AQA AS Level Biology Paper 2, June 2019: Question 3

Mark scheme

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Question Marking Guidance Mark Comments

Watson and Crick Correct spelling

03.1 OR 1 Ignore first/given/fore

Crick and Watson; names

03.2 DNA polymerase; 1 Correct spelling

Role of single-stranded DNA fragments 3

1. Template;

2. Determines order of nucleotides/bases;

3. Ignore forms

03.3 complementary bases

Role of DNA nucleotides

3. Forms complementary pairs / A – T, G - C 3. Accept sequence/

chain for strand

OR

Forms complementary (DNA) strand;

TOTAL 5

How to answer it

DNA Structure and Replication Study Guide

What this question tests

This question assesses your factual recall of historical scientific discoveries regarding DNA, key terminology regarding enzymes involved in DNA synthesis, and your deep understanding of the molecular mechanisms underpinning semi-conservative DNA replication (specifically the roles of templates and free nucleotides).

Question 03.1 (1 mark)

Historical Models of DNA

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication.

✅ Correct Answer

Watson and Crick (or Crick and Watson)

💡 Key Knowledge

James Watson and Francis Crick deduced the double helix structure of DNA in 1953, utilizing data from Rosalind Franklin, Maurice Wilkins, and Erwin Chargaff.

🧠 Exam Technique

Keep scientist recall concise. You do not need first names or initials. Examiners will accept either order as long as both surnames are present.

❌ Common Errors

Including incorrect pairings like "Watson and Franklin" or misspelling surnames (e.g., "Watson and Creek").

Mark breakdown: 1 mark awarded for naming both Watson and Crick correctly.

Question 03.2 (1 mark)

Enzymes in DNA Replication

A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with identical single-stranded DNA fragments and a solution containing DNA nucleotides. Name the enzyme used in this DNA replication.

✅ Correct Answer

DNA polymerase

💡 Key Knowledge

DNA polymerase is the key enzyme responsible for joining individual nucleotides together via phosphodiester bonds during DNA strand elongation.

🧠 Exam Technique

Be precise with biological terminology. Do not confuse this with transcription enzymes like RNA polymerase, or unwinding enzymes like DNA helicase.

❌ Common Errors

Writing just "polymerase" without specifying DNA polymerase, or writing "DNA helicase" (helicase unwinds the double helix, but here single-stranded fragments were already provided).

Mark breakdown: 1 mark for accurately identifying DNA polymerase with correct spelling.

Question 03.3 (3 marks)

Mechanisms of Semi-Conservative Replication

Use your knowledge of semi-conservative replication of DNA to suggest: 1. the role of the single-stranded DNA fragments. 2. the role of the DNA nucleotides.

✅ Correct Answer

Role of single-stranded fragments: Acts as a template; determines the order/sequence of nucleotides/bases.
Role of DNA nucleotides: Forms complementary pairs (A to T, C to G) OR forms the complementary (DNA) strand.

💡 Key Knowledge

Semi-conservative replication relies on complementary base pairing. Each original strand acts as a template guide so that new strands are exact copies of the parent molecule.

🧠 Exam Technique

This is a multi-part question worth 3 marks. Make sure you clearly address both components requested: the template strand function and the incoming nucleotide function.

❌ Common Errors

Vague phrasing such as "holding the DNA together" or stating nucleotides "provide energy" in this specific context without linking to base-pairing or strand formation.

Mark breakdown: 1 mark for template role, 1 mark for determining base sequence, and 1 mark for forming complementary pairs/strands.

Topics

Biology · 3.1 Biological molecules