Transcript Genetics of Viruses & Bacteria

Genetics of Viruses
& Bacteria
Viruses
 Nucleic
acid in protein coat
 Infectious
 Discovered through studies of tobacco
mosaic disease
Viruses
 DNA
virus or RNA virus
 Can be single or double stranded
 Linear or circular nucleic acid
Viruses
– protein shell
that encloses genome
 Made of protein
subunits (capsomeres)
 Accessory structures to
enhance infecting ability
Viral envelope –
membrane from host
cell covering capsid
 Capsid
Viruses
 Bacteriophage
(phage)
 Virus that infects bacteria
 Capsid has polyhedral head &
tail attached
 Reproduce only within host cell
 Host range: types of cells viruses
infect
Based on recognition of
receptors
Tissue specific
Viral Infection
 Depends
on virus type
 DNA viruses – DNA polymerase of host
cell to make viral template & new
genome
 RNA viruses – viral-encoded
polymerases use RNA as template
 Both divert resources from host
 Reproduce in two ways:
1) Lytic Cycle
 DNA
viruses
 Phage reproductive cycle  host death
 If only this way  phage is “virulent
phage”
 Virus docks with receptors  injects
DNA  kills host DNA  makes
copies  bursts out of cell
2) Lysogenic Cycle
 Replicates
genome without host death
 Can do both  “temperate phage”
 DNA incorporates into host DNA
 It is reproduced as host reproduces
 When chromosomes circle up, triggers
lytic cycle
LYTIC AND
LYSOGENIC CYCLES
Which is this?
Animal Virus Replication
– viral DNA inserts into host
genome for replication (may activate
or inactivate throughout organism’s
life)
 RNA viruses
 Retrovirus
Enzyme – reverse
transcriptase
RNA used as template to make
 Provirus
Animal Virus Replication
DNA
integrates
(provirus)
Host transcribes viral
DNA into mRNA &
RNA for new
infection sites
Example: HIV
FYI…Other Infectious Agents
 Prion
 Infectious
protein
 Misfolded
 Causes
cell
mistakes in regular proteins in
Bacteria
 One
circular DNA molecule (double
stranded)
 Chromosome packed in one region
– nucleoid
 Also have plasmids
 Binary fission for reproduction
 Mutation & genetic recombinations
increase diversity
Genetic Recombination
3
steps
 1) Transformation
Alteration of bacterial cell’s
genotype by uptake naked, foreign
DNA from surrounding
environment by
surface proteins
Genetic Recombination
Foreign
allele incorporated into
bacterial chromosome
Now have recombinant
Genetic Recombination
 2)
Transduction
Phages carry bacterial genes from
one host cell to another
Two types
Generalized
Specialized
Genetic Recombination
 3)
Conjugation
Direct transfer of genetic
material between two
bacterial cells that are joined
One way (“Male” to “Female”)
“Male” hooks “female” & donates DNA
Male has special DNA (F factor)
within chromosome or as plasmid
Plasmids
 Small,
circular, self-replicating DNA
molecule separate from bacterial
chromosomes
 Beneficial for recombination; not
necessary for survival
 R plasmid allows bacteria to be
antibiotic resistant
Transposons
 Transposable
genetic
elements
 “Jumping genes”
 Section or copy of
section moves
within genome
 Results in recombination
Bacterial Gene Expression
 Adjust
enzymatic activity by use of
operons
Operons
 Operator
 Located
within promoter (where
RNA polymerase binds) or
between promoter & coding
genes
 Controls access of RNA
polymerase to the genes (so
controls if gene is on)
 Operator + Promoter + Genes
(DNA to make enzymes) = Operon
Operons
 Operon
switched off by protein
called repressor
 Binds to operator; blocks RNA
polymerase
 Reversible
 Corepressor: small molecule
cooperating with repressor to turn
operon off
Trp Operon
Function – make tryptophan when none is
present
 If tryptophan is present…
 No need to make trp
 Repressor is activated by the trp
(corepressor)
 Operon is blocked and switched off
 Opposite if trp is absent
Trp operon

Trp Operon
 Example

of repressible operon
Transcription inhibited when
repressor binds allosterically to
regulatory protein
Lac Operon
 Example
of inducible operon
 Stimulated when binding of inducer to
repressor
 Inactivates the repressor
 Lac operon
Lac
Operon
Quiz
Lac operon
Function – make enzymes to break down
lactose and allolactose
 If lactose is present…
 Need to make enzymes to break it down
 Lactose & allolactose (inducers) binds to
repressor to inactivate it
 Operon works & transcription occurs
 Opposite if lactose is absent

Lac Operon
 Operons
switched on by presence of
another molecule (“volume control”)
 Ex) cAMP