AQA A-Level Biology Paper 1, June 2023: Question 10

15 marks · Medium difficulty · Extended Answer

Describe quaternary protein formation, DNA and chromosome structure, and other processes causing genetic variation.

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Question

Question text

10.1 Describe how a quaternary protein is formed from its monomers.

Do not include the process of translation in your answer.

[5 marks]

10.2 Describe the structure of DNA and the structure of a chromosome.

[6 marks]

10.3 Mutation can result in an increase in genetic variation within a species.

Describe and explain the other processes that result in increases in genetic variation

within a species.

[4 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

3. Accept alpha helix

1. Amino acids joined by peptide bond(s);

OR β-pleated sheet

2. (By) condensation reaction(s); for ‘secondary

structure’

3. Secondary structure is formed by hydrogen o

4. Accept 3 for

bonding;

tertiary

4. Tertiary structure formed by interactions 5 4. and 5. Accept for

10.1 (between R groups); (5 x ‘interactions’,

5. Quaternary structure contains >1 polypeptide AO1) hydrogen bonds OR

disulfide bridges OR

OR ionic bonds OR

hydrophobic OR

Quaternary structure formed by hydrophilic

interactions/bonds between polypeptides; interactions

5. Ignore peptide

1. Polymer of nucleotides; 1. Accept

‘polynucleotide’

2. (Nucleotide) consists of deoxyribose,

phosphate and an organic/nitrogenous base;

3. Phosphodiester bonds (between nucleotides); 2. Accept ‘phosphoric

acid’ for phosphate

4. DNA double helix held by H bonds

6 max 5. Ignore bases

10.2 identified with letters

(6 x

2 strands held by H bonds; (A, T, G, C)

AO1)

5. (Hydrogen bonds/pairing) between adenine,

5. Reject adenosine

thymine and cytosine, guanine;

and cysteine

6. DNA is associated with histones/proteins;

Accept correctly

7. (During mitosis/when visible) chromosome annotated diagram(s)

consists of two chromatids joined at a for equivalent marking

centromere; points

1. Independent segregation of homologous If no marks awarded

Chromosomes/pairs; accept one principle

mark ‘Meiosis

2. Crossing over between homologous

producing cells that

chromosomes/pairs;

are genetically

3. Random fertilisation of gametes; different (from one

another)’

4. (Produces) new combinations of alleles;

1. For ‘independent’

accept ‘random’

1. For ‘segregation’

accept ‘assortment’

26 2. Accept ‘within

bivalent’ for ‘between

4 homologous pair’

10.3 (4 x

AO1) 3. Ignore ‘random

mating’

3. Accept ‘random

fusion’ for ‘random

fertilisation’

Accept as an

additional mark point

5. (Produces) new

combinations of

maternal and

paternal

chromosomes

Ignore reference to

epigenetics

How to answer it

Proteins, DNA Structure, and Genetic Variation Study Guide

What this question tests

This sequence of questions assesses core biological knowledge across three major topics: Protein Structure (polypeptide folding levels from monomers), Nucleic Acids & Chromosomes (polynucleotide structure and eukaryotic chromosome organisation during cell division), and Genetic Variation (meiotic processes and random fertilisation excluding mutations). It rigorously tests AO1 recall and precise biological terminology.

Question 10.1 (5 Marks)

Describe how a quaternary protein is formed from its monomers. Do not include the process of translation in your answer.

✅ Correct Answer / Mark Scheme

Amino acids are joined together by peptide bonds.
Condensation reactions occur during peptide bond formation (releasing water).
Secondary structure (alpha helices / beta-pleated sheets) is formed and maintained by hydrogen bonding.
Tertiary structure is formed through interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic/hydrophilic interactions) between R groups.
Quaternary structure contains more than one polypeptide chain (or is formed by interactions/bonds between multiple polypeptides).

💡 Key Knowledge

You must trace the hierarchy of protein structure sequentially: monomers (amino acids) → primary → secondary → tertiary → quaternary. Never skip levels when asked to describe formation.

🧠 Exam Technique

Pay close attention to negative constraints: "Do not include the process of translation" means mentioning ribosomes, tRNA, mRNA, or codons will waste time and risk confusion, though it won't directly negate valid points unless incorrect.

❌ Common Errors

Confusing bonds: stating that peptide bonds hold tertiary or quaternary structures together. (Peptide bonds are strictly primary structure).
Vague descriptions of tertiary bonds—always specify that bonds form between R groups.

Mark allocation: 5 marks total (5 × AO1). Awarded point-by-point based on the marking guidance above.

Question 10.2 (6 Marks)

Describe the structure of DNA and the structure of a chromosome.

✅ Correct Answer / Mark Scheme

DNA is a polymer of nucleotides (or polynucleotide).
Each nucleotide consists of deoxyribose sugar, a phosphate group, and an organic/nitrogenous base.
Nucleotides are joined by phosphodiester bonds.
DNA forms a double helix held together by hydrogen bonds between complementary base pairs.
Specific base pairing occurs between adenine and thymine, and between cytosine and guanine.
DNA is associated with histones / proteins (forming chromatin).
When visible during mitosis, a chromosome consists of two sister chromatids joined at a centromere.

💡 Key Knowledge

A complete description must bridge the molecular level (nucleotides, phosphodiester bonds) and the structural macroscopic level (double helix, histones, sister chromatids linked by a centromere).

🧠 Exam Technique

Max 6 marks. Make sure you cover both parts of the prompt: DNA structure and chromosome structure. Examiners frequently see students write extensively about DNA bases and completely forget to describe what a chromosome looks like.

❌ Common Errors

Writing "adenosine" instead of adenine (adenosine is a nucleoside, not a nitrogenous base).
Confusing phosphodiester bonds (backbone) with hydrogen bonds (between strands).

Mark allocation: 6 marks max (6 × AO1).

Question 10.3 (4 Marks)

Mutation can result in an increase in genetic variation within a species. Describe and explain the other processes that result in increases in genetic variation within a species.

✅ Correct Answer / Mark Scheme

Independent segregation of homologous chromosomes during meiosis (producing new combinations of maternal and paternal chromosomes).
Crossing over between homologous chromosomes (within a bivalent) during meiosis I, creating new combinations of alleles.
Random fertilisation of gametes (or random fusion of gametes), producing unique combinations of alleles in the zygote.

💡 Key Knowledge

Meiosis and sexual reproduction shuffle existing alleles rather than creating new ones (which mutation does). Ensure you name the specific meiotic stages and mechanisms correctly.

🧠 Exam Technique

Note the word "other" in the question stem—do not discuss mutations! If zero marks are awarded for specific mechanisms, examiners will accept the principle mark: "Meiosis producing cells that are genetically different from one another."

❌ Common Errors

Using imprecise terminology like "random mating" instead of "random fertilisation of gametes".
Stating crossing over happens between sister chromatids (it must be between non-sister chromatids of a homologous pair, though the mark scheme credits "between homologous chromosomes/pairs").

Mark allocation: 4 marks total (4 × AO1).

Topics

Biology · 3.1 Biological molecules · 3.4 Genetic information, variation and relationships between organisms

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.