Unit 3--Lecture 17 (Genetics of Bacteria and Archaea) Flashcards
The Transforming Principle
Experiments of Griffith
Infections of mice with strains of Streptococcus
Rough and smooth colony types
—smooth: dead
—rough: healthy
—heat killed smooth: healthy
—heat killed smooth & rough: dead
Phenotype due to capsule protection
Avery, MacLeod, and McCarty extended Groffith’s results
DNA is the transforming material
Competence
Competence: the ability to take up exogenous (naked) DNA
Some bacteria naturally competent
Genetic Transformation
Competent cells (natural or artificial)
Naked DNA taken up, incorporated, and expressed
Conjugation
Plasmid-directed transfer requires cell contact
Hfr Strains
Integrated F factor
Conjugal transfer
Generalized Transduction
Involves a lytic phage
Specialized Transduction
Involves a lysogenic phage
Defense Against Transferred DNA
Bacteria cut entering DNA at specific restriction sites
Bacteria add methyl groups to DNA
—-prevents restriction at those sites
Telling Them Apart
Transformation: needs naked DNA
—-DNase will inhibit transformation
Conjugation: needs cell contact
—-0.2 um membranes will disrupt conjugation
Transduction: involves bacteriophage
—-unaffected by either condition
Recombination
Entering DNA replaces chromosomal DNA
—-DNA enters via transformation, conjugation, transduction
—-also used to repair damaged DNA
—-requires specific recombination proteins
—-RecA
—-RecBCD
—-RuvAB
Mutations
Mutation: mistake during replication or damage to DNA
Wrong bases incorporated
—-transition
—-transversion
Indels are bad
Mutations
Mutants
Mutagens
Mutagens Cause Mutations
Electromagnetic radiation
—-UV light, gamma rays, x-rays
Point Mutations
A single base is altered in the sequence
Silent mutation: TAT to TAC»_space; Tyr to Tyr
Missense mutation: TAT to TTT»_space; Tyr to Phe
Nonsense mutation: TAT to TAA»_space; Tyr to Stop
5-Bromouracil Mutagenesis
A = T
A = 5BU
G = 5BU
G = C
A becomes G
Transition
Measuring Mutagen Strength
Ames test was created by Bruce Ames
Salmonella typhimurium used to test mutagens
His- mutant strain grown in the absence of histidine
Look for reversions to His+
DNA Repair
Mismatch repair:
–mispaired base cut out of strand
–strand without methyl groups is newer
–assumes to be in error
Thymidine dimers:
–induced by UV
–cut out by UvrAB complex
Damaged bases:
–excised by specific enzymes
–excised by specific enzymes
–replaced by DNA Polymerase I
Recombinational repair:
–occurs just after strand has replicated
–undamaged strand is copied
–replaced damaged strand
–catalyzed by RecA recombinase
SOS repair:
–extensive DNA damage inactivates LexA
–activation of many repair genes
–rapid polymerization of DNA
–error-prone, but better than no repair
