D1.3 Mutation and gene editing
Mutation and gene editing explain how DNA sequence changes arise, affect proteins, create variation and can be studied or altered deliberately.
- Syllabus
- First assessment 2025
- Topic
- D1.3
- Level
- SL
Learning objectives
D1.3.1Gene mutations• Gene mutations are changes in the base sequence of DNA• Main types are substitution, insertion, deletion, and duplicationD1.3.2Base substitution consequences• Base substitutions can create SNPs and change codons• Degeneracy can make substitutions silent, missense, or nonsenseD1.3.3Insertion and deletion consequences• Insertions or deletions not in multiples of three cause frameshifts• Frameshifts alter downstream codons and often disrupt protein functionD1.3.4Causes of mutation• Mutations can arise from replication errors, repair errors, or chromosome damage• Mutagens include chemicals, ionizing radiation, and ultraviolet radiationD1.3.5Randomness in mutation• Mutations occur randomly with respect to organism need or advantage• Mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposureD1.3.6Consequences in germ vs. somatic cells• Germ-line mutations can be inherited by offspring• Somatic mutations affect only descendant body cells and can contribute to cancerD1.3.7Mutation as source of variation• Mutation is the original source of new alleles and genetic variation• Many mutations are neutral or harmful, but variation supplies material for natural selection
A Mutation Changes DNA Sequence, Not a Whole Organism at Once
A gene mutation is a change in the order or number of DNA bases within a gene. It creates a new sequence; its later effect depends on where the change occurs.
- Substitution: one nucleotide is replaced by another.
- Insertion: one or more nucleotides are added.
- Deletion: one or more nucleotides are lost.
- Duplication: a DNA segment is copied and repeated.
Mutation names describe the DNA change. They do not by themselves tell you whether a protein, cell or organism will be affected.
One Base Substitution Can Be Silent, Missense or Nonsense
A base substitution replaces one nucleotide without changing sequence length. In a population, a one-base variant present among individuals is a single-nucleotide polymorphism (SNP).
| Coding outcome | Why it happens | Possible protein effect |
|---|---|---|
| silent | altered codon still specifies the same amino acid because the code is degenerate | primary structure unchanged |
| missense | altered codon specifies a different amino acid | folding or function may change |
| nonsense | altered codon becomes a stop codon | translation ends early |
A substitution outside a coding sequence can still matter if it changes a promoter or other regulatory sequence; some substitutions have no detectable effect.
An Indel Shifts the Reading Frame Unless Its Size Is a Multiple of Three
A ribosome reads nucleotides in non-overlapping groups of three. Inserting or deleting a number of bases not divisible by three changes the grouping from that point onward: a frameshift.
- A one- or two-base indel usually changes many downstream codons and may create an early stop.
- A three-base indel adds or removes one codon without shifting later codon boundaries.
- A larger in-frame indel can still disrupt function by adding or removing amino acids.
Do not call every insertion or deletion a frameshift. Test whether the number of bases changed is a multiple of three.
Mutation Is Random Relative to Need, but Its Probability Is Not Uniform
A mutation does not arise because an organism needs a useful trait. A change can occur before an environmental challenge and later prove harmful, neutral or advantageous.
Mutation probability can still vary with:
- base identity and local sequence context
- how often a region is copied or transcribed
- access to proofreading and repair systems
- dose and type of mutagen exposure
Random relative to need is not the same as equally likely at every base. Natural selection is the non-random filter; it does not direct the earlier mutation.
Cell Lineage Determines How Far a Mutation Can Travel
| Mutation location | Cells that can inherit it | Biological reach |
|---|---|---|
| germ-line lineage | gametes and potentially the offspring formed from them | can pass between generations |
| somatic cell | mitotic descendants of that body cell | remains within the individual and its cell clone |
A somatic mutation can contribute to cancer if it affects control of cell division, DNA repair or cell death. Further mutations and selection among cell clones can then expand the abnormal lineage.
A somatic mutation is not inherited by the person's offspring merely because many body cells carry it; it must enter the germ line to cross generations.
