Revise Mutation and gene editing for IB Biology (D1.3)

A gene mutation is a change in the base (nucleotide) sequence of a gene. It may be as small as a single base or involve several bases. Mutations arise in two broad ways: spontaneously, as rare mistakes made by DNA polymerase during replication that escape proofreading, and by induction, when a mutagen in the environment increases the error rate. Either way the change is permanent once it is copied into a new DNA strand, and if it occurs in a cell line that gives rise to daughter cells it is passed on to them. The key point for everything that follows is that the effect of a mutation is decided at the level of the codon: how the change alters the triplet code determines whether the protein changes at all, and if so, how.

Gene mutations are classified by what happens to the bases. In a substitution, one base is replaced by a different base — this changes a single codon and never shifts the reading frame. In an insertion, one or more bases are added; in a deletion, one or more bases are removed. Insertions and deletions matter far more than their size suggests, because the ribosome reads mRNA in fixed groups of three from a set starting point. Add or remove a number of bases that is not a multiple of three and every codon after the change is re-grouped — a frameshift.

A substitution has one of three outcomes, and which one occurs depends entirely on the new codon. A silent (neutral) mutation produces a codon that still codes for the same amino acid — possible because the genetic code is degenerate, so several codons specify the same amino acid — and the protein is unchanged. A missense mutation produces a codon for a different amino acid, changing one residue in the polypeptide; depending on where it sits and how different the new amino acid is, this may barely matter or may disrupt the protein's folding and function. A nonsense mutation produces a stop codon, ending translation early and giving a shortened, usually non-functional protein. Insertions and deletions that cause a frameshift usually devastate the protein: every amino acid downstream is wrong and a premature stop codon commonly appears, so the polypeptide is almost always non-functional.

Most mutations are spontaneous — rare copying errors during replication. Their rate is raised by mutagens: ionising radiation (such as X-rays and gamma rays) and ultraviolet light, which damage DNA directly, and chemical mutagens such as those in tobacco smoke and certain industrial compounds, which react with bases and cause mispairing. It is essential to be precise about what mutagens do. A mutagen increases the probability that a mutation occurs; it does not decide which gene mutates or steer the change towards anything useful. In this sense mutation is random and undirected with respect to the organism's needs: an environment that would benefit from a particular allele does not make that allele more likely to arise. This is the single most important conceptual point in the topic, and the one examiners most often test.

Whether a mutation can be inherited depends on which cells it occurs in. A somatic mutation arises in a body cell; it is copied to that cell's descendants within the individual — and can, for example, contribute to cancer — but it cannot be passed to offspring. A germ-line mutation arises in a gamete or in the cells that produce gametes; it can be inherited, and if it is, it will be present in every cell of the offspring. Only germ-line mutations are heritable, and only they contribute to the variation on which evolution acts. This distinction also underpins the ethics of gene editing: changing somatic cells affects one person, whereas changing germ-line cells changes all future generations.

Sexual reproduction — meiosis and fertilisation — reshuffles existing alleles into new combinations, but it cannot create an allele that did not already exist. Mutation is the ultimate source of new alleles, and therefore of all genetic variation. Most mutations are neutral or harmful, but occasionally one is beneficial in a particular environment. Natural selection then raises the frequency of that beneficial allele over generations. So mutation and selection play distinct roles: mutation is the undirected source of new variation, and selection is the directional filter. Without mutation there would be no new variation for selection to act on, and evolution would have no raw material.

CRISPR-Cas9 lets scientists make a change at a chosen point in the genome — in effect, a deliberate mutation. It was discovered as a bacterial defence system against viruses and adapted for use in other cells. It has two working parts: a guide RNA (gRNA), a short RNA engineered to be complementary to the target DNA sequence, and Cas9, a nuclease enzyme that cuts DNA. The gRNA base-pairs with its complementary sequence in the genome, which brings Cas9 to precisely that locus; Cas9 then makes a double-strand break. The cell's own repair machinery seals the break — and if scientists also supply a corrected DNA template, the cell can copy it during repair, changing the sequence to the desired version.

Gene editing has broad potential: correcting the single-gene mutations behind disorders such as sickle-cell anaemia and cystic fibrosis, engineering crops for higher yield or disease resistance, and creating precise research models. But the same power raises hard questions. The central ethical line is between somatic editing (affecting only the treated individual, and broadly comparable to other therapies) and germ-line editing (heritable, affecting all future generations). Further concerns include off-target edits, unequal access, consent for people not yet born, and the difference between treating disease and non-therapeutic 'enhancement'. These are why heritable germ-line editing in humans is widely restricted even where somatic gene-editing research is permitted.

This is an 'explain' question, so the marks are awarded analytically: each distinct valid biological point is worth one mark, up to the maximum. Method-style points (M) credit correct reasoning about the mechanism, answer points (A) credit a correct stated outcome, and error-carried-forward (ECF) means that if you set up one step wrongly but reason correctly from it, the later points can still be earned. Study how each mark below is tied to a specific named idea, not to loose phrasing.

Mutation ties this topic to the rest of the course. It is the source of the new alleles that population genetics and natural selection then act on; the substitution/missense/nonsense logic depends directly on transcription, translation and the degenerate genetic code; and CRISPR-Cas9 is applied molecular biology built on complementary base-pairing and enzyme specificity. Master the codon-level reasoning here — trace a base change to its codon, then to the amino acid, then to the protein — and both the exam calculations and the bigger evolutionary picture fall into place.