Amino acids, proteins and DNAAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Amino acids, proteins and DNA topic test

Total 54 marks

Name

Class

Date

1
Serine, HOCH₂CH(NH₂)COOH, is a white crystalline amino acid that is soluble in water and decomposes at about 228 °C without boiling.
(a)
Which structure represents serine in the crystalline solid?
[1 mark]
AHOCH₂CH(NH₂)COOH
BHOCH₂CH(NH₃⁺)COOH
CHOCH₂CH(NH₃⁺)COO⁻
DHOCH₂CH(NH₂)COO⁻
(b)
Which statement best explains why serine has a much higher melting point than an amine of similar relative molecular mass?
[1 mark]
AThere are strong ionic attractions between the zwitterions in the solid
BThere are only weak London forces between the molecules
CCovalent bonds form between serine molecules in the solid
DThe molecule contains a chiral centre
(c)
Give the structure of the main organic species formed when serine is dissolved in (i) an excess of dilute hydrochloric acid and (ii) an excess of aqueous sodium hydroxide.
[2 marks]
Total for question 1: 4 marks
2
Silk fibres are made from the protein fibroin. Fibroin has long regions in which glycine and alanine residues alternate along the polypeptide chain, and these regions are folded into a pleated sheet that makes silk strong.
(a)
What is meant by the primary structure of a protein?
[1 mark]
AThe coiling of the chain into an α-helix
BThe sequence of amino acids joined by peptide links
CThe overall three-dimensional folding of a polypeptide chain
DThe arrangement of several polypeptide chains side by side
(b)
Which statement correctly describes the pleated sheet in fibroin?
[1 mark]
AIt is held by sulfur–sulfur bonds between cysteine residues
BIt is held together by covalent peptide links between neighbouring chains
CIt is held together by ionic bonds between the end groups of the chains
DIt is held by hydrogen bonds between N–H and C=O groups of peptide links in neighbouring sections of chain
(c)
State the conditions used to hydrolyse a protein into its amino acids and name the bond that is broken.
[2 marks]
Total for question 2: 4 marks
3
The painkiller ibuprofen is sold as an equal mixture of two enantiomers. Only one enantiomer, (S)-ibuprofen, is effective: it binds to the active site of the enzyme cyclooxygenase and stops it working, which reduces inflammation.
(a)
Explain why only one enantiomer of ibuprofen binds to the active site of the enzyme.
[3 marks]
(b)
Explain how ibuprofen reduces the activity of cyclooxygenase, and how computers can help chemists to design new drugs that act in this way.
[4 marks]
Total for question 3: 7 marks
4
Insulin is a protein hormone made of two polypeptide chains held together by sulfur–sulfur bonds between cysteine residues. Each chain contains regions of α-helix. A researcher completely hydrolyses a sample of insulin and then analyses the amino acids that result.
(a)
Describe the primary, secondary and tertiary structures of an insulin chain and explain what holds the secondary and tertiary structures in place.
[6 marks]
(b)
Describe how the researcher could hydrolyse insulin and then use thin-layer chromatography (TLC) to identify the amino acids present.
[6 marks]
Total for question 4: 12 marks
5
A short section of one strand of DNA has the base sequence adenine, thymine, guanine, cytosine, read in one direction. In a cell the strand is paired with a complementary strand to form part of a double helix.
(a)
What is the base sequence of the complementary strand opposite this section, read in the same direction?
[1 mark]
AThymine, adenine, cytosine, guanine
BThymine, adenine, guanine, cytosine
CAdenine, thymine, cytosine, guanine
DCytosine, guanine, adenine, thymine
(b)
Adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine through three. What is the total number of hydrogen bonds between the two strands in this four-base section?
[1 mark]
A8
B9
C10
D12
(c)
Explain why adenine pairs only with thymine and guanine pairs only with cytosine.
[2 marks]
Total for question 5: 4 marks
6
A chemist analyses a mixture of amino acids by TLC. The solvent front moves 8.0 cm from the baseline. Spot X moves 3.2 cm and spot Y moves 5.6 cm. Under these conditions the reference Rf values are: glycine 0.30, serine 0.40 and valine 0.70. The spots are colourless until the plate is treated with a developing agent.
(a)
What is the Rf value of spot Y?
[1 mark]
A0.40
B1.43
C0.30
D0.70
(b)
Which reagent is used to locate the colourless spots of amino acids on the plate?
[1 mark]
AAnhydrous copper(II) sulfate
BNinhydrin
CAcidified potassium dichromate(VI)
DAqueous silver nitrate
(c)
Identify spots X and Y and explain why Rf values, rather than distances moved by the spots, are used to identify amino acids.
[2 marks]
Total for question 6: 4 marks
7
A biochemist makes a dipeptide in which valine, (CH₃)₂CHCH(NH₂)COOH, is the residue with the free amino group and serine, HOCH₂CH(NH₂)COOH, is the residue with the free carboxylic acid group.
(a)
Draw the structure of the dipeptide, identify the peptide link and name the small molecule lost when the link forms.
[3 marks]
(b)
The dipeptide is heated under reflux with 6 mol dm⁻³ hydrochloric acid. Give the structures of the two organic products as they exist in the acid solution and explain why they have these structures.
[4 marks]
Total for question 7: 7 marks
8
A student is revising nucleic acids. She is told that DNA has two complementary strands wound into a double helix, and that the anticancer drug cisplatin, cis-Pt(NH₃)₂Cl₂, a platinum(II) complex, works by binding to DNA in dividing cells.
(a)
Describe how a DNA nucleotide is made up, how nucleotides join to form a single strand, and how two strands are held together in a double helix.
[6 marks]
(b)
Explain how cisplatin prevents DNA replication in cancer cells and why it also has adverse effects on patients. Evaluate whether its use can be justified.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).