coli cell injects its DNA through their tails as seen in electron micrograph T

Coli cell injects its dna through their tails as seen

This preview shows page 59 - 62 out of 75 pages.

T = 0 , cell attachment – T4 bacteriophage attaches to E. coli cell & injects its DNA (through their tails, as seen in electron micrograph) ~ T = 2, mRNA synthesis – synthesis of phagespecific mRNA begins ~ T = 6, DNA replication – replication of phage DNA begins, host DNA has been degraded by phageencoded nucleases ~ T = 14, assembly begins – DNAfilled heads, tails lacking tails fibers, & assembled tail fibers first appear ~ T = 17, assembled phage – first intact phage particles assembled ~ T = 25, host cell Lysis progeny released – host bacterium lysed, releasing about 300 progeny phage Bacteriophage T4 genetics: ~ Phage particle can be quantitated – phage count proportional to plaque count ~ Gene mapping can be performed by coinfection with 2 different mutants & screening for recombinants ~ Temperature sensitive mutants (grow at 25C but not 42C) Bacteriophage Lambda (l ): ~ Coliphage ~ dsDNA genome ~ Consists of 48 502 bp (~ 50 genes) ~ Linear DNA in head circularizes after host entry in tail (immediately) ~ Replicates by lytic or lysogenic pathway ~ Lytic pathway: Similar to T4 Replicates DNA Synthesizes viral proteins Assembles particles Host lysis ~ Lysogenic pathway: DNA inserts into host chromosome & replicates with host DNA Prophage – integrated state of l chromosome
Image of page 59
Lysogenics state = phage proteins not expressed Occasionally l prophage excises from host chromosome & enters lytic pathway Bacteriophage l lifecycle: ~ Lysogenic pathway: Integration into host chromosome Replication as part of host chromosome Capable of reactivation lytic pathway ~ Lytic pathway: Replication Assembly Cell Lysis l DNA integration occurs by site specific recombination: ~ Attachment sites: AttP on l chromosome attB on bacterial chromosome mediated by l integrase (l int gene) recombination occurs through 15 bp sequence common to both attP & attB - GCTTTTTTACTA - CGAAAAATGAT Excision also occurs through sitespecific recombination between sequences & requires action of l integrase & l excicase (l xis gene) - Note: excision occurs 1 in every 1000 replications Genetics of Bacteria: ~ Escherichia coli (strain K12) ~ Model organism for studying bacterial genetics ~ Single circular chromosome (4.64 million nucleotides, 4 377 genes) ~ Also harbor separate, small circular DNA molecules called plasmids ~ Plasmids: 2 to 10 000 nucleotides, only few genes Present in single or multiple copies (10 – 20) Single or multiple different plasmids/cell Replicate independently of bacterial chromosomes Use bacterial enzymes for replication Can transfer between bacteria Express genes required for plasmid transfer (F plasmids) Often contain genes providing resistance to cellular stresses (R plasmids) Isolation of bacterial mutants: ~ 3 methods ~ Mutants unable to utilize specific energy sources
Image of page 60
Example: wildtype Lac + & mutant Lac (unable to utilize lactose as sole energy source) ~ Mutants unable to synthesize essential metabolite Example: wildtype Leu + (prototroph) & mutant Leu (auxotroph) for leucine synthesis ~ Mutants resistant to drugs & antibiotics
Image of page 61
Image of page 62

You've reached the end of your free preview.

Want to read all 75 pages?

  • Fall '13
  • DNA, DNA Transcription & RNA Processing

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture