Transcript Virus - My CCSD

Genetics of Viruses and Bacteria
Viral structure
 Virus:
“poison” (Latin); infectious particles
consisting of a nucleic acid in a protein
coat (there are MANY, MANY types of
viruses)
 Composition of virus
 Capsid: protein shell that encloses the
viral genome (the protein subunits are
called capsomeres)
 DNA or RNA that is inserted into infected
cells
Examples of viruses
Virus structure (cont.)
 Other
accessories for viruses/virus
types:
 Membranous envelope that
allows a virus to “fool” a cell
membrane and allow the virus to
enter the cell (viral envelope)
 Bacteriophage (phage): viruses
that are able to infect bacteria
General features of viral
reproduction
 Viruses
are intracellular parasites
 They need a host cell to reproduce
 They lack enzymes, ribosomes and all
other machinery needed to make
proteins
 Viruses can only infect a limited range of
cells (host range)
 This is why diseases are usually species
or tissue specific
Lytic Cycle


The lytic cycle is a viral reproductive strategy that
results in the death of the host cell
 Attachment: virus binds to a specific receptor
site on the outer membrane
 Injection: the viral DNA/RNA is inserted into the
cell membrane
 Synthesis: the viral DNA directs the production
of viral proteins and the synthesis of viral
nucleotides
 Assembly: the synthesized viral material is
assembled
 Release: the viral particles are released from
the organism, thereby destroying the host cell
Virulent virus (phage reproduction only by the lytic
cycle)
Lytic cycle
Lysogenic Cycle
 Genome
replicated w/o destroying the host cell
 Very similar to the lytic cycle
 Key differences:
 Genetic material of virus becomes incorporated
into the host cell DNA by recombination (uses
crossing-over)at a specific chromosomal loci
 The incorporated viral DNA is known as a
prophage
 Once the prophage synthesizes its material, it
circulates in the cell
 Temperate virus (phages capable of using the lytic
and lysogenic cycles)
 May give rise to lytic cycle
Lysogenic cycle
Animal Viruses
 Viruses
that infect animals are extremely
varied
 They can be double stranded or single
stranded
 They can be made of DNA or RNA
 They can have an outer membrane (viral
envelope) or not
 PURPOSE: The reason for the extreme
variability in viral composition is to enter cells
and utilize their reproductive machinery
Retroviruses (class of
RNA Viruses
 Retroviruses:
a class of RNA virus
that can use an RNA template to
transcribe its nucleotides into the
DNA template
 Uses an enzyme called reverse
transcriptase
 One deadly example of a
retrovirus is HIV
This is the virus that leads to the
disease known as AIDS
Retrovirus (HIV)
HIV (cont.)
Unlike
a prophage in bacteria,
the integrated viral DNA
(provirus) is a permanent part
of the cells genotype
The cell will continue to
synthesize the virus for the life of
the cell
How do we fight viruses?
 Viruses
are extremely damaging
 They utilize our own cellular
machinery to produce, infect and
destroy our own cells
 With the creation of vaccines
(harmless variants of pathogenic
microbes), we can condition our
body to destroy the infection
before it can result in illness
Why do we still have viruses?
 With
the advent of vaccination, a lot of
diseases have become extinct (polio or
small pox)
 Yet, viruses have a high level of
mutation
 They are constantly changing to “fool”
your bodies immune system
 Even the influenza virus (flu) mutates
every year so that you must get a new
flu vaccine each season
 Also we do not understand enough
about some viruses to create a vaccine
Viroids and prions


Viroids: tiny, naked
circular RNA that infect
plants; do not code for
proteins, but use cellular
enzymes to reproduce;
stunt plant growth
Prions: “infectious
proteins”; “mad cow
disease”; trigger chain
reaction conversions; a
transmissible protein
Bacterial genetics
 Nucleoid:
region in
bacterium densely
packed with DNA
(no membrane)
 Plasmids: small
circles of DNA
(separate from
bacterial genome)
 Reproduction:
binary fission
(asexual)
Bacterial DNA-transfer
processes
 Transformation:
genotype alteration by the uptake
of naked, foreign DNA from the environment
 Transduction: phages that carry bacterial genes
from 1 host cell to another
 Generalized: random transfer of host cell
chromosome
 Specialized: incorporation of prophage DNA
into host chromosome
 Conjugation: direct transfer of genetic material;
cytoplasmic bridges; pili; sexual
Bacterial Plasmids
 Small,
circular, self-replicating DNA
separate from the bacterial
chromosome
 F (fertility) Plasmid: codes for the
production of sex pili (F+ or F-)
 R (resistance) Plasmid: codes for
antibiotic drug resistance
Transposable elements
 Transposable
elements: nucleotide sequences
that can move from one site in a chromosome or
plasmid to another site
 Insertion sequence: (only in bacteria) can move
one gene from one site to another
 Transposons: transposable genetic element; piece
of DNA that can move from location to another in
a cell’s genome (chromosome to plasmid, plasmid
to plasmid, etc.); “jumping genes”
 This allows genetic information to be
incorporated or passed on to other bacteria
Incorporation of a plasmid
Operons (the basic idea)
 For
many proteins, there is a segment of DNA
where all of the necessary genes are grouped
together
 Therefore, you only need a single promoter site
where RNA polymerase can begin to transcribe the
DNA code
 Near the promoter site is a stretch of DNA that
controls whether RNA polymerase can bind. This is
called the operator
 The promoter site, the operator and the stretch of
DNA that codes for the protein(s) is called the
operon
Operons (the trp operon)


An example of an operon is the tryptophan (trp) operon in E. coli that
produces the amino acid, trp
The way it works
 Trp operon is usually ‘on’ . . . RNA polymerase has access to the
promoter
 To stop the production of trp, the operon has to be turned ‘off’
 A protein called the trp repressor binds to the operator and blocks
the attachment of RNA polymerase
 This repressor protein is specific to the trp operator site and stops
transcription
 The trp repressor is the product of another regulatory gene with its
own operon
 When trp is absent, the repressor is inactive and the production of trp
proceeds normally
 When trp is present in higher concentrations, it acts as a corepressor
 It binds with the repressor protein and “activates” it so that it can
bind to the operator and turn off transcription
Repressible operon
 The
trp operon is called a repressible
operon
 This means that the trp operon is usually in
the “on” condition . . . it can transcribe
the DNA normally
 Transcription can only be inhibited when
trp binds with the repressor protein
 This allows the repressor protein to bind
to the operator and prevent
transcription
Inducible operon
 In
an inducible operon, the operon is
usually “off”
 It is not possible to transcribe the DNA
 There must be some sort of signal
(molecule) that can turn the operon on
 An example of an inducible operon is the
lactose (lac) operon
Operons (the lac operon)




In E. coli, the enzyme beta-galactosidase is needed to
break lactose into glucose and galactose
Normally, E. coli does not have a large amount of this
enzyme present
The operon to create beta-galactosidase is normally in
the “off” position
 A regulatory gene, lacI, creates a repressor protein
that is normally bound to the operator of the lac
operon
When lactose is present, it will bind to the repressor
protein and inactivate it
 Since this molecule is needed to start DNA
transcription, it is called an inducer