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Genetics of Bacteriophages

Genetics of Bacteriophages. 1. Lytic growth of bacterio phages. 2. Morphology of selected bacteriophages. 3. Scaffolded assembly. Assembly of complex phages is assisted by scaffolding proteins. These are removed once assembly is complete. 4. ϕ X174.

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Genetics of Bacteriophages

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  1. Genetics of Bacteriophages 1

  2. Lytic growth of bacterio phages 2

  3. Morphology of selected bacteriophages 3

  4. Scaffolded assembly. Assembly of complex phages is assisted by scaffolding proteins. These are removed once assembly is complete 4

  5. ϕX174 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 Single-stranded DNA bacteriophages 5

  6. ϕX174 Single-stranded DNA bacteriophages 6

  7. After the single-stranded ϕX174DNA enters the cell, it is converted into a double-stranded molecule (replicative form, RF) by synthesis of the complementary (or ‘minus’) strand (step A). • The minus strand provides a template for the production of further copies of the plus strand (step B). • These are in turn converted to the double-stranded replicative form by synthesis of the complementary (minus) strand (step C). 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 Single-stranded DNA bacteriophages 8

  9. M13 Single-stranded DNA bacteriophages 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 RNA-containing phages 10

  11. Bacteriophage T4(165Kb) Double-stranded DNA phages 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 B. subtilis phage SPO1 13

  14. Replication of bacteriophage lambda DNA. After infection, the cohesive ends are ligated to form a circular molecule which replicates (theta mode) to generate more circular DNA. Later, replication switches to the rolling circle mode, generating multiple length linear DNA for packaging into phage particles 14

  15. Cutting λ DNA at the cos sites 15

  16. LysogenyStructure of the attachment sites of l. (a) Integration requires site-specific recombination between attP (on the phage) and attB (on the chromosome). (b) After integration, the sites at either end of the prophage (attL and attR) have the same core sequence (shaded) as attP and attB but different flanking sequences 16

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  19. 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 20

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