16 6 Ultraviolet (UV) radiation damages DNA and is used in microbiology to produce aseptic conditions. The UV radiation changes the bonding found in DNA. This change in bonding creates thymine dimers in which covalent bonds form between adjacent thymine nucleotides. Thymine dimers bend the DNA backbone. This prevents replication of DNA. The diagram shows the change in bonding caused by UV radiation.  thymine dimer A T C C T T G C T A G G A A C G (a) Name the bonds that are lost when thymine dimers are formed. (1) .................................................................................................................................................................................................................................................................................... (b) Explain why preventing the replication of DNA produces aseptic conditions. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 17 Turn over (c) Aseptic conditions are used when growing bacteria in laboratories. Justify two methods, other than UV radiation, for ensuring aseptic conditions. 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(d) The effectiveness of UV radiation was investigated, using three species of pathogenic bacteria. The table provides information about the bacteria used in this investigation. Bacteria Gram staining Infected part of body P. aeruginosa negative blood S. aureus positive lung E. coli negative intestine (i) Compare and contrast the structure of the wall of Gram positive bacteria and Gram negative bacteria. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 18 (ii) In this investigation, living bacteria of each species were exposed to UV radiation for four hours. The numbers of living bacteria were counted each hour during the four-hour period. The graph shows the results of this investigation. Key P. aeruginosa S. aureus E. coli 0 1 2 3 4 Time / h Number of bacteria per cm3 108 109 107 106 105 104 103 102 101 0 Calculate the percentage change in numbers of living E. coli after two hours of exposure to UV radiation. (2) Answer .............................................................. % 19 Turn over (iii) A student concluded from this investigation that if food is exposed to UV radiation, it would remove all risk of food poisoning. Criticise the validity of this conclusion. (4) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 6 = 14 marks)