Turn over 7 4 Bacteria are the host cells for λ (lambda) phage viruses. (a) Which is a description of a λ phage? (1) A DNA virus with a complex protein capsid B DNA virus with a helical protein capsid C RNA virus with a complex protein capsid D RNA virus with a helical protein capsid (b) The image shows phage viruses attacking a bacterium. bacterium phage (Source: © nobeastsofierce Science/Alamy Stock Photo) The length of this bacterium is 1.7 µm. Calculate the length of the labelled phage. Give your answer in nanometres (nm). (2) Answer .............................................................. nm 8 (c) Viruses can be cultured and a growth curve can be produced. The flow chart shows how this can be done. Grow the host cells of the virus on agar Add the viruses to the host cells View the cells under a microscope and count the number of lysed (burst) cells Repeat counts at regular intervals The graph shows a growth curve for viruses. 6 5 4 3 2 1 0 20 40 60 80 100 120 Time after adding viruses to cells / min Log10 number of lysed cells Turn over 9 (i) Explain why there was a delay before the number of lysed cells started to increase. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Calculate the mean rate of increase in the actual number of lysed cells between 50 minutes and 90 minutes. (2) Answer .............................................................. cells min–1 (iii) A sketch has been made of this growth curve. Time after adding viruses to cells Log10 number of lysed cells Complete this sketch to predict the shape of the growth curve after 120 minutes, assuming there is an excess of host cells. (2) (Total for Question 4 = 10 marks) 10 BLANK PAGE