Biology Help UKAQA AS & A-level practice

3.1.5 Nucleic acids: DNA and RNA

Free AQA AS and A-level Biology questions on Nucleic acids: DNA and RNA (3.1.5). Try sample MCQs with clues and explanations, then start a topic quiz.

Specification route
3.1.5
Question bank
21 questions
Course stage
AS / Year 12

Practise this topic

21 standard questions. No sign-in required.

Quick 5Practise 10Full 20

Practise Biology maths and data skills

Sessions use up to the requested number of available questions. Standard practice uses levels 1–5.

Sample questions

QUESTION 1 · 3.1.5 · LEVEL 4

A DNA strand contains $20\%$ adenine. According to specific complementary base pairing, what is the percentage of cytosine in this DNA molecule?

  • $30\%$
  • $20\%$
  • $40\%$
  • $60\%$
Show clue

Adenine pairs with Thymine, and Cytosine pairs with Guanine. Total bases = $100\%$.

Show answer and explanation

Answer: $30\%$

If adenine is $20\%$, then thymine is also $20\%$ (total $40\%$). The remaining $60\%$ must be divided equally between cytosine and guanine, meaning cytosine is $30\%$.

QUESTION 2 · 3.1.5 · LEVEL 5

After bacteria grown with $^{15}N$ are transferred to $^{14}N$, their DNA forms a single intermediate-density band after one generation. Which replication model is ruled out by this result?

  • The conservative model, because it predicts separate heavy parental-DNA and light newly synthesised-DNA bands after one generation.
  • The semi-conservative model, because it predicts only light DNA after one generation.
  • The dispersive model, because it predicts separate heavy and light DNA bands after one generation.
  • Both the semi-conservative and dispersive models, because both predict separate heavy and light bands after one generation.
Show clue

After one generation, semi-conservative and dispersive replication can both give DNA of intermediate density; conservative replication predicts two bands.

Show answer and explanation

Answer: The conservative model, because it predicts separate heavy parental-DNA and light newly synthesised-DNA bands after one generation.

A single intermediate-density band after one generation rules out conservative replication, which predicts one heavy parental-DNA band and one light newly synthesised-DNA band. At this stage, both semi-conservative and dispersive models remain compatible; the second-generation pattern distinguishes them.

QUESTION 3 · 3.1.5 · LEVEL 3

Which statement correctly describes the structural differences between DNA and RNA?

  • DNA is a double helix with long polynucleotide chains, whereas RNA is a relatively short, single polynucleotide chain.
  • DNA contains the sugar ribose, whereas RNA contains the sugar deoxyribose.
  • DNA nucleotides are joined by hydrogen bonds, whereas RNA nucleotides are joined by phosphodiester bonds.
  • DNA holds genetic information in the ribosomes, whereas RNA transfers it from the nucleus.
Show clue

Consider the stability and size required for their respective functions in the cell.

Show answer and explanation

Answer: DNA is a double helix with long polynucleotide chains, whereas RNA is a relatively short, single polynucleotide chain.

A DNA molecule is a double helix with two long polynucleotide chains. An RNA molecule is a relatively short polynucleotide chain. DNA has deoxyribose; RNA has ribose.

QUESTION 4 · 3.1.5 · LEVEL 2

Why did many scientists initially doubt that DNA carried the genetic code?

  • The relative simplicity of DNA, with only four different bases, seemed insufficient to carry complex genetic instructions.
  • DNA was only found in prokaryotic cells, not eukaryotic cells.
  • DNA was easily destroyed by high temperatures, making it too unstable.
  • Proteins were not yet discovered in the nucleus.
Show clue

Proteins are made of 20 different amino acids, making them seem like much better candidates for a complex code.

Show answer and explanation

Answer: The relative simplicity of DNA, with only four different bases, seemed insufficient to carry complex genetic instructions.

DNA contains only four different bases. Early researchers therefore questioned whether such a chemically simple polymer could carry enough information for heredity, and proteins were initially considered stronger candidates.

QUESTION 5 · 3.1.5 · LEVEL 4

If a chemical inhibitor specifically prevents the action of DNA polymerase, what would be the immediate effect on a dividing cell?

  • Phosphodiester bonds would not form between adjacent nucleotides, preventing the synthesis of a new DNA strand.
  • The DNA double helix would not be able to unwind and separate into two strands.
  • Hydrogen bonds would not break between the complementary base pairs.
  • Free DNA nucleotides would not be attracted to the exposed bases on the template strand.
Show clue

Identify what specific part of replication DNA polymerase is responsible for.

Show answer and explanation

Answer: Phosphodiester bonds would not form between adjacent nucleotides, preventing the synthesis of a new DNA strand.

The role of DNA polymerase is in the condensation reaction that joins adjacent nucleotides (forming phosphodiester bonds). Helicase unwinds and breaks hydrogen bonds. Attraction of free nucleotides happens spontaneously via hydrogen bonding.

QUESTION 6 · 3.1.5 · LEVEL 2

What are the three components of a DNA nucleotide?

  • Deoxyribose, a phosphate group, and one of the organic bases adenine, cytosine, guanine or thymine
  • Ribose, a phosphate group, and one of the organic bases adenine, cytosine, guanine or uracil
  • Deoxyribose, an amine group, and a nitrogen-containing organic base
  • A pentose sugar, a sulfate group, and an organic base
Show clue

DNA contains a specific sugar and does not contain uracil.

Show answer and explanation

Answer: Deoxyribose, a phosphate group, and one of the organic bases adenine, cytosine, guanine or thymine

The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.

QUESTION 7 · 3.1.5 · LEVEL 1

Which organic base is found in RNA nucleotides but never in DNA nucleotides?

  • Uracil
  • Thymine
  • Adenine
  • Cytosine
Show clue

This base pairs with adenine in RNA molecules.

Show answer and explanation

Answer: Uracil

The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.

QUESTION 8 · 3.1.5 · LEVEL 2

What type of bond forms between two nucleotides in a condensation reaction?

  • Phosphodiester bond
  • Peptide bond
  • Hydrogen bond
  • Glycosidic bond
Show clue

This bond forms the strong sugar-phosphate backbone of nucleic acids.

Show answer and explanation

Answer: Phosphodiester bond

A condensation reaction between two nucleotides forms a phosphodiester bond.

Practise this topic

21 standard questions are available (levels 1–5). No sign-in required.

Start 20-question quizUse all standard questions

Related AQA Biology questions

3.1.4Proteins and enzymes26 questions 3.1.6ATP26 questions 3.1.3Lipids22 questions 3.1.7Water22 questions