Abi Starr
Chapter 14.4 – Gene mutation
Any change to the quantity or structure of DNA of an organism is known as a mutation. Mutations
arising in body cells are not passed on to the next generation. Mutations occurring during the
formation of gametes may be inherited, often producing sudden and distinct differences between
individuals. Therefore, they are the basis of discontinuous variation. Any change to one or more
nucleotide bases or any rearrangement of the bases in DNA is known as a gene mutation.
A sequence of triplets on DNA is transcribed into mRNA and is then translated into a sequence of
amino acids that make up a polypeptide. It follows that any changes to one or more bases in the
DNA triplets could result in a change in the amino acid sequence of the polypeptide. There are a
number of ways in which the DNA bases can change; two examples of this are substitution and
deletion.
Substitution of bases
This type of gene mutation involves a nucleotide in a DNA molecule being replaced by another
nucleotide that has a different base. Depending on which new base is substituted for the original,
there are three possible consequences. As an example, let us take the triplet of bases GTC that code
for the amino acid glutamine. A change to a single base could result in one of the following:
A nonsense mutation – this occurs if the base change results in the formation of one of
three stop codons that mark the end of a polypeptide chain. If the first base is replaced with
adenine, then GTC becomes ATC. This triplet is transcribed as UAG in mRNA. UAG is a stop
codon. As a result, the production of the polypeptide would be stopped prematurely. The
final protein would almost certainly be significantly different and therefore could not
perform its usual function.
A mis-sense mutation – This arises when the base change results in a different amino acid
being coded for. In the example above, if cytosine is replaced by guanine, then GTC become
GTG. GTG is a triplet code for the amino acid Histidine and this then replaces the original
amino acid (glutamine). The polypeptide produced will differ in just one amino acid. The
significance of this difference will depend on the role of the original amino acid. If it was
important in forming bonds that determine the tertiary structure of the final protein, then
the replacement amino acid may not form the same bonds. Therefore, the protein may be a
different shape and therefore may not function properly. For example, if the protein is an
enzyme, its active site may no longer be complementary to the substrate and it will not
catalyse the reaction.
A silent mutation – this occurs when the substituted base, although different, still codes for
the original amino acid. This is due to the degenerate nature of the genetic code, in which
most amino acids have more than one codon. For instance, if the third base in the example
above is replaced by thymine, then GTC becomes GTT. However, as both triplet codes code
for glutamine there is no change in the polypeptide produced and so the mutation will have
no effect.
Page 1 of 3
Chapter 14.4 – Gene mutation
Any change to the quantity or structure of DNA of an organism is known as a mutation. Mutations
arising in body cells are not passed on to the next generation. Mutations occurring during the
formation of gametes may be inherited, often producing sudden and distinct differences between
individuals. Therefore, they are the basis of discontinuous variation. Any change to one or more
nucleotide bases or any rearrangement of the bases in DNA is known as a gene mutation.
A sequence of triplets on DNA is transcribed into mRNA and is then translated into a sequence of
amino acids that make up a polypeptide. It follows that any changes to one or more bases in the
DNA triplets could result in a change in the amino acid sequence of the polypeptide. There are a
number of ways in which the DNA bases can change; two examples of this are substitution and
deletion.
Substitution of bases
This type of gene mutation involves a nucleotide in a DNA molecule being replaced by another
nucleotide that has a different base. Depending on which new base is substituted for the original,
there are three possible consequences. As an example, let us take the triplet of bases GTC that code
for the amino acid glutamine. A change to a single base could result in one of the following:
A nonsense mutation – this occurs if the base change results in the formation of one of
three stop codons that mark the end of a polypeptide chain. If the first base is replaced with
adenine, then GTC becomes ATC. This triplet is transcribed as UAG in mRNA. UAG is a stop
codon. As a result, the production of the polypeptide would be stopped prematurely. The
final protein would almost certainly be significantly different and therefore could not
perform its usual function.
A mis-sense mutation – This arises when the base change results in a different amino acid
being coded for. In the example above, if cytosine is replaced by guanine, then GTC become
GTG. GTG is a triplet code for the amino acid Histidine and this then replaces the original
amino acid (glutamine). The polypeptide produced will differ in just one amino acid. The
significance of this difference will depend on the role of the original amino acid. If it was
important in forming bonds that determine the tertiary structure of the final protein, then
the replacement amino acid may not form the same bonds. Therefore, the protein may be a
different shape and therefore may not function properly. For example, if the protein is an
enzyme, its active site may no longer be complementary to the substrate and it will not
catalyse the reaction.
A silent mutation – this occurs when the substituted base, although different, still codes for
the original amino acid. This is due to the degenerate nature of the genetic code, in which
most amino acids have more than one codon. For instance, if the third base in the example
above is replaced by thymine, then GTC becomes GTT. However, as both triplet codes code
for glutamine there is no change in the polypeptide produced and so the mutation will have
no effect.
Page 1 of 3
