Types of Mutations

Mutations

  • A mutation is a change to the genetic material of a cell.

  • Can be the replacement, subtraction of addition of nucleotides or the duplication or rearrangement of a section of a chromosome

  • Can be neutral, harmful, or beneficial.

  • Mutations can occur spontaneously and randomly or can be induced by mutagens.

  • Mutations in gonad cells may be inherited.

  • Mutations in body cells (somatic cells) are not inherited

Types of Mutations

  • Point Mutation: affects only one or very few nucleotides in a gene sequence

  • Chromosome Mutation: a missing, an extra or an irregular portion of chromosomal DNA. It can be from an atypical number of chromosomes or a structural abnormality in one or more chromosomes

Point Mutation

  • Changes the base sequence of a single gene and may form a new allele

  • Can occur spontaneously or can be induced by mutagens

  • During DNA replication, sometimes one base is replaced by another nitrogenous base which in turn changes the mRNA – this means the amino acids and then proteins produced may be affected

Point Mutation Types

  • Silent Mutation: changes in the DNA sequence that do not cause a change in amino acid and has no effect on the protein being produced. E.g. GCC and GCA both code for valine so a change in the 3rd base will have no effect on the amino acid sequence

  • Missense Mutation: a point mutation that results in an amino acid change.

  • Nonsense Mutation: a point mutation that changes a codon for an amino acid into a stop codon. This causes translation to stop, shortening the polypeptide chain that is being synthesised = non- functional protein

Point Mutation - Base Substitution

  • In a base pair substitution, one nucleotide and its complementary partner replace another pair of nucleotides

  • Change can be a silent mutation, a missense mutation or a nonsense mutation

  • May result in a different amino acid being inserted into the polypeptide

  • Sickle cell anaemia is caused by a single base substation – CTC is changed to CAC (and the complementary strand changes from GAG to GTG) – it is a missense mutation as amino acid glutamate is swapped for valine which alters the shape of the haemoglobin molecule

Point Mutation - Frame Shift Mutation

  • Frame shift mutation occurs when extra bases are inserted or deleted from a strand of DNA and shifts the entire ‘reading frame’ of RNA

  • Can lead to the creation of a whole sequence of incorrect amino acids and the production of a non-functional protein

  • The nucleotide sequence is read in multiples of 3 (code for each amino acid) so the deletion or insertion of 1 or 2 nucleotide will change the reading sequence for the rest of the gene sequence

  • The deletion or insertion of 3 bases (or a multiple of 3) will only affect one amino acid