Structure of DNA and RNA - AQA A-Level Biology (Paper 1) Exam Questions

Practise 17 exam-style questions on Structure of DNA and RNA for AQA A-Level Biology (Paper 1). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

17 questions
1–8 marks each
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17 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Describe how a phosphodiester bond is formed between two nucleotides.

Model answer

A condensation reaction occurs between the phosphate group of one nucleotide and the pentose (deoxyribose or ribose) of the other nucleotide, releasing a molecule of water.

Explanation

Like glycosidic and peptide bonds, the phosphodiester bond is formed by condensation and broken by hydrolysis. It links the sugar of one nucleotide to the phosphate of the next, so a chain of alternating sugars and phosphates (the sugar-phosphate backbone) is built up with the bases sticking out. A common error is to say the bond forms between bases - bases are joined only by hydrogen bonds, between the two strands of DNA.

Mark scheme

1 mark
Condensation (reaction) / a molecule of water is released
Reject hydrolysis. Ignore the names of enzymes.
1 mark
Between the phosphate (group) of one nucleotide and the pentose / deoxyribose / ribose / sugar of the other (nucleotide)
Accept 'joins the sugar of one nucleotide to the phosphate of the next'. Do not accept 'between the bases', 'between two phosphates' or any reference to hydrogen bonds forming.

Question 2

2 marks

DNA and RNA are information-carrying molecules found in all living cells.

Give the role of RNA in carrying genetic information.

Name the two types of biological molecule from which ribosomes are formed.

Model answer

RNA transfers genetic information from DNA to the ribosomes.
Ribosomes are formed from RNA and protein.

Explanation

In all living cells DNA holds the genetic information, and RNA transfers that information from the DNA to the ribosomes. Ribosomes themselves are made of RNA (ribosomal RNA) and proteins. A common error is to say ribosomes contain DNA, or to give RNA the role of storing information.

Mark scheme

1 mark
(RNA) transfers / carries genetic information / a copy of the information from DNA to the ribosomes
Do not accept 'RNA stores / holds the genetic information' (this is the role of DNA). Accept 'carries the code from DNA to the ribosomes'.
1 mark
RNA AND protein
Both needed. Accept rRNA or ribosomal RNA for RNA. Accept polypeptide for protein. Do not accept DNA, mRNA or tRNA. List rule: if more than two molecules are given, a wrong extra answer (e.g. DNA or lipid) cancels the mark.

Question 3

3 marks

DNA molecules are very stable, so the genetic information they carry is rarely changed. However, the two strands of a DNA molecule can be separated easily, for example when DNA is copied.

Explain how the structure of a DNA molecule allows it to be both stable and easily separated into two strands.

Model answer

The sugar-phosphate backbone of each strand is held together by strong covalent phosphodiester bonds, and the bases are on the inside of the double helix, where they are protected. There are very many hydrogen bonds between the base pairs, which together hold the strands firmly. But each hydrogen bond is weak, so the two strands can be separated easily when the hydrogen bonds are broken.

Explanation

Stability comes from the covalent phosphodiester bonds of the backbones, the protected position of the bases and the large number of hydrogen bonds. Separation is easy because each hydrogen bond is weak, so the strands can be 'unzipped' without breaking the backbone. A common error is to describe hydrogen bonds as strong.

Mark scheme

1 mark
Sugar-phosphate backbone held together by (strong) phosphodiester / covalent bonds
Max 2 if the answer does not refer to hydrogen bonds being individually weak. Max 3 overall. Accept 'phosphodiester bonds are strong / hard to break'.
1 mark
Bases are on the inside of the (double) helix, protected by the sugar-phosphate backbone
Accept 'the double helix protects the bases'.
1 mark
Many hydrogen bonds (between the bases) together make the molecule stable
Do not accept 'strong hydrogen bonds'.
1 mark
Each hydrogen bond is weak, so the strands can be separated easily / with little energy
Do not accept 'the phosphodiester bonds are broken to separate the strands'.

Question 4

5 marks

Compare the structure of a DNA molecule with the structure of an RNA molecule.

Model answer

Similarities: both DNA and RNA are polymers of nucleotides. In both, each nucleotide contains a pentose, a phosphate group and a nitrogen-containing base, and the nucleotides are joined by phosphodiester bonds. Both contain the bases adenine, cytosine and guanine.

Differences: DNA contains the sugar deoxyribose but RNA contains ribose. DNA contains thymine but RNA contains uracil instead. A DNA molecule is a double helix of two polynucleotide chains held together by hydrogen bonds between complementary bases, but an RNA molecule is a single polynucleotide chain. DNA molecules are very long, whereas RNA molecules are relatively short.

Explanation

'Compare' means give similarities and differences, so include both. State each difference as a contrast ('DNA has deoxyribose but RNA has ribose') rather than describing one molecule only. Name bases in full; letters are not credited in a description of structure.

Mark scheme

1 mark
Both are polymers of nucleotides / polynucleotides
Max 4 if the answer gives no similarity between DNA and RNA. Max 5 overall. Each difference must state what DNA has AND what RNA has (or be a clear comparative). Bases must be named: letters alone are insufficient.
1 mark
(In both) each nucleotide contains a pentose, a phosphate (group) and a (nitrogenous / organic) base
1 mark
(In both) nucleotides are joined by phosphodiester bonds
Accept 'both formed by condensation reactions between nucleotides'.
1 mark
Both contain adenine, cytosine and guanine
All three bases must be named.
1 mark
DNA contains deoxyribose AND RNA contains ribose
1 mark
DNA contains thymine AND RNA contains uracil
Names needed; 'T and U' alone is insufficient.
1 mark
DNA is double-stranded / a double helix AND RNA is a single (polynucleotide) chain / single-stranded
Accept 'DNA has two polynucleotide chains held by hydrogen bonds and RNA has one'.
1 mark
DNA molecules are (very) long AND RNA molecules are (relatively) short
Accept 'DNA has many more nucleotides than RNA'.

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