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3. Morphology of selected bacteriophages 3

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5. ϕ X174 The phage fX174 is an icosahedral phage that contains a circular single- stranded DNA molecule of 5386 nucleotides. It codes for 11 proteins, each of which has been identified. Adding together the size of all those proteins comes to 2330 amino acids, which would require 6990 nucleotides (3 2330) – substantially more than the total length of the genome Firstly the genes are very tightly packed – there is very little non-coding sequence in the genome. In most cases, the end of one gene is directly adjacent to (or slightly overlaps with) the start of the next. Secondly, one of the proteins (A*) corresponds to the C-terminal region of protein A 1 5

6. ϕ X174 6

7. Replication of single-strand bacteriophages 7

8. M13 M13, represent another type of single-stranded phage. These are ‘male-specific’ phages, since they infect the cell by attaching to the tips of the pili specified by the F plasmid As the replication cycle proceeds, one of the phage proteins that is produced is able to bind to the single-stranded DNA and divert it into the production of phage particles by targeting it to the cell membrane where it is extruded from the cell, with phage coat proteins being polymerized around it during its passage through the membrane 8

9. M13 9

10. MS2(3600 N) Another type of male-specific phage is represented by MS2 This is an icosahedral RNA- containing phage which attaches to the sides of the F- pili, rather than to the tip as M13 does. It is an extremely simple phage, containing some 3600 nucleotides,coding for just three genes: a coat protein, a maturation protein and a replicase 10

11. Bacteriophage T4(165Kb) 11

12. Terminal redundancy of bacteriophage T4. Multiple length linear DNA is the substrate for packaging into phage particles. The amount of DNA packaged in longer than the genome size, leading to terminal redundancy (a sequence of about 1600 base pairs at one end of the molecule is repeated in the same orientation at the other end) 12

13. Control of phage development 13

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19. 20 Control of the lytic/lysogenic switch in λ. CII stimulates transcription of the cI repressor gene from PE; CIII stabilizes CII. CI represses PL and PR and stimulates PM allowing further synthesis of repressor. Cro represses PL, PR and PM

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