D1.3 Mutation and gene editingIB Biology SL: Revision notes

Section 1

Gene mutations and their types

A gene mutation is a structural change to a gene at the molecular level: a change in its base sequence. There are three types:

Substitution — one base is replaced by another.
Insertion — one or more extra bases are added.
Deletion — one or more bases are lost.

Key termsgene mutationsubstitutioninsertiondeletion

Section 2

Consequences of base substitutions

A base substitution changes one codon. Because the genetic code is degenerate (most amino acids have more than one codon), the new codon may code for the same amino acid (a silent change), a different amino acid (which may or may not affect the protein's shape), or a stop codon (giving a truncated polypeptide). A single-nucleotide polymorphism (SNP) is a position in the genome where individuals differ by one base; SNPs are the result of base substitution mutations.

Key termsdegeneratesingle-nucleotide polymorphism (SNP)stop codon

Common mistake

Don't say a substitution always changes the protein. Degeneracy means it may change one amino acid or none.

Section 3

Consequences of insertions and deletions

Inserting or deleting a number of bases that is not a multiple of three causes a frameshift: every codon downstream is read differently, often creating an early stop codon. The polypeptide is very likely to cease to function. Insertions or deletions of whole codons keep the reading frame, but major insertions or deletions still usually destroy function because a large part of the amino acid sequence is changed.

Key termsframeshift

Section 4

Causes of gene mutation and randomness

Mutations arise from errors in DNA replication or repair, and their rate is raised by mutagens:

Chemical mutagens — e.g. benzo[a]pyrene and nitrosamines in tobacco smoke, mustard gas, aflatoxins.
Mutagenic radiation — ultraviolet light, X-rays, gamma rays and alpha particles from radioactive sources such as radon.

Mutation is random: it can occur anywhere in the genome, although some bases have a higher probability of mutating than others. No natural mechanism is known for deliberately changing a particular base to change a trait.

Key termsmutagenchemical mutagenrandom mutation

Exam tip

When an environment favours a mutant (e.g. a drug), say the environment selects mutants that already arose; it did not cause them.

Section 5

Germ cells and somatic cells

Mutations in germ cells (cells that develop into gametes) can be inherited by offspring and may cause genetic disease. Mutations in somatic cells (all other body cells) are not passed to offspring but can lead to cancer if they affect genes that control cell division.

Key termsgerm cellsomatic cell

Section 6

Mutation as the source of genetic variation

Gene mutation is the original source of all genetic variation: it creates new alleles. Most mutations are harmful or neutral for an individual, but rare beneficial ones are selected, so in the long term mutation is essential for evolution by natural selection.

Key termsgenetic variationallele

Section 7

NOS: commercial genetic tests

Direct-to-consumer genetic tests can reveal variants linked to future health and disease risk. A raised risk is a probability, not a diagnosis. Without expert interpretation (for example a genetic counsellor) results may be misunderstood, causing anxiety, false reassurance or poor decisions.

Key termsgenetic testrisk

That's the notes covered.

Carry on to the next subtopic.