Genetics of Viruses and
Bacteria
Virus Size

Virus
Bacterium
Animal
cell

0.25 µm
Animal cell nucleus
Bacteria are
prokaryotes with
cells much smaller
and more simply
organized than
those of
eukaryotes
Viruses are smaller
and simpler than
bacteria
The Discovery of Viruses:
Scientific Inquiry



Tobacco mosaic disease
stunts growth of tobacco
plants and gives their
leaves a mosaic coloration
In the late 1800s,
researchers hypothesized
that a particle smaller than
bacteria caused the disease
In 1935, Wendell Stanley
confirmed this hypothesis
by crystallizing the
infectious particle, now
known as tobacco mosaic
virus (TMV)
Viruses: “Non-living”
Entities
Small packages of nucleic acids in a
protein coat
 Are NOT cells—no cytoplasm and do
not perform metabolic reactions
 Obligate intracellular parasites—
dependent upon other cells for
replication

Types of Viruses

DNA viruses


Genome is DNA
RNA viruses
Genome is RNA
 Smaller than DNA viruses
 Lack of proofreading—leads to 
evolution rate

Viruses have a Specific Host
Range

Recognition by complementary fit
between external viral protein and
specific cell surface receptor sites
http://pathmicro.med.sc.edu/mhunt/rep1.jpg
Bacteriophages


Viruses that infect
bacteria
Set in motion a
genetic takeover of
bacteria, such as
Escherichia coli
Viruses Replicate Inside Living
Cells

Obligate intracellular parasites
Viruses lack enzymes needed for
metabolism and have no structures to
make proteins
 Use cells own machinery to replicate
viruses


Pathogen: agent that causes disease

Viruses damage cells during replication
Reproductive Cycles of
Phages


Phages are the
best understood
of all viruses
Phages have two
reproductive
mechanisms: the
lytic cycle and
the lysogenic
cycle
Lytic Cycle
Results
Lysogenic
Cycle
in the Replicates
death of the
without
host cell
destroying the
host
Virulent
phage
Temperate
virus
Bacterial
The viral DNA
defenses—
incorporated
restriction
into the host
enzymes cut
DNA
up certain
(prophage)
phage DNA
Lytic Cycle
Virulent Viruses-reproduce only by lytic cycle
Temperate Viruses-reproduces by lytic cycle or
lysogenic cycle
Reproductive Cycles of
Animal Viruses
•
Two key variables in classifying
viruses that infect animals:
DNA or RNA?
 Single-stranded or double-stranded?

Class/Family
Envelope
Examples/Disease
I. Double-stranded DNA (dsDNA)
Adenovirus
No
Respiratory diseases, animal
tumors
Papovavirus
No
Herpesvirus
Yes
Poxvirus
Yes
Papillomavirus (warts, cervical
cancer): polyomavirus (animal
tumors)
Herpes simplex I and II (cold
sores, genital sores); varicella
zoster (shingles, chicken pox);
Epstein-Barr virus
(mononucleosis, Burkitt’s
lymphoma)
Smallpox virus, cowpox virus
Class/Family Envelop Examples/Disease
e
II. Single-stranded DNA (ssDNA)
Parvovirus
No
B19 parvovirus (mild rash)
III. Double-stranded RNA (dsRNA)
Reovirus
No
Rotavirus (diarrhea),
Colorado tick fever virus
Class/Family
Envelope
Examples/Disease
IV. Single-stranded RNA (ssRNA); serves as mRNA
Picornavirus
No
Rhinovirus (common cold);
poliovirus, hepatitis A virus, and
other enteric (intestinal) viruses
Severe acute respiratory
syndrome (SARS)
Coronavirus
Yes
Flavivirus
Yes
Yellow fever virus, West Nile
virus, hepatitis C virus
Togavirus
Yes
Rubella virus, equine encephalitis
viruses
Class/Family
Envelope
Examples/Disease
V. ssRNA; template for mRNA synthesis
Filovirus
Yes
Ebola virus (hemorrhagic fever)
Orthomyxovirus
Yes
Influenza virus
Paramyxovirus
Yes
Measles virus; mumps virus
Rhabdovirus
Yes
Rabies virus
VI. ssRNA; template for DNA synthesis
Retrovirus
Yes
HIV (AIDS); RNA tumor viruses
(leukemia)
Animal Viruses
1.
2.
3.
4.
5.
6.
7.
Glycoproteins on viral envelope
recognize/bind specific receptors
on host cell
Viral envelope fises with cell’s
plasma membrane, and the
capsid and viral genome enter
the cell
Cellular enzymes remove capsid
Viral genome serves as template
for replication of RNA strands
a. Templates for new RNA
b. Serve as mRNA for protein
synthesis
Vesicles transport glycoproteins
to cell’s plasma membrane
Capsid forms around viral
genome
Virus buds from the cell



