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 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhy does the DNA double helix have the same diameter along its whole length, whatever the base sequence?
- Each base pair is made of two purines
- Each base pair has one purine and one pyrimidine
- Every base pair is held by the same number of hydrogen bonds
- Histone proteins hold the strands a fixed distance apart
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Answer: B [1]
Question 3
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 4
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 5
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 6
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]
Question 7
Paper 1B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanIn an experiment like that of Hershey and Chase, T2 bacteriophages labelled with either ³⁵S or ³²P were allowed to infect E. coli. After blending and centrifuging, the radioactivity in the pellet and supernatant was measured. (a) State which molecule each isotope labels. [1] (b) Analyse the results to deduce which molecule enters the bacteria. [2] (c) Suggest why the supernatant still contains some ³²P. [1]
| Isotope | Pellet (bacteria) / % | Supernatant / % |
|---|---|---|
| ³⁵S | 20 | 80 |
| ³²P | 85 | 15 |
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- (a) ³⁵S labels protein and ³²P labels DNA [1]
- (b) most ³²P (85%) is in the pellet / with the bacteria [1]
- (b) most ³⁵S (80%) is in the supernatant / stays outside [1]
- (b) so DNA enters the bacteria (and protein does not), so DNA is the genetic material [1]
- (c) some viruses had not injected their DNA / not all viruses infected bacteria / some bacteria burst or stayed in the supernatant OWTTE [1]
- [max 4]: max 1 for (a), max 2 for (b), max 1 for (c)
Question 8
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean(a) Describe the structure of a nucleosome. [2] (b) Explain how Chargaff's data falsified the tetranucleotide hypothesis. [2]
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- (a) DNA wrapped around a core of eight histone proteins [1]
- (a) an additional histone protein attached to linker DNA holds the DNA in place [1]
- (a) positive histones attract negatively charged DNA phosphates [1]
- (b) tetranucleotide hypothesis: DNA is a repeating sequence of the four bases, so all four in equal amounts [1]
- (b) Chargaff found base proportions differ between species / A + T is not equal to G + C [1]
- (b) only A = T and G = C, so the bases are not all equal and the hypothesis is false [1]
- [max 4]: max 2 for (a), max 2 for (b)