wrong base, and errors that escape proofreading become permanent mutations.
Randomness
- Mutation can strike any gene in any cell at any time, and the organism's needs do not make a helpful mutation more likely.
Section 3 of 6Consequences for the Protein
The effect depends on where in the gene the change falls and what it does to the protein shape.
Silent
- Because the genetic code is degenerate, a new codon may still code for the same amino acid, so the protein is unchanged.
Missense
- The new codon codes for a different amino acid;
- in sickle cell disease GAG becomes GTG in the gene, so glutamic acid is replaced by valine.
Nonsense
- The new codon is a stop codon, so translation ends early and a shortened polypeptide is made.
Germ line versus somatic
- Only mutations in cells that form gametes can be inherited,
- whereas somatic mutations affect that body and may contribute to cancer.
Section 4 of 6Mutation and Variation
Original source
- Every new allele first arose by mutation, so mutation supplies the raw material on which natural selection acts.
Meiosis
- Crossing over and independent assortment shuffle existing alleles into new combinations in the gametes.
Random fertilisation
- Which sperm fuses with which egg is chance, so each zygote carries a unique combination of alleles.
Genetic testing
- Consumer genetic tests report variants linked to disease risk, but results need expert interpretation to avoid needless anxiety.
Section 5 of 6Worked Example
Question
- The mRNA AUG GCU UAC GGA codes for Met-Ala-Tyr-Gly.
- One adenine is inserted after the first codon.
- Give the new amino acid sequence and explain the effect.
- Original codonsAUG GCU UAC GGA.
- After inserting A following AUG the sequence reads AUG AGC UUA CGG A.
- Read in tripletsAUG = Met, AGC = Ser, UUA = Leu, CGG = Arg.
- The final single base A is an incomplete codon.
Answer New sequence Met-Ser-Leu-Arg: every amino acid after the first has changed because the reading frame shifted, which will almost certainly give a non-functional protein.
Section 6 of 6Exam Checklist & Tips
4.3.3 – Key Points for the Exam
- Gene mutation A change in the base sequence of a gene that produces a new allele.
- Frameshift An insertion or deletion not divisible by three that alters every codon after it.
- Mutagen An agent, such as radiation or a chemical, that increases the rate of mutation.
- Germ line The cells that give rise to gametes, so mutations in them can pass to offspring.
- SNP A single base position at which the DNA sequence differs between individuals in a population.
Common Mistakes
- Claiming that mutations occur because an organism needs them; they are random with respect to need.
- Thinking all mutations are harmful, when many are silent or neutral and a few are beneficial.
- Forgetting that somatic mutations are not passed to offspring.
- Calling a three-base deletion a frameshift; the reading frame is kept.
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