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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Recall the three components of a nucleotide.
- S2
The three components are a pentose sugar, a phosphate group, and an organic base.
- S3
Step 2: Identify the specific components for DNA.
- S4
For 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).
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Recall the three components of a nucleotide.
- S2
The three components are a pentose sugar, a phosphate group, and an organic base.
- S3
Step 2: Identify the specific components for DNA.
- S4
For 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)
Common mistakes
- Confusing deoxyribose with ribose. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)3 marks