IB Biology HL D1.3 Mutation and gene editing Notes

A gene mutation is a change in the base sequence of DNA. Start by classifying what changed: a substitution replaces one base, an insertion adds base(s), a deletion removes base(s), and a duplication repeats a section.

Classify mutation by what happens to bases.

A base substitution replaces one base and can create a single nucleotide polymorphism, or SNP. Its effect depends on the codon: because the genetic code is degenerate, a substitution may be silent, missense, or nonsense.

Substitution effect depends on the genetic code.

Insertions and deletions are most disruptive when they are not in multiples of three. Because codons are read in triplets, adding or removing one or two bases shifts the reading frame, changing downstream codons and often disrupting protein function.

A frameshift changes how all later triplets are read.

Mutations can arise without an external mutagen, for example from replication errors, repair errors, or chromosome damage. Mutagens increase mutation risk and include chemicals, ionizing radiation, and ultraviolet radiation.

Mutation causes include internal errors and external mutagens.

Mutation is random with respect to what the organism needs. A mutation does not appear because it would be useful; instead, mutation creates variation and selection may later increase useful variants. Mutation rate can still vary with DNA sequence, gene expression, repair, and mutagen exposure.

Mutation appears first; selection filters later.

The consequence of a mutation depends on the cell lineage. Germ-line mutations occur in cells that lead to gametes and can be inherited by offspring; somatic mutations occur in body cells, affect only descendant body cells, and can contribute to cancer.

Cell lineage determines inheritance.

Mutation is the original source of new alleles and genetic variation. Many mutations are neutral or harmful, but without mutation there would be no new DNA variants for natural selection to act on.

Mutation supplies variation before selection.

Gene knockout is a functional test: make a specific gene non-functional, then observe the phenotype. Model organisms such as mice, Drosophila, zebrafish, and Arabidopsis are useful because knockout libraries let scientists compare normal and knockout phenotypes systematically.

Knockout tests gene function by loss of function.

CRISPR-Cas9 gene editing works by targeting and cutting. A guide RNA base-pairs with a complementary DNA target sequence and directs Cas9 to that site; after Cas9 cuts, DNA repair can lead to deletion, replacement, insertion, or gene disruption.

Target first, cut second, edit through repair.

Conserved sequences remain similar across species or over long evolutionary times. If a sequence changes very little, that suggests essential function, lower mutation rate, or strong purifying selection removing harmful changes.

Conserved positions point to functional importance or selection.

Gene mutations are changes in the base sequence of DNA; main types are substitution, insertion, deletion, and duplication. Base substitutions can create SNPs and change codons; degeneracy can make substitutions silent, missense, or nonsense. Insertions or deletions not in multiples of three cause frameshifts that alter downstream codons and often disrupt protein function. Mutations can arise from replication errors, repair errors, or chromosome damage; mutagens include chemicals, ionizing radiation, and ultraviolet radiation. Mutations occur randomly with respect to organism need or advantage; mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposure. Germ-line mutations can be inherited by offspring; somatic mutations affect only descendant body cells and can contribute to cancer. Mutation is the original source of new alleles and genetic variation; many are neutral or harmful, but variation supplies material for natural selection.

Use this for SL/core questions about mutation types, substitution consequences, frameshifts, mutation causes, randomness, germ-line versus somatic effects, cancer risk, and variation for natural selection.

Gene knockout makes a specific gene non-functional to investigate phenotype; model organisms such as mice, Drosophila, zebrafish, and Arabidopsis support KO libraries. Guide RNA directs Cas9 to a complementary DNA target sequence; Cas9 cutting enables deletion, replacement, insertion, or gene disruption. Conserved sequences remain similar across species or long evolutionary times; conservation suggests essential function, lower mutation rate, or strong purifying selection.

Use this for HL questions about knockout evidence, model organisms and KO libraries, CRISPR-Cas9 gene editing mechanism, and conserved-sequence interpretation.