Question 1
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 meanWhich feature is shared by all viruses?
- An envelope derived from host cell membrane
- A capsid made of protein
- Double-stranded DNA
- The enzyme reverse transcriptase
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Answer: B [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 meanDuring the lysogenic cycle of bacteriophage lambda, what happens to the viral DNA?
- It is packed into new capsids straight away.
- It is transcribed to make enzymes that lyse the cell.
- It is integrated into the host chromosome and replicated when the host divides.
- It is broken down by host enzymes.
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Answer: C [1]
Question 3
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 meanThe viral load (number of copies of HIV RNA per cm³ of blood) was monitored in two groups of patients. One group took a single antiviral drug and the other took a combination of three drugs with different targets. The data are for practice. (a) Calculate the percentage decrease in viral load in the single-drug group between week 0 and week 8. [1] (b) Describe the trend in viral load in the single-drug group after week 8. [1] (c) Explain the change in viral load in the single-drug group after week 8. [2] (d) Suggest why the viral load stayed low in the combination group. [2]
| Time / weeks | Single drug / copies per cm³ | Three-drug combination / copies per cm³ |
|---|---|---|
| 0 | 100 000 | 100 000 |
| 4 | 2 000 | 1 500 |
| 8 | 1 500 | 200 |
| 12 | 9 000 | below 50 |
| 16 | 40 000 | below 50 |
| 24 | 90 000 | below 50 |
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- (a) (100 000 − 1 500) ÷ 100 000 × 100 = 98.5% [1]
- (b) increases from 1 500 to 90 000 copies per cm³ / rises back almost to the starting level by week 24 [1]
- (c) HIV has a high mutation rate / reverse transcriptase makes errors / does not proofread [1]
- (c) some variants are resistant to the drug [1]
- (c) the drug selects for resistant variants, which multiply / natural selection [1]
- max 2 for (c)
- (d) a variant would need mutations giving resistance to all three drugs at the same time [1]
- (d) this is very unlikely / the probability is very low [1]
- (d) the drugs act on different targets, so replication is blocked at more than one stage [1]
- max 2 for (d)
Question 4
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 meanCompare and contrast the structure of bacteriophage lambda and HIV. [4]
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- both have a protein capsid [1]
- both have nucleic acid as genetic material / both lack cytoplasm [1]
- lambda has double-stranded DNA; HIV has single-stranded RNA [1]
- lambda is not enveloped; HIV is enveloped in host cell membrane [1]
- HIV has glycoproteins in its envelope that bind host receptors; lambda binds the host with its tail [1]
- lambda has an icosahedral head and a tail; HIV has a cone-shaped capsid [1]
- HIV carries reverse transcriptase / integrase / protease; lambda carries no enzymes [1]
- max 4, with at least one similarity for full marks
Question 5
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanDescribe the lytic cycle of bacteriophage lambda. [5]
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- the phage tail attaches to specific receptors on E. coli [1]
- viral DNA is injected; the capsid stays outside [1]
- the viral DNA circularises [1]
- viral genes are transcribed by host RNA polymerase [1]
- and translated by host ribosomes to make capsid proteins / viral enzymes [1]
- the host supplies ATP / energy, nucleotides and amino acids [1]
- viral DNA is replicated many times [1]
- new phages are assembled / DNA is packed into heads [1]
- viral enzymes break down the cell wall [1]
- the cell lyses / bursts, releasing many (about 100) phages [1]
- released phages infect other cells [1]
- max 5
Question 6
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 meanOutline the evidence that viruses have several origins. [3]
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- viruses are very diverse in structure / in genetic material (DNA or RNA, single- or double-stranded) [1]
- suggesting they did not all descend from one common ancestor [1]
- their shared features may be the result of convergent evolution [1]
- because all share the same extreme obligate parasitism / way of life [1]
- the genetic code is shared with living organisms, suggesting origins from cells [1]
- some may have escaped from cells (e.g. as plasmids), others reduced from parasitic cells [1]
- max 3
Question 7
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 meanExplain why a new influenza vaccine is produced each year. [3]
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- influenza has an RNA genome with a high mutation rate / its RNA polymerase does not proofread [1]
- mutations change the surface antigens haemagglutinin / neuraminidase [1]
- this is antigenic drift [1]
- antibodies / memory cells from earlier vaccines no longer bind / recognise the new strain [1]
- segments from two strains can mix in one host (antigenic shift), producing a new subtype [1]
- the vaccine is matched to the strains predicted to circulate [1]
- max 3