DNA & Gene Expression (Edexcel International AS Biology)

Exam Questions

45 mins4 questions
1a
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6 marks

Acute hepatic porphyria (AHP) is a very rare genetic disorder.

A drug has been developed to treat AHP.

This drug was tested in a clinical trial involving 94 patients from 18 countries.

The drug was given to 48 of the patients. The other 46 patients were a control group.

(i)

Comment on the design of this clinical trial.

(2)

(ii)

Each patient was given 2.5 mg of the drug per kg of body mass, once a month.

The drug is available as a solution with a concentration of 189 mg cm−3.

Calculate the volume of drug that was given each month to a patient with a body mass of 64 kg.

(2)

(iii)

Nausea was experienced by 27 % of the patients receiving this drug.

Calculate the number of patients who experienced nausea.

(2)

1b
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8 marks

This drug is a double-stranded RNA molecule.

(i)
The diagram shows part of the base sequence on one of the RNA strands.

Complete the diagram to show the base sequence on the other RNA strand.
(2)
q6b-unit-1-oct-2021-edexcel-ial-biology
(ii)
Describe the bonding in this double-stranded RNA molecule.

(3)

(iii)
In AHP, toxic porphyrin molecules build up.

The synthesis of the haem component of haemoglobin involves several steps.

Each step in the synthesis of haem is catalysed by a different enzyme.

This drug works by interfering with the mRNA copies from the gene coding for one of these enzymes.

Explain how the action of this drug helps patients with AHP.

(3)

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2a
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3 marks

The polynucleotide DNA is composed of mononucleotides linked together.

Two polynucleotides form a DNA molecule.

The diagram shows part of a DNA molecule.

q4a-unit-1-oct-2020-edexcel-ial-biology

(i)

Draw a circle around one mononucleotide that includes the base labelled R.

(1)

(ii)

Which row of the table identifies the bonds labelled S, T and U?

(1)

      S T U
  A hydrogen phosphodiester covalent
  B hydrogen covalent phosphodiester
  C phosphodiester hydrogen covalent
  D phosphodiester covalent hydrogen

(iii)

The base labelled P is adenine.

Which is the base labelled Q?

(1)

  A cytosine
  B guanine
  C thymine
  D uracil

2b
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6 marks

Meselson and Stahl carried out experiments that provided evidence for the semi‐conservative replication of DNA.

Heavy nitrogen (N15) and light nitrogen (N14) were used in these experiments.

The flow chart summarises part of one experiment performed by Meselson and Stahl.

q4b-unit-1-oct-2020-edexcel-ial-biology

After each stage, a sample of DNA was taken from the bacteria and the DNA molecules separated using a density gradient in a tube.

The heavier DNA molecules form bands lower down the gradient than the lighter DNA molecules.

The height of each band is proportional to the percentage of DNA molecules in the sample.

(i)

Complete the diagram to show the results of this experiment.

Use the dotted lines to help you to position the bands on the diagram.

The first one has been done for you.

(5)

q4bi-unit-1-oct-2020-edexcel-ial-biology

(ii)

The experiments of Meselson and Stahl disproved the dispersive theory of DNA replication.

The diagram shows the expected results if the dispersive theory was correct.

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Which diagram would show the bands of DNA molecules on the density gradient at stage 2, if the dispersive theory was correct?

(1)

q4bii-2-unit-1-oct-2020-edexcel-ial-biology

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3a
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3 marks

An average‐sized human chromosome contains a DNA molecule with about 300 million nucleotides, arranged in pairs.

Describe the structure of a nucleotide pair.

3b
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2 marks

Before a cell can divide, the DNA molecule has to replicate. This takes place in the S phase of the cell cycle.

Human DNA replicates at a rate of 50 nucleotides per second.

Calculate how long it would take, to the nearest hour, for 150 million nucleotides to replicate.

Assume that replication starts at one end of the molecule and continues to the other end.

3c
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5 marks

In human cells, replication of DNA occurs at several sites along the molecule.

These sites are called replication bubbles.

The image shows a replication bubble.

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The diagram shows how the DNA is replicated in one replication bubble.

q3c-2-unit-1-january-2022-edexcel-ial-biology
(i)

Explain the role of DNA polymerase in a replication bubble.

Use the information in the diagram and your own knowledge to support your answer.

(2)

(ii)

In human cells, S phase lasts about 10 hours.

Suggest why each DNA molecule is replicated using many replication bubbles

(3)

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4a
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6 marks

A gene contains the genetic code for the sequence of amino acids in a polypeptide chain.

The table shows the genetic codes found in DNA.

Genetic
code
Amino
acid
Genetic
code
Amino
acid
Genetic
code
Amino
acid
Genetic
code
Amino acid

AAA
AAG

Lysine CAA
CAG
Glutamine GAA
GAG
Glutamic
acid
TAC
TAT
Tyrosine
AAC
AAT
Asparagine CAT
CAC
Histidine GAC
GAT
Aspartate TCA
TCC
TCG
TCT
Serine
ACA
ACC
ACG
ACT
Threonine CCA
CCC
CCG
CCT
Proline GCA
GCC
GCG
GCT
Alanine TGG Tryptophan
AGA
AGG
Arginine CGA
CGC
CGG
CGT
Arginine GGA
GGC
GGG
GGT
Glycine

TGC
TGT

Cysteine
AGC
AGT
Serine CTA
CTC
CTG
CTT
Leucine GTA
GTC
GTG
GTT
Valine

TTA
TTG

Leucine
ATA
ATC
ATT
Isoleucine  

TTC
TTT

Phenylalanine
ATG Methionine  

The genetic codes TAA, TAG and TGA are stop codons, that do not code for amino acids.

Explain the nature of the genetic code.

Use information in the table to support your answer.

4b
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6 marks

The diagram shows the sequence of nucleotide bases in part of a DNA template (antisense) strand.

Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Base A T G G C T T G C C C G A T C C T A

(i)

Give the sequence of amino acids that is coded for by these bases.

(1)

(ii)

Explain the possible effects on a protein if there is a substitution mutation in the 9th base in this DNA strand.

Use information in the table to support your answer.

(5)

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