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Micro - Bacterial Genetics (i) (DNA rep steps (Elongation (on lagging…
Micro - Bacterial Genetics (i)
intro
genetic variation processes drive increased virulence + increased antibiotic resistance
bacterial genome
1 chromo
circular
dsDNA in a helix
antiparallel
4000 genes, 5 million bps
plasmids
small
circular
extrachromosomal
mobile
replicate independently
can be copied + transferred between cells
transfers phenotypic advantages (virulence, resistance, metabolic genes)
genetic exchange
plasmids with MDR genes common in hospital bacteria
maintenance driven by antibiotic selective pressure
hence nosocomial infections difficult to Tx
strict WC bp - increases fidelity (exactness)
topology: supercoiled via DNA gyrase (topoisomerase 2)
this enzyme also unravels supercoiling - essential to expose DNA for rep
inhibited by quinolones (e.g. ciprofloxacin)
rep = semiconservable (1 template, 1 new)
has operons
cluster of genes under control of a single promotor
rare in eukaryotes
allows coordinated expression of genes with related functions
e.g. the lac operon
controls genes involved in utilising lactose
-vely controlled (off til switched on when glucose isn't available)
LacL: repressor, produced most of time (binds to promote to block RNAP binding)
LacZ: Codes for beta-galactosidase - converts lactose to glucose + galactose
LacY+A: transport lactose into cell
when operon switched on inducer is produced (binds to LacL + prevents it binding to promoter)
DNA rep steps
Initiation
begins at origin of rep (oriC)
helicase unwinds dsDNA to expose ssDNA
Elongation
rep occurs in both directions from fork (leading + lagging strand)
DNAP adds complimentary bases (reads 5'-3')
on lagging strand RNA primase lays down multiple RNA primers (guides DNAP) between Okazaki fragments
discontinuous rep
@ end DNA ligase links fragments
Proof reading
corrects errors/altered nucleotide sequences
v low mutation rate (10^-8 - 10^-11 per base)
by DNAP
Termination
Transcription
initiated by promotor (upstream)
RNAP
binds to promotor + unwinds dsDNA ahead
transcribes DNA to mRNA
rifampicin
RNA synthesis inhibitor
forms stable complex with bacterial RNAP + prevents transcription initiation
doesn't affect mammalian RNAP :smiley:
bacteria adapt to changing environment by changing gene expression @ transcription level (switched on or off)
Translation
requirements
mRNA
ribosomes
tRNA
protein synthesis inhibitors
bind irreversibly to ribosome so mRNA can't
bactericidal
aminoglycosides (e.g. kanamycin)
macrocodes (e.g. erythromycin)
tetracyclines
streptomycin
chloramphenicol
bacteria only make proteins as required
Mutations
most common source of genetic variation
spontaneous or induced by mutagens
3 types
substitution
frameshift
insertion
deletion
nonsense -> early stop codon -> truncated protein