1. 1
   Three viruses are compared. Bacteriophage lambda infects the bacterium Escherichia coli; it has a protein head of fixed shape containing double-stranded DNA, attached to a protein tail, and has no outer membrane. SARS-CoV-2, a coronavirus, contains a single strand of RNA packed with protein, surrounded by a lipid envelope that carries protruding spike glycoproteins. HIV contains two copies of single-stranded RNA and the enzyme reverse transcriptase inside a cone-shaped protein core, which is itself surrounded by a lipid envelope with glycoproteins. None of the three contains cytoplasm or ribosomes.

   (a)

   Which structural feature is shared by all three viruses?

   [1 mark]

   • AA lipid envelope derived from a host cell
   • BDouble-stranded DNA as the genetic material
   • CThe enzyme reverse transcriptase
   • DA capsid made of protein

   (b)

   What is the origin of the lipid envelope of HIV?

   [1 mark]

   • AIt is host cell membrane taken by the virus as it buds out of the cell
   • BIt is synthesised by enzymes coded for and carried inside the virus
   • CIt is formed from the cell wall of a bacterium
   • DIt is formed when the capsid proteins fold into a bilayer

   (c)

   Outline why the features shared by viruses do not necessarily indicate that all viruses have a single common origin.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A culture of Escherichia coli was infected with bacteriophage lambda so that 2.0 × 10⁶ bacterial cells were each infected at the same moment. Samples were taken every 5 minutes and the number of free (extracellular) phage particles was measured. For the first 45 minutes, no increase in free phages was detected. Between 45 and 60 minutes the number rose sharply, and from 60 minutes onwards it stayed constant at 1.8 × 10⁸, by which time all the infected bacteria had burst.

   (a)

   Why were no free phages detected during the first 45 minutes?

   [1 mark]

   • AThe phages had attached to the bacteria but not injected their DNA
   • BThe bacteria were destroying the phage DNA with enzymes
   • CThe phage DNA was inside the bacteria, being replicated and used to make phage proteins, before any new phages were released
   • DThe phages were reproducing by binary fission inside the bacteria

   (b)

   Which process releases the new phages from the bacteria at the end of the lytic cycle?

   [1 mark]

   • AThe phages bud out, taking bacterial membrane with them
   • BThe bacterial cell is lysed by enzymes coded for by the phage
   • CThe bacterium divides and the phages are released into the daughter cells
   • DThe phages are secreted by exocytosis

   (c)

   Calculate the mean number of phages released by each infected bacterium.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In some Escherichia coli cells infected by bacteriophage lambda, the phage DNA is inserted into the bacterial chromosome at a specific site. A phage gene in the inserted DNA produces a repressor protein that switches off the phage genes needed for making new phages. These bacteria grow and divide normally for many generations and are not killed if more lambda phages are added. When a culture of these bacteria was exposed to ultraviolet light, which damages DNA, most cells burst within about an hour, releasing new phages. Lambda follows this pathway more often when several phages infect each cell or when the bacteria are short of nutrients.

   (a)

   Outline the lysogenic cycle of bacteriophage lambda and explain why these bacteria are not killed when more lambda phages are added.

   [3 marks]

   (b)

   Explain the effect of ultraviolet light on the culture and suggest why the ability to switch between the lysogenic and lytic cycles is an advantage to lambda.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   HIV copies its RNA genome into DNA using reverse transcriptase, an enzyme with no proofreading, which makes about one error per 10,000 bases copied. The HIV genome is about 9,700 bases long, and an untreated patient produces around 10¹⁰ new virus particles per day. In the early 1990s, patients treated with a single drug, zidovudine, had virus resistant to it within about six months; today a combination of three drugs taken together can keep the virus suppressed for decades, although there is still no vaccine. Influenza viruses also have an RNA genome copied without proofreading, divided into eight separate segments. The composition of the seasonal influenza vaccine is reviewed twice a year, and its effectiveness varies between about 10% and 60% from season to season.

   (a)

   Using the information about HIV and influenza, explain why some viruses evolve very rapidly.

   [6 marks]

   (b)

   Discuss the consequences of rapid viral evolution for the treatment and prevention of HIV and influenza.

   [6 marks]

   Total for question 4: 12 marks

End of questions