Mutations (College Board AP® Biology): Study Guide

Types of mutation

• A mutation is a change in the DNA sequence

• Alterations in DNA can change the type or amount of protein produced, which can in turn alter the phenotype of a cell or organism

• Mutations can be:

  • beneficial; these result in useful new traits

  • detrimental; mutations that disrupt gene function, leading to disease or loss of function

  • neutral; mutations that have no effect on phenotype

Point mutations

• A point mutation is a change to a single nucleotide in DNA—usually a one-base substitution, but sometimes a one-base insertion or deletion

  • Substitution mutations involve the switching of one nucleotide for another

  • One-base insertion mutations involve addition of a single nucleotide

  • One-base deletion mutations involve deletion of a single nucleotide

• Because of the degenerate nature of the genetic code, it is possible for substitution mutations to be silent, meaning that there is no change to the amino acid sequence

• Insertion or deletion of nucleotides, however, shifts the reading frame, meaning that they alter every codon downstream from the mutation; this is known as a frameshift mutation

  • Frameshift mutations are more likely to lead to major changes in amino acid sequence

• Frameshift mutations are more likely to lead to:

  • a missense mutation: an amino acid is changed and the resulting protein is altered

  • a nonsense mutation: the substitution creates a premature stop codon leading to a shortened, non-functional protein

Example: cystic fibrosis and the CFTR gene

• Cystic fibrosis is a genetic disorder of cell membranes resulting from a mutation in the CFTR gene

  • There are multiple mutations that can result in cystic fibrosis, some of which are point mutations

• The mutation results in problems with the CFTR protein

  • It results in disruptions to ion transport in cells, particular those in the respiratory tract and reproductive organs

  • The body produces large amounts of thick, sticky mucus in these areas

  • Over time, this may damage the lungs and stop them from working properly

• Cystic fibrosis is caused by a recessive allele

  • This means individuals who are heterozygous won’t be affected by the disorder, but are carriers

  • Individuals who are homozygous recessive will have the disorder

Example: adaptive melanism in pocket mice and the MC1R gene

• Adaptive melanism in rock pocket mice (Chaetodipus intermedius) is the evolution of dark fur that camouflages them in volcanic rock environments, increasing their chances of survival

  • dark-colored fur is said to be melanic

• The change in phenotype is caused by mutations in the MC1R (melanocortin 1 receptor) gene

  • The MC1R gene encodes a protein that regulates melanin production in hair and skin cells

  • Mouse individuals with MC1R mutations produce more pigment proteins, leading to darker fur and therefore better camouflage

  • Over generations, natural selection favors mice with dark fur in these darker environments