A-Level Biology Topic 6: Nucleic Acids and Protein Synthesis MCQs – 30 Practice Questions with Answers (Cambridge 9700)

Free practice for Cambridge A-Level Biology 9700 on nucleic acids and protein synthesis, with every question followed by a short explanation to help you check your understanding.

Structure of Nucleotides and DNA

Q1. What are the three parts of a nucleotide?

  • (A) An amino acid, a fatty acid and glycerol
  • (B) A sugar, a base and a lipid
  • (C) A sugar, a phosphate and a base
  • (D) A hexose sugar, water and a protein
Show Answer

Answer: C – A nucleotide is made of a pentose sugar, a phosphate group and a nitrogenous base.

Q2. Which bases are purines, with a double ring structure?

  • (A) Adenine and guanine
  • (B) Cytosine and thymine
  • (C) Cytosine and uracil
  • (D) Thymine and uracil
Show Answer

Answer: A – Adenine and guanine are purines with a double ring structure.

Q3. Which bases are pyrimidines, with a single ring structure?

  • (A) Adenine and guanine only
  • (B) Cytosine, thymine and uracil
  • (C) Adenine, cytosine and guanine
  • (D) Guanine, thymine and uracil
Show Answer

Answer: B – Cytosine, thymine and uracil are pyrimidines with a single ring structure.

Q4. Which base is found in RNA but not in DNA?

  • (A) Adenine
  • (B) Guanine
  • (C) Cytosine
  • (D) Uracil
Show Answer

Answer: D – Uracil replaces thymine in RNA, so it is found in RNA but not in DNA.

Q5. How are the two strands of a DNA molecule arranged?

  • (A) Both run in the same direction, side by side
  • (B) They are joined end to end in one long chain
  • (C) They run antiparallel
  • (D) They cross over at every base pair
Show Answer

Answer: C – DNA strands are antiparallel, with one running 5 to 3 and the other 3 to 5.

Q6. Which base pairs are complementary in DNA?

  • (A) Adenine with thymine, and cytosine with guanine
  • (B) Adenine with guanine, and cytosine with thymine
  • (C) Adenine with cytosine, and guanine with thymine
  • (D) Adenine with uracil, and cytosine with guanine
Show Answer

Answer: A – In DNA, adenine pairs with thymine and cytosine pairs with guanine.

Q7. How does hydrogen bonding differ between base pairs in DNA?

  • (A) Both pairs have the same number of bonds
  • (B) A-T pairs have more bonds than C-G pairs
  • (C) No hydrogen bonds join the pairs
  • (D) C-G pairs have more hydrogen bonds than A-T pairs
Show Answer

Answer: D – C-G pairs are joined by three hydrogen bonds, while A-T pairs are joined by two.

Q8. What type of bond links successive nucleotides in a DNA strand?

  • (A) A hydrogen bond
  • (B) A phosphodiester bond
  • (C) A peptide bond
  • (D) A glycosidic bond
Show Answer

Answer: B – Nucleotides in a strand are linked by phosphodiester bonds.

Q9. What is ATP, in structural terms?

  • (A) A phosphorylated nucleotide
  • (B) A short polypeptide chain
  • (C) A type of fatty acid
  • (D) A disaccharide sugar
Show Answer

Answer: A – ATP is a phosphorylated nucleotide.

Q10. How does messenger RNA differ from DNA in its basic structure?

  • (A) It is double stranded and contains thymine
  • (B) It is made of amino acids
  • (C) It is single stranded, with uracil
  • (D) It has no sugar component
Show Answer

Answer: C – Messenger RNA is single stranded and uses uracil in place of thymine.

DNA Replication and the Genetic Code

Q11. What does semi-conservative replication mean?

  • (A) Both original strands are kept together in one molecule
  • (B) Both strands of each new molecule are newly made
  • (C) Only one strand of DNA is ever copied
  • (D) One old strand and one new strand
Show Answer

Answer: D – Semi-conservative replication means each new molecule has one original strand and one newly made strand.

Q12. During which phase of the cell cycle does DNA replication occur?

  • (A) G1 phase
  • (B) S phase
  • (C) G2 phase only after mitosis
  • (D) Telophase
Show Answer

Answer: B – DNA replication occurs during the S phase of the cell cycle.

Q13. In which direction does DNA polymerase add new nucleotides?

  • (A) Only in the 5 to 3 direction
  • (B) Only in the 3 to 5 direction
  • (C) In both directions at once
  • (D) From the middle outwards
Show Answer

Answer: A – DNA polymerase adds nucleotides only in the 5 to 3 direction.

Q14. Why is one new strand made continuously and the other in short sections?

  • (A) Because the two strands have different bases
  • (B) Because ligase works on only one strand
  • (C) DNA polymerase works only 5 to 3
  • (D) Because only one strand contains genes
Show Answer

Answer: C – The antiparallel strands mean one strand can be copied continuously while the other is copied in sections.

Q15. What is the role of DNA ligase in replication?

  • (A) It unwinds the double helix
  • (B) It removes the original strands
  • (C) It adds new nucleotides to the growing strand
  • (D) It joins lagging-strand sections
Show Answer

Answer: D – DNA ligase joins the short sections of the lagging strand into a continuous strand.


Q16. What is a gene?

