AS Level Biology chapter 6: Nucleic acids and protein synthesis MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

26 exam-style questions on Nucleic acids and protein synthesis, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

26 questions · 2 topics · 10 per test

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What chapter 6 covers

Ten Nucleic acids and protein synthesis questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. A double-stranded DNA fragment has 100 base pairs, of which 30 are A–T pairs. How many hydrogen bonds hold the two strands together?

  1. A
    250
  2. B
    300
  3. C
    230
  4. D
    270
Show answer

Answer: D. 30 A–T pairs × 2 = 60; 70 C–G pairs × 3 = 210; 60 + 210 = 270.

2. A sample of double-stranded DNA contains 18% adenine. What percentage of its bases are guanine?

  1. A
    18%
  2. B
    64%
  3. C
    36%
  4. D
    32%
Show answer

Answer: D. A = T = 18%, so A + T = 36%. The remaining 64% is shared equally by G and C: 32% each.

3. Why is the lagging strand made in short fragments during DNA replication?

  1. A
    DNA polymerase adds nucleotides only to the 3′ end of a new strand, so on this template it must work away from the fork
  2. B
    DNA polymerase can only add nucleotides to the 5′ end of a new strand, so it has to work towards the fork
  3. C
    helicase unwinds the double helix in stages, so this template becomes available only in short lengths
  4. D
    DNA ligase can only join nucleotides to the 5′ end of a strand, so this template must be copied in sections
Show answer

Answer: A. New DNA can only be built 5′ to 3′. On the lagging strand template this means polymerase works away from the replication fork, making short fragments that are later joined by DNA ligase.

4. Which three components make up a DNA nucleotide?

  1. A
    ribose, phosphate and a nitrogenous base
  2. B
    deoxyribose, phosphate and a nitrogenous base
  3. C
    deoxyribose, an amino acid and a nitrogenous base
  4. D
    glucose, phosphate and a nitrogenous base
Show answer

Answer: B. A DNA nucleotide is a phosphate group, the pentose sugar deoxyribose and one of four bases. RNA nucleotides contain ribose.

5. Bacteria are grown for many generations with heavy nitrogen (15N), then moved to light nitrogen (14N) for two rounds of replication. What DNA is then present?

  1. A
    all intermediate (hybrid) density
  2. B
    half heavy and half light
  3. C
    half intermediate and half light
  4. D
    all light
Show answer

Answer: C. After one round every molecule is hybrid. In the second round each hybrid gives one hybrid and one light molecule: half and half.

6. A drug blocks RNA polymerase in a eukaryotic cell. What is the direct effect of the drug on the cell?

  1. A
    DNA replication stops because the new strands cannot be joined together
  2. B
    no new mRNA is made, so protein synthesis stops once existing mRNA has been broken down
  3. C
    introns are no longer removed, so the mRNA is longer but translation carries on as normal
  4. D
    tRNA can no longer bring amino acids to the ribosome, although mRNA production continues
Show answer

Answer: B. RNA polymerase makes mRNA by transcription of the template strand. Without it no new mRNA is made, so once the existing mRNA is broken down no new polypeptides can be built. DNA replication uses DNA polymerase.

7. How many hydrogen bonds form between the bases in each C–G pair and each A–T pair?

  1. A
    C–G two; A–T three
  2. B
    C–G three; A–T two
  3. C
    both form two
  4. D
    both form three
Show answer

Answer: B. Cytosine and guanine are held by three hydrogen bonds; adenine and thymine by two.

8. In a eukaryotic cell, the primary transcript of a gene is longer than the mRNA that leaves the nucleus. Why?

  1. A
    non-coding introns are removed and the exons are joined together
  2. B
    the mRNA is cut in half and only the half with the start codon leaves the nucleus
  3. C
    the stop codon is removed before the mRNA leaves the nucleus, which shortens it
  4. D
    exons are removed and the introns are joined together to form the mature mRNA
Show answer

Answer: A. Introns are non-coding and are cut out; the coding exons are spliced together to form the mature mRNA.

9. Which change to the DNA of a gene is least likely to change the polypeptide produced?

  1. A
    insertion of two nucleotides near the start of the gene, altering the reading frame
  2. B
    deletion of one nucleotide near the start of the gene, altering the reading frame
  3. C
    substitution of one nucleotide that changes a triplet into a stop codon
  4. D
    substitution of one nucleotide in the third position of a triplet
Show answer

Answer: D. Many amino acids have more than one triplet differing only at the third base, so the change may be silent. Single insertions or deletions shift the reading frame.

10. What is meant by the statement that the two strands of DNA are antiparallel?

  1. A
    the strands run in opposite directions, one 5′ to 3′ and the other 3′ to 5′
  2. B
    the strands are joined by phosphodiester bonds between bases instead of hydrogen bonds
  3. C
    the strands wind in opposite senses, one as a right-handed and the other as a left-handed helix
  4. D
    the strands run in the same direction, so both are read from the 5′ end to the 3′ end at once
Show answer

Answer: A. The sugar–phosphate backbones point in opposite directions, so a 5′ end of one strand lies opposite the 3′ end of the other.

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Nucleic acids and protein synthesis: questions students ask

How many Nucleic acids and protein synthesis MCQs are there?

26, each with the answer and a short explanation, across 2 topics: Structure of nucleic acids and replication of DNA, Protein synthesis. Every chapter test draws ten of them, so you can take it many times.

Is the AS Level Biology chapter 6 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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