The Genetics of Viruses and Bacteria
Gene Expression
Chapter 17 and Chapter 35

Discovery – researchers discovered viruses by
studying the TMV (tobacco mosaic virus)
◦ Infected sap was sprayed on other plants; Beijerinck
concluded that the pathogen was reproducing
because the concentration of it was undiluted in
each new generation of infected plants,
◦ Stanley crystallized the pathogen known as TMV

Viral structure
 Viral Genomes – include double strand DNA, single
stranded DNA, double strand RNA, or single stranded
RNA
 Linear or circular
 May have four genes or several hundred
 Capsids and Envelopes
 Capsid – protein coat that encloses the viral genome
 Rod-shaped, polyhedral, or complex
 Composed of small protein spherical subunits call
capsomeres
 Envelope – membrane that coats viral capsids
 Helps virus infect host
 Bacteriophages – most complex virus with a icosohedral
head; named T1 – T7

Viral Reproduction
 Host Range – virus contain viral proteins that fit into
specific cell surface receptor sites of the host
 Some ranges may be broad while others are small or even
one host ranges
 Mechanisms – viruses commandeer their host cell
machinery and produce copies of themselves
 Patterns of viral genome replication (viral protein
production)
 DNA to DNA = virus will use hot cells DNA polymerase to
copy its DNA genome
 RNA to RNA = most host cells do not have an enzyme to
copy RNA, the virus cell will carry RNA replicase, an enzyme
that uses viral RNA as a template produce complementary
RNA
 RNA to DNA to RNA – viral cell carries reverse transcriptase,
an enzyme that transcribes DNA from an RNA template

Lytic Cycle – virulent phages reproduce only
by this mechanisms; virulent phages lyse the
host cells resulting in host cell death
 Phage attaches to cell surface
 Viral surface proteins recognize receptor sites of the
host cell
 Phage contract sheath and inject viral genome
 ATP stored in phage tailpiece powers contraction
 Capsid ghost is left behind; capsid with no genetic
material present
 Hydrolytic enzymes destroy host cell’s DNA
 Host cell transcribes then translates viral proteins

Phage genome directs the host cell to
produce phage components: DNA and capsid
proteins
 Using nucleotides from its degraded genome, the
host cell makes copies of the phage genome
 Host assembles tail fibers, phage tails, and
polyhedral heads
 Phage components assemble spontaneously
through weak hydrogen bond interactions

Cell lyses and releases phage particles
 Lysozymes specified by the viral genome digest
bacteria cell wall
 Lytic cycle takes 20–30 minutes and may increase
the population a hundredfold

Lysogenic Cycle – coexistence of host and phage
 Viral replication where the viral genome becomes
incorporated into the bacterial genome
 Phage  binds to bacteria surface
 Phage  injects DNA into host
  DNA forms a circle and either begins lytic or lysogenic
cycle
  DNA crosses over into the bacterial DNA and becomes a
prophage
 Prophage genes are copied along with bacteria genome. As
cell divides the prophage is passed to each new daughter
cell
 Lysogenic Cell – host cell carrying a prophage in its
chromosome
 Excision (exiting) of the prophage from the bacteria
chromosome may begin the lytic cell

Animal Viruses
◦ Provirus – viral DNA that inserts into a host cell
chromosome (animals chromosome)
◦ Retrovirus – RNA virus that uses reverse
transcriptase to transcribe DNA from the viral RNA
(HIV)
◦ Emerging Viruses – make a sudden impact; most
likely an existing virus that has expanded its host
range



Bacterial chromosome is a circular, double
stranded DNA found in the nucleoid region
Plasmids – extra chromosomal units found in
most bacteria that contain extra genes
(double stranded rings)
Binary Fission – reproduction preceded by
DNA replication

Genetic Recombination – gene transfer
between bacteria and other sources of DNA
◦ Transformation – gene transfer during which a
bacterial cell assimilates foreign (naked) DNA from
its surroundings (Avery’s experiment, chapter 16)
◦ Transduction – gene transfer from one bacteria to
another by a bacteriophages
 Host cell DNA is packaged along with phage DNA
during the lytic cycle and that phage infects another a
host

Conjugation – direct transfer of genes
between two cells that are temporarily joined
via a sex pilli (ability to form sex pilli is found
in the genes of the F plasmid)
 Characteristics of Plasmids
 Contain a few genes, not necessary for survival
 Replicate independently or in synchrony of the
bacterial chromosome
 No extracellular stage, unlike viruses
 F plasmid – “fertility” plasmid consisting of 25 genes
which are involved in production of sex pilli
 Replicates in synchrony of the bacterial chromosome
 Each daughter cell will be F+
 R plasmid – “resistance” plasmid carrying up to ten genes
fro antibiotic resistance
 Increased antibiotic use has produced many pathogenic
resistant strains of bacteria

Transposons – pieces of DNA that move from
different locations on the chromosome
(McClintock, 1940’s)
◦ Causes mutations by interrupting the transcription
of mRNA, and therefore, disrupting the translation
of that protein
◦ Increase/decrease protein production by inserting
within a regulatory gene sequence that controls
transcription rates

Operon – a system that allows for the turning
on or the turning off of metabolic activites
◦ Operator – the DNA switch found within the
promoter region
◦ Repressor – protein that may shut the operon down
by binding to the operator so the RNA polymerase
can not bind to the promoter to transcribe RNA
 Product of a regulator gene

TRP Operon – regulates the production of
repressible enzymes by producing the amino
acid tryptophan
 Five genes encode the polypeptides that make the
enzymes
 When “on”, RNA polymerase binds to the DNA and
transcribes the gene’s; inactive repressor; no
tryptophan present
 When “off”, tryptophan (corepressor) binds to the
inactive repressor making it active allowing it to bind to
the operator; tryptophan present
 When tryptophan is present, it inhibits its own
production by activating the repressor

LAC Operon – production of enzymes to take up
and metabolize lactose
 Three genes code for the enzymes necessary to
metabolize lactose
 When “off”, lactose is absent, repressor will bind to
operator stopping the transcription by RNA polymerase
to produce the enzymes
 When “on”, lactose present, repressor in inactive
because of an inducer (allolactose) that will bind to the
repressor rendering it inactive
 Allolactose is an isomer of lactose that enters the cell
and induces the binding of the active repressor to the
operator