What is a gene mutation?
A change to the base sequence of DNA on chromosomes. This can arise spontaneously during DNA replication.
What is substitution of bases?
A nucleotide in a section of a DNA molecule is replaced by another nucleotide that has a different base.
What are the 3 possible outcomes of base substitution?
1) Formation of a stop codon
2) Change to the tertiary structure of the enzyme so enzyme-substrate complexes can’t form.
3) No effect on the polypeptide produced.
Describe the formation of a stop codon from base substitution.
1) Substitution can cause the formation of a stop codon.
2) This marks the end of the polypeptide chain.
3) As a result the production of the polypeptide coded for by the section of DNA would be stopped prematurely.
4) The protein would not be able to perform its normal function.
Describe how a base substitution can have no effect on the polypeptide.
1) Substitution causes the formation of a different codon.
2) Codon forms the same amino acid due to the degenerate nature of the genetic code.
3) Mutation will have no effect on the polypeptide produced.
Describe how a base substitution can lead to the formation of a non-functional protein/enzyme.
1) Substitution causes the formation of a codon for a different amino acid.
2) Changes the sequence of amino acids in primary structure.
3) Changes positions of hydrogen, ionic and disulfide bonds in tertiary structure.
4) Active site changes shape.
5) The substrate will be unable to bind to active site and form an enzyme-substrate complex.
What is an example of a medical condition caused by base substitution?
Sickle Cell Anaemia
What is a deletion of bases?
The loss of 1 or more nucleotide bases from a DNA sequence.
What does a deletion mutation cause?
Frame shift to the left.
How can a deletion mutation lead to the production of a non-functional protein/enzyme?
1) Frame shift to the left
2) Most triplets are different and code for different amino acids
3) Polypeptide is different.
Name the 6 types of gene mutation.
1) Substitution
2) Deletion
3) Addition
4) Duplication
5) Inversion
6) Translocation
Describe the addition mutation.
1) One or more bases are inserted into the DNA sequence.
2) Frame shift to the right.
Describe the duplication of bases.
1) One or more nucleotides are repeated.
2) Frame shift to the right.
Describe the inversion of bases.
1) A group of bases become detached from the DNA sequence.
2) They rejoin at the same position but in the reverse order.
Describe the translocation of bases.
1) A group of bases become separated from the DNA sequence on one chromosome.
2) They become inserted into the DNA sequence of a different chromosome.
3) Lead to an abnormal phenotype.
What are some effects of the translocation of bases?
Cancer
Reduced fertility
What are mutagenic agents?
Factors that increase the rate of mutation.
What are 2 examples of mutagenic agents?
High energy ionising radiation
Chemicals
Describe high energy ionising radiation.
Alpha and beta particles are examples.
Also x-rays and UV light.
This form of radiation can disrupt the structure of DNA.
Describe how chemicals are mutagenic agents.
Nitrogen dioxide is an example that may directly alter the structure of DNA or interfere with transcription.
How can a mutation be beneficial?
It may produce the genetic diversity necessary for natural selection and speciation.
How can a mutation be negative?
It may produce an organism that is less well suited to its environment.
Explain why not all gene mutations affect the order of amino acids.
1) Some substitutions change only 1 codon which could still code for the same amino acid as the genetic code is degenerate.
2) Some mutations occur in introns which do not code for amino acids as they are removed during splicing.
Explain why a change in amino acid sequence is not always harmful.
1) May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)
2) May positively change the properties of the protein, giving the organism a selective advantage
