9.1 Mutations

• A mutation is any change to the quantity or the base sequence of the DNA of an organism
• Mutations occurring during the formation of gametes may be inherited, often producing sudden and distinct difference between individuals
• A gene mutation is any change to one or more nucleotide bases or a change in the sequence of the bases in DNA, resulting in the formation of a new allele
• Any changes to one or more bases in the DNA triplets could result in a change in the amino acid sequence of the polypeptide
• Gene mutations can arise spontaneously during DNA replication and include base substitution and base deletion
• Base substitution is a type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
• Types of gene mutations:

  • Base substitution
  • Base deletion
• Example of base substitution:

  1. GTC codes for the amino acid glutamine
  2. If the final base, cytosine, is replaced by guanine, then GTC becomes GTG
  3. GTG is a DNA triplet that codes for the amino acid histidine thus replacing the original amino acid glutamine
  4. The polypeptide produced will differ in a single amino acid
• The significance of this base substitution depends on the role of the original amino acid:

  • If it's important in forming bonds that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds
  • The protein may then be a different shape and therefore not function properly
• Example of the significance of base substitution:

  For example, if the protein is an enzyme, its active site may no longer be complementary to fit the substrate and it will no longer catalyse the reaction
• The degenerate nature of the genetic code means most amino acids have more than one codon:

  • Therefore, the effect of the base substitution mutation is different if the new triplet of bases still codes for the same amino acid as before
• Example of degenerate in base substitution:

  1. If the cytosine in GTC is replaced by thymine to become GTT
  2. There is no change in the polypeptide produced and so the mutation will have no effect

  • This is because both DNA triplets code for glutamine
• Base deletion is a gene mutation which arises when a nucleotide is lost from the normal DNA sequence
• In base deletion, the loss of a single nucleotide from thousands in a typical gene is a minor change however still has consequences
• Consequences of base deletion:

  • The amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function correctly
  • This is because the sequence of bases in DNA is read in units of 3 bases (triplet)
  • One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base - this is called frame shift
• Chromosome mutations are changes in the structure or number of whole chromosomes
• Chromosome mutations can arise spontaneously and take two forms:

  • Changes in whole set of chromosomes
  • Changes in the number of individual chromosomes
• • Changes in whole sets of chromosomes occur when organisms have 3 or more sets of chromosomes rather than the usual two
  • This is called polyploidy and occurs mostly in plants
• • Changes in the number of individual chromosomes occurs when individual homologous pairs of chromosomes fail to separate during meiosis
  • This is called non-disjunction
  • This results in a gamete having one more or fewer chromosome
  • When fertilised with a normal gamete, the resultant offspring has more or fewer chromosomes in all body cells
  • An example is down's syndrome