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

A gene mutation is a structural change to the base sequence of a gene. Substitutions swap one base for another. Because the code is degenerate, a substitution may leave the amino acid unchanged, change one amino acid, or create a stop codon. Single-nucleotide polymorphisms (SNPs) are the result of base substitutions. Insertions and deletions not in multiples of three cause a frameshift, so the polypeptide is very likely to stop functioning; major insertions or deletions also usually destroy function.

Mutations arise from errors in DNA replication or repair and are made more frequent by mutagens: chemicals (e.g. benzo[a]pyrene in tobacco smoke, nitrosamines, mustard gas) and radiation (UV, X-rays, gamma rays). Mutation is random — it can happen anywhere in the genome, though some bases mutate more readily, and there is no natural mechanism for deliberately changing a base to change a trait. Mutations in germ cells can be inherited; mutations in somatic cells are not, but can cause cancer. Mutation is the original source of all genetic variation: mostly harmful or neutral to individuals, but essential for evolution by natural selection.

NOS: commercial genetic tests give information about future disease risk, which can be misunderstood without expert interpretation.

Gene knockout is a technique for investigating what a gene does by changing it so it is inoperative, then observing the effect on the organism. For some model species used in research, such as mice, a library of knockout organisms exists, with a line for many individual genes. You do not need to know the details of the techniques.

CRISPR sequences are used to make a guide RNA whose base sequence is complementary to the target DNA. The enzyme Cas9 carries the guide RNA to the matching sequence and cuts both DNA strands. As the cell repairs the cut, bases may be inserted or deleted (inactivating the gene), or a supplied DNA template can be used to insert a new sequence.

Successful example: exa-cel (Casgevy), approved in 2023 for sickle cell disease. A patient's blood stem cells are removed, edited with CRISPR–Cas9 so that they make fetal haemoglobin, and returned. Most treated patients in trials had no severe pain crises afterwards.

Some potential uses of CRISPR raise ethical issues that must be addressed before implementation: editing embryos changes the germ line so edits are inherited by people who cannot consent; off-target cuts may cause harm; and editing could be used for enhancement. Scientists across the world work under different regulatory systems, so there is an international effort to harmonise regulation of genome editing technologies such as CRISPR.

Conserved sequences are identical or similar across a species or group of species; highly conserved sequences stay identical or similar over long periods of evolution (e.g. histone H4). Two hypotheses explain this:

1. Functional requirements — the gene product must have an exact structure, so almost any change is harmful and removed by natural selection.
2. Slower rates of mutation in that part of the genome.

Evidence such as many silent third-base differences in DNA but few amino acid differences supports the functional-requirement hypothesis.