D1.3.3—Insertion and deletion consequences
Insertions and deletions can shift the reading frame when bases are added or removed outside complete codon triplets during gene mutation.
- Syllabus
- First assessment 2025
- Objective
- D1.3.3
- Level
- HL
Insertions and Deletions Can Shift the Reading Frame
Insertions and deletions are likely to stop a polypeptide functioning when they shift its reading frame or change a large section of its sequence.
Ribosomes read mRNA in triplets. Adding or removing a number of bases that is not a multiple of three regroups every downstream codon, often changing many amino acids and creating an early stop codon.
Not a multiple of three → frameshift and changed downstream codons. Multiple of three → no frameshift, but amino acids are added or removed. A major insertion or deletion can still disrupt structure and function even without a frameshift.
Deleting one base near the start of a coding sequence shifts the triplet grouping for most of the remaining mRNA and is therefore likely to produce a non-functional polypeptide.
A three-base insertion or deletion avoids a frameshift, but it is not automatically harmless because the added or missing amino acid may be important.
Core Mutation Effects
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.
Concept essentials
- Insertions add bases to a DNA sequence.
- Deletions remove bases from a DNA sequence.
- Frameshifts occur when inserted or deleted bases are not in multiples of three.
- A frameshift changes downstream codons and can strongly disrupt protein function.