Genetic Mutations (AQA A Level Biology): Revision Note

Gene 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

• As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for

• Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed (as the genetic code is degenerate)

• Mutations in the DNA base sequence can occur due to the

  • insertion, deletion or substitution of a nucleotide

  • inversion, duplication or translocation of a section of a gene

Addition of bases

• A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation

• An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases

  • This is because every group of three bases in a DNA sequence codes for an amino acid

• An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence

  • This is sometimes known as a frameshift mutation

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Deletion of bases

• A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence is known as a deletion mutation

• Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for

• Like an insertion mutation, a deletion mutation also causes a frame-shift by changing the groups of three bases further on in the DNA sequence

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Substitution of bases

• A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base is known as a substitution mutation

• Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the triplet (group of three bases) in which the mutation occurs; it will not have a knock-on effect

• Substitution mutations can take three forms:

  • Silent mutations – the mutation does not alter the amino acid sequence of the polypeptide (this is because the genetic code is degenerate)

  • Missense mutations – the mutation alters a single amino acid in the polypeptide chain

  • Nonsense mutations – the mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function

Inversion within a gene section

• Usually occurs during crossing-over in meiosis

• The DNA of a single gene is cut in two places

• The cut portion is inverted 180°, then rejoined to the same place within the gene

• The result is that a large section of the gene is 'backwards' and therefore multiple amino acids are affected

• Inversion mutations frequently result in a non-functional protein

  • In some cases, an entirely different protein is produced

• 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

• Over time, 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

• Similarly to inversion, a gene is cut in two places

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

• The result is that the cut gene is now non-functional due to having a section missing, and the gene that has gained the translocated section is also likely to 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

• Similarly, 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 to replicate when it contains faulty DNA