genetic mutations

• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
• mutations occur continuously and spontaneously

  • Errors in the DNA often occur during DNA replication
• • Mutations in the DNA base sequence can occur due to the insertion, deletion or substitution of a nucleotide or due to the inversion, duplication or translocation of a section of a gene
• insertion of nucleotides

  • occurs when new base randomly inserted into the DNA sequence
  • changes original amino acid coded for by original base triplet creates new triplet bases
  • frameshift mutation
• deletion of nucleotides

  • occurs when a base randomly deleted from DNA sequence
  • changes amino acid that would have been coded for
  • frameshift mutation
• substitution of nucleotides

  • when a random base in DNA sequence is swapped for a different base
  • will only change the amino acid coded for if the triplet does not code for the same original amino acid
  • three forms:
  • silent mutations: does not alter amino acid sequence (since some codons code for same amino acids- degenerate)
  • missense mutations: alters a single amino acid
  • nonsense mutations: creates a premature stop codon so causes polypeptide chain to be incomplete and affects final protein structure and function
• inversion within a gene section

  • usually occurs when crossing-over in meiosis
  • DNA of single gene cut in two places
  • The cut portion is inverted 180° then rejoined to the same place within the gene
  • The result is a large section of the gene is 'backwards' and therefore multiple amino acids are affected
  • Inversion mutations frequently result in a non-functional protein
  • The mutation is often harmful because the original gene can no longer be expressed from that chromosome
  • If the other chromosome in the pair carries a working gene the effect of the mutation may be lessened
• Duplication of a gene

  • A whole gene or section of a gene is duplicated so that two copies of the gene/section appear on the same chromosome
  • The original version of the gene remains intact and therefore the mutation is not harmful
  • Overtime, the second copy can undergo mutations which enable it to develop new functions
  • Duplication mutations are an important source of evolutionary change
  • Alpha, beta and gamma haemoglobin genes evolved due to duplication mutations
• Translocation of a gene section

  1. Section of gene cut in two places

  2. The section of the gene that is cut off attaches to a separate gene

  3. The result is the cut gene is now non-functional due to having a section missing and the gene that has gained the translocated section is likely to also be non-functional
• If a section of a proto-oncogene is translocated onto a gene controlling cell division


  It could boost expression and lead to tumours
• If a section of a tumour suppressor gene is translocated and the result is a faulty tumour suppressor gene


  This could lead to the cell continuing replication when it contains faulty DNA
• Mutations occur spontaneously and randomly during DNA replication
• DNA base sequence determines


  Sequence of amino acids that make up a protein
• Mutations in a gene


  Can sometimes lead to a change in the amino acid sequence coded for by the gene
• Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed
• Degenerate genetic code


  More than one triplet code codes for the same amino acid
• Some mutations will not cause a change in the amino acid sequence
• Substitution mutations

  Mutations that usually have a smaller effect on the resultant polypeptide
• Some gene mutations change all base triplets downstream from (after) the mutation, this will result in a non-functional polypeptide
• Insertion and deletion mutations


  Result in a frameshift
• • Exposure to mutagenic agents can increase the rate of mutation, they include
  • High energy ionising radiation, such as alpha, beta or gamma radiation
  • Chemicals, such as nitrogen dioxide or benzopyrene from tobacco smoke
•  The effect of gene mutations on polypeptides 

  • Most mutations do not alter the polypeptide or only alter it slightly so that its appearance or function is not changed
  • However, a small number of mutations code for a significantly altered polypeptide with a different shape
  • This may affect the ability of the protein to perform its function. For example:
  • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
  • A structural protein (like collagen) may lose its strength if its shape changes
• The effect of gene mutations on phenotype

  • Polypeptides / proteins affect the phenotype of an organism via specific cellular mechanisms
  • If a mutation causes a major alteration in a polypeptide then cellular mechanisms could be affected, which may impact the phenotype of the organism
  • For example, a mutation in the TYR gene in humans affects the structure of an enzyme that is needed for the production of the pigment melanin
  • The phenotype of the human is affected by the lack of melanin
  • Individuals with the mutation have albinism; very pale skin and hair