Topic 8.1 - The Alteration Of The Sequence Of Bases In DNA Can Alter The Structure Of Proteins

Question 1

What is a gene mutation? (2)

Answer 1

1) A change in the base sequence of DNA

2) Can arise spontaneously during DNA replication (interphase)

Question 2

What is a mutagenic agent?

Answer 2

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

Question 3

Explain how a gene mutation can lead to the production of a non-functional protein / enzyme (5)

Answer 3

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

4) So changes tertiary structure of protein

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

Question 4

What is substitution?

Answer 4

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

Question 5

What is addition?

Answer 5

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

Question 6

What is deletion?

Answer 6

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

Question 7

What is duplication?

Answer 7

A sequence of DNA bases / nucleotides is repeated / copied

Question 8

What is inversion?

Answer 8

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

Question 9

What is translocation?

Answer 9

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

Question 10

Explain why not all gene mutations affect the order of amino acids (2)

Answer 10

1) Some substitutions change only 1 triplet code / codon which could still code for the same amino acid
- As the genetic code is degenerate

2) Some occur in introns which do not code for amino acids

Question 11

Explain why a change in amino acid sequence is not always harmful (2)

Answer 11

1) May not change tertiary structure of protein

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

Question 12

Explain what is meant by a frame shift (3)

Answer 12

1) A frameshift occurs when gene mutations change the number of nucleotides / bases by any number not divisible by 3

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

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