A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Nucleotides in DNA and RNA — mark scheme explained
The short answer
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are crucial information-carrying molecules in all living cells. DNA holds genetic information, while RNA transfers this information from DNA to the ribosomes for protein synthesis.
The question
Identify the components of a nucleotide in DNA.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For identification questions, marks are typically awarded for correctly naming components or structures. For explanatory questions, marks are given for clear and concise descriptions that demonstrate a deep understanding of the topic. For descriptive questions, marks are awarded for providing accurate details without necessarily explaining the underlying principles.
What the command words demand
- Identify
- Recognize and name specific components or structures.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear account of the structure or process without necessarily providing an explanation.
- Compare
- Highlight similarities and differences between two or more concepts or structures.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- Recall the three components of a nucleotide.3 marksThe three components are a pentose sugar, a phosphate group, and an organic base.
- Identify the specific components for DNA.0 marksFor DNA, the pentose sugar is deoxyribose, the phosphate group remains the same, and the organic bases are adenine (A), cytosine (C), guanine (G), and thymine (T).
Final answer: Pentose sugar: Deoxyribose; Phosphate group; Organic bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T)
Work through every step correctly and you earn all 3 marks.
Another worked example
Explain the base pairing rules in DNA.
- Recall the four organic bases in DNA.0 marksThe four organic bases are adenine (A), cytosine (C), guanine (G), and thymine (T).
- State the base pairing rules.2 marksAdenine pairs with Thymine, and Cytosine pairs with Guanine.
Final answer: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G)
Work through every step correctly and you earn all 2 marks.
Common mistakes
Confusing deoxyribose with ribose.
Why it happens: Students may mix up the pentose sugars in DNA and RNA, leading to incorrect answers when identifying components of nucleotides.
Fix: Always remember that deoxyribose is found in DNA, while ribose is found in RNA. Deoxyribose has one less oxygen atom compared to ribose.
Misremembering base pairing rules in DNA.
Why it happens: Students might confuse which bases pair with each other, leading to incorrect answers when applying base pairing rules.
Fix: Practice the base pairing rules: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).
Forgetting that RNA has uracil instead of thymine.
Why it happens: Students may mistakenly think that RNA uses the same bases as DNA, leading to confusion when identifying organic bases in RNA.
Fix: Remember that RNA uses adenine (A), cytosine (C), guanine (G), and uracil (U) instead of thymine (T).
Misunderstanding the role of hydrogen bonds in DNA.
Why it happens: Students may not fully grasp how hydrogen bonds contribute to the stability of the double helix structure, leading to incomplete or incorrect explanations.
Fix: Understand that hydrogen bonds between complementary base pairs (A-T and C-G) hold the two polynucleotide chains together in DNA, providing structural stability.
Confusing the formation of phosphodiester bonds with other types of bonds.
Why it happens: Students may mix up the type of bond formed between nucleotides, leading to incorrect answers when describing polynucleotide chain formation.
Fix: Always remember that a phosphodiester bond forms between the 3' carbon of one pentose sugar and the 5' carbon of the next nucleotide during condensation reactions.
Failing to explain the double helix structure of DNA clearly.
Why it happens: Students may struggle to articulate the double helix structure and its significance, leading to vague or incorrect explanations.
Fix: Practice describing the double helix structure: two polynucleotide chains held together by hydrogen bonds between complementary base pairs (A-T, C-G). Emphasize that this structure provides stability and allows for accurate replication of genetic information.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Identify Nucleotide Components | Name the three parts of a DNA nucleotide and their specific DNA forms. | 3 |
| State Base Pairing | Explain the complementary base pairing rules that hold the two DNA strands together. | 2 |
| Describe Biological Process | Describe how nucleotides join by condensation reactions to build a DNA polynucleotide chain. | 4 |
| Compare DNA And RNA | Identify structural similarities and differences between DNA and RNA molecules. | 4 |
| Total across these question types | 13 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.