Biology Help UKAQA AS & A-level practice

3.8.1 Alteration of the sequence of bases in DNA

AQA A-level Biology 3.8.1 practice on Alteration of the sequence of bases in DNA, with free MCQs, clues and worked explanations drawn from the The control of gene expression section of specification 7402.

Specification route
3.8.1
Question bank
23 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.8.1 · LEVEL 1

Which type of gene mutation involves a section of DNA breaking off and reattaching in the reverse order?

  • Inversion
  • Translocation
  • Duplication
  • Substitution
Show clue

The sequence of bases is flipped backwards.

Show answer and explanation

Answer: Inversion

Gene mutations might arise during DNA replication and include addition, deletion, substitution, inversion, duplication and translocation of bases. An inversion involves a sequence of bases becoming separated and rejoining in the reverse order.

QUESTION 2 · 3.8.1 · LEVEL 3

Why does a base addition mutation usually have a more severe effect on a polypeptide than a base substitution?

  • A base addition changes the nature of all base triplets downstream from the mutation, resulting in a frame shift.
  • A base addition physically prevents RNA polymerase from binding to the DNA.
  • A base addition always introduces an immediate stop codon, prematurely ending translation.
  • A base addition causes the entire chromosome to undergo non-disjunction during meiosis.
Show clue

Adding a single letter changes how every subsequent group of three is read.

Show answer and explanation

Answer: A base addition changes the nature of all base triplets downstream from the mutation, resulting in a frame shift.

Some gene mutations change the nature of all base triplets downstream from the mutation, ie result in a frame shift. Addition and deletion mutations alter the reading frame, whereas substitution may only alter a single amino acid (or none due to the degenerate code).

QUESTION 3 · 3.8.1 · LEVEL 2

What is a 'translocation' mutation?

  • A section of bases separates from one DNA molecule and becomes inserted into the DNA sequence of a different, non-homologous chromosome.
  • A section of DNA breaks off and reattaches to the same chromosome in reverse order.
  • One or more bases are repeated directly after their original sequence.
  • A base is swapped for a different base during DNA replication.
Show clue

The prefix 'trans-' means across or changing location.

Show answer and explanation

Answer: A section of bases separates from one DNA molecule and becomes inserted into the DNA sequence of a different, non-homologous chromosome.

Gene mutations might arise during DNA replication and include... translocation of bases. Translocation involves a group of bases moving from one chromosome to a different chromosome.

QUESTION 4 · 3.8.1 · LEVEL 1

Which type of gene mutation occurs when one or more bases are repeated in the DNA sequence?

  • Duplication
  • Inversion
  • Translocation
  • Substitution
Show clue

This mutation copies a section of the gene and inserts it right next to the original.

Show answer and explanation

Answer: Duplication

Gene mutations might arise during DNA replication and include addition, deletion, substitution, inversion, duplication and translocation of bases. Duplication involves one or more bases being repeated.

QUESTION 5 · 3.8.1 · LEVEL 1

How do mutagenic agents, such as UV radiation and certain chemicals, affect gene mutations?

  • They significantly increase the rate at which spontaneous mutations occur.
  • They completely stop DNA polymerase from proofreading the DNA strand.
  • They exclusively cause translocation mutations by physically breaking chromosomes.
  • They specifically target and mutate recessive alleles while leaving dominant alleles intact.
Show clue

Mutations happen naturally all the time; these agents just act as a catalyst.

Show answer and explanation

Answer: They significantly increase the rate at which spontaneous mutations occur.

Gene mutations occur spontaneously and the mutation rate is increased by mutagenic agents.

QUESTION 6 · 3.8.1 · LEVEL 2

Which combination of gene mutations will always result in a frame shift in the reading of the DNA sequence?

  • Addition and deletion
  • Substitution and inversion
  • Duplication and substitution
  • Inversion and translocation
Show clue

A frame shift occurs when the total number of bases changes, disrupting the reading of triplets downstream.

Show answer and explanation

Answer: Addition and deletion

Gene mutations include addition, deletion, substitution, inversion, duplication and translocation. Some gene mutations change the nature of all base triplets downstream (frame shift). Addition and deletion permanently alter the grouping of the triplets.

QUESTION 7 · 3.8.1 · LEVEL 2

A substitution mutation occurs in a gene, but the encoded polypeptide is completely unchanged. What is the most likely reason for this?

  • The genetic code is degenerate, so the mutated triplet codes for the exact same amino acid as the original triplet.
  • The mutation occurred in an exon, which is always spliced out before translation.
  • DNA polymerase immediately repairs all substitution mutations during the $G_2$ phase of the cell cycle.
  • The ribosome skips over the mutated triplet, leaving a small but unnoticeable gap in the protein.
Show clue

Multiple different three-letter 'words' in DNA can mean the exact same 'thing' in a protein.

Show answer and explanation

Answer: The genetic code is degenerate, so the mutated triplet codes for the exact same amino acid as the original triplet.

Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. This is often called a 'silent' mutation.

QUESTION 8 · 3.8.1 · LEVEL 4

A substitution mutation changes a DNA triplet from $TTC$ to $ATC$. This changes the mRNA codon from $AAG$ (coding for Lysine) to $UAG$ (a stop codon). What is the consequence of this?

  • Translation will terminate prematurely, resulting in a severely truncated and likely non-functional polypeptide.
  • Translation will continue normally, as the degenerate code will replace the stop codon with a similar amino acid.
  • The mutation causes a frame shift, altering every amino acid from that point onwards.
  • RNA polymerase will detach immediately, meaning no mRNA is ever produced.
Show clue

A stop codon acts as a full stop at the end of a sentence.

Show answer and explanation

Answer: Translation will terminate prematurely, resulting in a severely truncated and likely non-functional polypeptide.

Gene mutations... Some gene mutations change only one triplet code. If a substitution creates a premature stop codon (a nonsense mutation), the ribosome will detach early, producing a shortened, usually non-functional protein.

Practise this topic

23 questions are available. No sign-in required.

Start 20-question quizUse all available questions