8.1 The alteration of the sequence of bases in DNA can alter the structure of proteins

What is a gene mutation?

• A change in the base sequence of DNA

• Can arise spontaneously during DNA replication (interphase)

What is a mutagenic agent? Provide examples.

• A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles.

Explain how a gene mutation can lead to the production of a non-functional enzyme.

1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA

2. So changes sequence of amino acids in the encoded polypeptide

3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)

4. So changes tertiary structure (shape) of protein

5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

What is a substitution mutation?

• A base / nucleotide is replaced by a different base / nucleotide in DNA

What is an addition mutation?

• 1 or more bases / nucleotides are added to the DNA base sequence

What is a deletion mutation?

• 1 or more bases / nucleotides are lost from the DNA base sequence

What is a duplication mutation?

• A sequence of DNA bases / nucleotides is repeated / copied

What is an inversion mutation?

• A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order

What is a translocation mutation?

• A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome

Explain one reason why a gene mutation might not affect the order of amino acids.

• Some substitutions change only 1 triplet code / codon which could still code for the same amino acid

  • As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)

Explain another reason why a gene mutation might not affect the order of amino acids.

• Some occur in introns which do not code for amino acids

For an enzyme, what specific part changes shape to make it non-functional?

• The active site

Explain the structural reason why a change in amino acid sequence is not always harmful.

• May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don't change)

Explain the evolutionary reason why a change in amino acid sequence is not always harmful.

• May positively change the properties of the protein, giving the organism a selective advantage

What is a frameshift mutation?

• A frameshift occurs when gene mutations (eg. addition, deletion, duplication or translocation) change the number of nucleotides/bases by any number not divisible by 3

What is the immediate effect of a frameshift on the reading of the genetic code?

• This shifts the way the genetic code is read, so all the DNA triplets /mRNA codons downstream from the mutation change

What is the ultimate effect of a frameshift on the polypeptide produced?

• The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant

If a mutation adds or deletes a number of bases that is a multiple of 3, what is the result and why is it not a frameshift?

• If a multiple of 3 bases is added / removed there won't be a frameshift, but extra / less triplets will result in extra / less amino acids in the encoded polypeptide

What is one severe potential outcome of a frameshift mutation?

• A frameshift could also lead to production of a stop codon (that doesn't code for amino acids so terminates translation), resulting in a shorter polypeptide