8.1 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 (on a chromosome)

• can arise spontaneously during DNA replication (interphase)

What is a mutagenic agent?

a factor that increases rate of mutation, e.g. ultraviolet light or alpha particles

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

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

2. so change sequence of amino acids in the encoded polypeptide

3. so changes position of hydrogen / ionic / disulphide bonds

4. so changes tertiary structure

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

list the different types of gene mutation

1. substitution

2. addition

3. deletion

4. duplication

5. inversion

6. translocation

Describe a substitution gene mutation

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

Describe an addition gene mutation

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

Describe a deletion gene mutation

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

Describe a duplication gene mutation

a sequence of DNA bases / nucleotides is repeated / copied

Describe an inversion gene mutation

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

Describe a translocation gene mutation

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

Explain why not all gene mutation 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)

• some occurs 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

• may not change tertiary structure of protein, if position of ionic / disulphide / hydrogen bonds don’t change

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

Explain what is meant by frameshift

• occurs when mutation (addition, deletion, duplication, or translocation) change the number of nucleotides / bases by a number not divisible by three

• this shifts the way the genetic code is read, so all the DNA triplets / mRNA codons downstream from the mutation change (so significant effect)

Explain how mutation can lead to production of shorter polypeptides

• deletion or translocation → triplets / codons missing

• substitution, addition, deletion, duplication, inversion or translocation → premature stop triplet / codon so amino acids missing at end of polypeptides