3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins

What are gene mutations? (2)

A change in the base sequence of DNA

Can arise spontaneously during DNA replication (interphase)

What are mutagenic agents? (2)

- A factor that increases rate of mutation

- E.g. benzene, X-rays, UV light etc

What are the 6 types of gene mutations? (6)

- Substitution

- Deletion

- Addition

- Duplication

- Translocation

- Inversion

What is a substitution gene mutation? (1)

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

What is a deletion gene mutation? (1)

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

What is an addition gene mutation? (1)

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

What is a duplication gene mutation? (1)

A sequence of DNA bases / nucleotides is repeated / copied

What is a translocation gene mutation? (2)

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

What is an inversion gene mutation? (1)

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

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

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

Explain what is meant by 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

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

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

Explain why not all gene mutations 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

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

Explain 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)

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

How can a mutation change the length of a polypeptide

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.

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.