Biology revision – (Unit 20) Gene Expression
Gene mutations
Substitution of bases
- The type of gene mutation in which a nucleotide in a section of a DNA molecule is
replaced by another nucleotide that has a different base
There are 3 possible consequences – these depend on which new base is subbed in:
- Formation of one of the 3 stop codons – ending the polypeptide chain
o Production of the polypeptide coded for by the section of DNA would be
stopped prematurely
o The final protein would be different and wouldn’t perform normal function
- Formation of a codon for a different amino acid meaning the structure of the
polypeptide produced would differ in a single amino
o The protein produced would differ in shape and not function properly
Example of this is sickle cell anaemia
- Formation of a different codon but one that produces a codon for the same amino as
before
o This is because the genetic code is degenerate and so most aminos have more
than one codon
Has no effect if this happens
Deletion of bases
- Loss of a nucleotide base from a DNA sequence is called deletion
- Impact on the phenotype can be significant
- The one deleted base creates a frame shift as the reading frame that contains each 3
letters of the code has been shifted left by one letter
- The gene is now read in the wrong tree base groups and the coded info is altered
- Most triplets will then be different, as will the amino they code for
- Polypeptides will be different and lead to production of a non-functional protein
- One deleted base at start can alter every triplet in the sequence
Other types of gene mutation
Addition of bases:
- An extra base becomes inserted in the sequence
- This usually has a similar effect to a base deletion – results in a frame shift
- If 3 extra bases are added, there will not be a frame shift
o The resulting polypeptide will be different but not to the same extent as would
be seen with a frame shift
Duplication of bases:
- One or more bases are repeated
- Produces a frame shift
Inversion of bases:
- A group of bases become separated from the DNA sequence and rejoin at the same
position but in the inverse order
Gene mutations
Substitution of bases
- The type of gene mutation in which a nucleotide in a section of a DNA molecule is
replaced by another nucleotide that has a different base
There are 3 possible consequences – these depend on which new base is subbed in:
- Formation of one of the 3 stop codons – ending the polypeptide chain
o Production of the polypeptide coded for by the section of DNA would be
stopped prematurely
o The final protein would be different and wouldn’t perform normal function
- Formation of a codon for a different amino acid meaning the structure of the
polypeptide produced would differ in a single amino
o The protein produced would differ in shape and not function properly
Example of this is sickle cell anaemia
- Formation of a different codon but one that produces a codon for the same amino as
before
o This is because the genetic code is degenerate and so most aminos have more
than one codon
Has no effect if this happens
Deletion of bases
- Loss of a nucleotide base from a DNA sequence is called deletion
- Impact on the phenotype can be significant
- The one deleted base creates a frame shift as the reading frame that contains each 3
letters of the code has been shifted left by one letter
- The gene is now read in the wrong tree base groups and the coded info is altered
- Most triplets will then be different, as will the amino they code for
- Polypeptides will be different and lead to production of a non-functional protein
- One deleted base at start can alter every triplet in the sequence
Other types of gene mutation
Addition of bases:
- An extra base becomes inserted in the sequence
- This usually has a similar effect to a base deletion – results in a frame shift
- If 3 extra bases are added, there will not be a frame shift
o The resulting polypeptide will be different but not to the same extent as would
be seen with a frame shift
Duplication of bases:
- One or more bases are repeated
- Produces a frame shift
Inversion of bases:
- A group of bases become separated from the DNA sequence and rejoin at the same
position but in the inverse order
