Question 1
Paper 1A style
Which combination is found in RNA but not in DNA?
- Deoxyribose and thymine
- Ribose and thymine
- Ribose and uracil
- Deoxyribose and uracil
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Answer: C [1]
Question 2
Paper 1B style
The table shows the percentage of each base in the nucleic acid from four sources. Some values are missing. (a) Calculate the percentage of guanine in human DNA. [2] (b) Deduce the percentage of cytosine in wheat DNA, giving a reason. [1] (c) Identify which sample is RNA and explain one piece of evidence for your answer. [2]
| Source | Adenine | Thymine | Guanine | Cytosine | Uracil |
|---|---|---|---|---|---|
| Human DNA | 30 | 30 | ? | ? | 0 |
| Wheat DNA | 27 | 27 | 23 | ? | 0 |
| Bacterial DNA | 25 | 25 | 25 | 25 | 0 |
| Sample X | 29 | 0 | 22 | 18 | 31 |
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- (a) A + T = 60%, so G + C = 100 − 60 = 40% [1]
- (a) G = C, so guanine = 20% [1]
- (b) 23% because cytosine pairs with guanine / G = C in double-stranded DNA [1]
- (c) sample X is RNA [1]
- (c) contains uracil and no thymine [1]
- (c) A ≠ U and G ≠ C, so it is single-stranded / has no complementary base pairing between strands [1]
- [max 5]: ECF in (a) for G = half of (100 − A − T)
Question 3
Paper 2A style
Calculate the number of different base sequences possible for a DNA strand 8 nucleotides long, and state what this shows about the capacity of DNA for storing information.
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- 4⁸ = 65 536 [1]
- any length and any base sequence is possible so the capacity for storing information is (effectively) limitless / very large [1]
Question 4
Paper 2A style
Draw a labelled diagram of a section of RNA consisting of two nucleotides, and outline how they are joined.
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- each nucleotide drawn as circle (phosphate), pentagon (sugar) and rectangle (base) with base and phosphate on different corners of the sugar [1]
- labels: phosphate, ribose / pentose sugar, base [1]
- bases named as RNA bases (A, U, G or C), not T [1]
- phosphate of one nucleotide bonded to the sugar of the next, forming a sugar–phosphate backbone [1]
- joined by condensation / water released [1]
- the bond is covalent [1]
- [max 4]
Question 5
Paper 2B style
Explain the role of complementary base pairing in allowing genetic information to be replicated and expressed.
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- A pairs with T (or U in RNA) and G pairs with C [1]
- complementarity is based on hydrogen bonding between bases [1]
- in replication the two DNA strands separate and each acts as a template [1]
- free nucleotides pair with complementary bases on the template [1]
- producing two identical DNA molecules / the base sequence is copied accurately [1]
- in transcription mRNA is built with bases complementary to the template strand of DNA [1]
- uracil pairs with adenine in transcription [1]
- in translation tRNA anticodons pair with complementary mRNA codons [1]
- so amino acids are linked in the sequence coded by the gene [1]
- the genetic code is shared by (almost) all organisms, so base sequences are translated the same way [1]
- [max 6]