Mutations

Question 1

What is a gene mutation?

Answer 1

A mutation is a change in the base sequence of DNA. They can arise spontaneously during DNA replication during interphase.

Question 2

What is a mutagenic agent?

Answer 2

A factor that increases rate of mutation, e.g. ultraviolet light, alpha particle

Question 3

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

Answer 3

• There is a change in the sequence of base triplets in DNA which changes sequence of codons on mRNA
• This may change the sequence of amino acids in the polypeptide chain (may also be frameshift)
• This changes the position of hydrogen / ionic / disulphide bonds between amino acids
• Changes the tertiary structure and the shape of the protein
• If this is an enzymes, the active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Question 4

Describe the different types of gene mutations

Answer 4

• Substitution - A base is replaced by a different base in the DNA polynucleotide chain
• Addition - 1 or more bases are added to the DNA base sequence
• Deletion 1 or more bases are removed from the DNA base sequence
• Duplication - A sequence of DNA bases is repeated / copied
• Inversion - A sequence of bases detaches from the DNA sequence, then rejoins at the same position in the reverse order
• Translocation - A sequence of DNA bases detaches and is inserted at a different location within the same or a different chromosome

Question 5

Explain why not all gene mutations affect the order of amino acids

Answer 5

Some substitutions may only change 1 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 mutations may occur in introns which do not code for amino acids as they are removed during splicing

Question 6

Explain why a change in amino acid sequence is not always harmful

Answer 6

May not change the tertiary structure of the protein as the position of ionic / disulphide / H bonds may not change. Also may positively change the properties of the protein, giving the organism a selective advantage

Question 7

Explain what is meant by a frameshift

Answer 7

Occurs when mutations (addition, deletion, duplication or translocation) change the number of bases by a number not divisible by 3. This shifts the way the genetic code is read, so all the codons downstream from the mutation change (so significant effects)