  • (A) A whole chromosome
  • (B) A DNA sequence coding for a polypeptide
  • (C) A single amino acid
  • (D) A protein found in the nucleus of every living cell
Show Answer

Answer: B – A gene is a sequence of nucleotides in DNA that codes for a polypeptide.

Q17. In the genetic code, what does a triplet of DNA bases code for?

  • (A) A specific amino acid, or a start or stop signal
  • (B) A whole protein on its own
  • (C) A complete chromosome
  • (D) A type of lipid
Show Answer

Answer: A – Each triplet codes for a specific amino acid or corresponds to a start or stop codon.

Q18. What is meant by the genetic code being universal?

  • (A) Every organism uses the same DNA bases
  • (B) Every gene codes for the same protein
  • (C) Same triplets, same amino acids
  • (D) Every cell uses the same gene at all times
Show Answer

Answer: C – The code is universal because the same triplets specify the same amino acids across organisms.

Q19. What is a codon?

  • (A) A sequence of three amino acids
  • (B) The enzyme that copies DNA
  • (C) A section of a ribosome
  • (D) A sequence of three bases on messenger RNA
Show Answer

Answer: D – A codon is a sequence of three bases on messenger RNA.

Q20. Which strand of DNA is used during transcription?

  • (A) The strand that never codes for anything
  • (B) The transcribed, or template, strand
  • (C) Both strands at the same time
  • (D) A strand of RNA made earlier
Show Answer

Answer: B – Transcription uses the transcribed, or template, strand of DNA.

Transcription, Translation and Mutation

Q21. Which enzyme carries out transcription?

  • (A) DNA ligase
  • (B) DNA polymerase only
  • (C) RNA polymerase
  • (D) Peptidase
Show Answer

Answer: C – RNA polymerase carries out transcription to form RNA.

Q22. Where does transcription take place in a eukaryotic cell, and where does translation take place?

  • (A) Transcription in the nucleus, translation in the cytoplasm
  • (B) Both in the nucleus
  • (C) Both in the cytoplasm
  • (D) Transcription in the cytoplasm, translation in the nucleus
Show Answer

Answer: A – In eukaryotes transcription occurs in the nucleus and translation occurs in the cytoplasm.

Q23. What happens to the primary transcript in a eukaryotic cell before it becomes messenger RNA?

  • (A) It is copied back into DNA
  • (B) It is broken down into amino acids in the cytoplasm
  • (C) It gains extra chromosomes
  • (D) Introns are removed and exons joined
Show Answer

Answer: D – Introns are removed and exons are joined to form mature messenger RNA.

Q24. During translation, what does transfer RNA do?

  • (A) It copies DNA into RNA
  • (B) It carries an amino acid to the ribosome
  • (C) It joins exons together
  • (D) It unwinds the DNA double helix in the nucleus
Show Answer

Answer: B – Transfer RNA brings its amino acid to the ribosome and pairs its anticodon with a codon on messenger RNA.

Q25. Where are amino acids joined together into a polypeptide?

  • (A) In the nucleus
  • (B) At the cell surface membrane
  • (C) At the ribosome
  • (D) Inside a chromosome
Show Answer

Answer: C – Polypeptides are assembled at ribosomes during translation.

Q26. What is a gene mutation?

  • (A) A base-sequence change that may alter a polypeptide
  • (B) A change in the number of chromosomes only
  • (C) The loss of a whole cell
  • (D) The movement of a gene to another organism by cell division
Show Answer

Answer: A – A gene mutation is a change in the base sequence of DNA that may result in an altered polypeptide.

Q27. How can a substitution mutation affect a polypeptide?

  • (A) It always deletes the whole gene
  • (B) It always doubles the gene
  • (C) It always leaves the protein completely unchanged in every case
  • (D) It may change one amino acid or create a stop codon
Show Answer

Answer: D – A substitution changes one base, which may alter one amino acid, introduce a stop codon, or have no effect on the amino acid.

Q28. Why can a deletion or insertion of a single nucleotide have a large effect?

  • (A) It removes the ribosome
  • (B) It shifts the reading frame for all following triplets
  • (C) It stops transcription permanently
  • (D) It changes every base in the gene to thymine
Show Answer

Answer: B – Adding or removing one nucleotide shifts the reading frame, changing many following amino acids.

Q29. Which sequence correctly orders protein synthesis in a eukaryotic cell?

  • (A) Transcription, then modification, then translation
  • (B) Translation in the nucleus, transcription in the cytoplasm, splicing at the membrane
  • (C) Splicing in the cytoplasm, transcription at the ribosome, translation in the nucleus
  • (D) Translation at the ribosome, transcription in the nucleus, splicing in the cytoplasm
Show Answer

Answer: A – Transcription and modification occur in the nucleus, followed by translation at ribosomes in the cytoplasm.

Q30. A DNA triplet changes so that the corresponding messenger RNA codon now codes for a different amino acid. What has most likely happened?

  • (A) A chromosome has been lost
  • (B) A ribosome has been destroyed
  • (C) A gene mutation has altered the base sequence
  • (D) The cell has divided by mitosis
Show Answer

Answer: C – A change in base sequence that alters an amino acid is a gene mutation.

Work through any questions you missed again, then revise those areas before moving on to the next topic.