After entering the
cell, viral DNA uses
host nucleotides and
enzymes to replicate
itself
It uses host
materials and
machinery to
produce capsid
proteins
Viral DNA and
capsid proteins
assemble into new
virus particles,
which leave the cell
Plant Viruses

Plant viruses are
serious agricultural
pests

Protein RNA
Most plant viruses
• Have RNA genomes
• Enter their hosts via
wounds in the plant’s
outer layers
• Injuries, insects
feeding, contaminated
farming tools
• Once infected, virus
spreads through
plasmodesmata
Figure 10.19
RNA as Viral Genetic
Material
The broadest variety of RNA
genomes is found in viruses that
infect animals
 Retroviruses use reverse
transcriptase to copy their RNA
genome into DNA
 HIV is the retrovirus that causes
AIDS

•Genetic flow :
RNA  DNA
•2 identical strands of RNA
•Infects white blood cells
HIV virus
Vaccinations


Antibiotics don’t work—no metabolic
reactions to interfere with
Vaccines—harmless derivatives of
pathogenic microbes that stimulate the
immune system to mount defenses
against the actual pathogen


Parts of viruses, modified or killed viruses
are injected into the body
Allows immune system to make antibodies
against specific markers on the viral coat
• HIV mutates too fast for immune system to keep
up with
Influenza Vaccine

Influenza, also known as
the flu, is a contagious
disease that is caused by
the influenza virus. It
attacks the respiratory tract
in humans (nose, throat,
and lungs). The flu is
different from a
cold. Influenza usually
comes on suddenly and may
include these symptoms:
 Fever
 Headache
 Tiredness (can be
extreme)
 Dry cough
 Sore throat
 Nasal congestion
 Body aches
Emerging Viruses
Emerging viruses are those that
appear suddenly or suddenly come to
the attention of scientists
 Severe acute respiratory syndrome
(SARS) recently appeared in China
 Outbreaks of “new” viral diseases in
humans are usually caused by existing
viruses that expand their host territory

LE 18-11
Young ballet students in Hong
Kong wear face masks to
protect themselves from the
virus causing SARS.
The SARS-causing agent is a
coronavirus like this one
(colorized TEM), so named for
the “corona” of glyco-protein
spikes protruding form the
envelope.
Emerging Viruses
RNA viruses have unusually high
mutation rate
 Spread of virus from one host
species to another
 Dissemination of a virus from a
small, isolated populations to
widespread epidemics

Global View of HIV epidemic
as of 2008
http://www.who.int/hiv/facts/en/hiv_global2003sm.jpg
Viruses and Cancer

Tumor viruses can transform cells
into cancerous cells
Viral Group
Examples/
Diseases
Cancer Types
Retrovirus
HTLV-1/adult
leukemia
Leukemia
Herpesvirus
Epstein-Barr/
infectious
mononucleosis
Burkitt’s
lymphoma
Hepatitis B virus
Chronic Hepatitis Liver cancer
Viruses and Cancer

Virus inserts viral nucleic acids into host
cell DNA



Insertion is permanent-provirus never excises
Insertion for DNA tumor viruses
straightforward
Oncogenes-genes found in viruses or as
part of normal eukaryotic genome;
trigger transformation of a cell to a
cancerous state

Usually more than one must be activated to
transform a cell
Viroids





Smaller and simpler than viruses
Small, naked, circular RNA molecules that
do not code for proteins
Disrupt normal plant metabolism,
development, and growth by causing
errors in gene regulation
Affect many commercial plants—
tomatoes, potatoes, chrysanthemums
Thought to have originated from escaped
introns—sequences similar to self-splicing
introns
Chrysanthemum with
chrysanthemum
chlorotic mottle viroid
Green tomato infected with
tomato spotted wilt virus
Prions
Pathogens that are proteins
 Cause several degenerative brain
diseases (Scrapie in sheep, “Mad
Cow” disease, Creutzfeldt-Jakob
disease)

Prions: Hypothesis for
Propagation
INFECTIOUS PRION PROTEINS have a different shape,
which they impose on normal prion proteins in a chain reaction
that ends in sickness and death.
A hypothesis of how infectious
protein particles, or prions, cause
disease: PrPSc - an abnormal protein
- communicates with its normal twin
- PrPc - creating an abnormal form,
that will eventually harm neurons.
(Adapted by Leigh Coriale Design and Illustration, with permission, Science
[July 12], 1996, American Association for the Advancement of Science.)
Kuru Infected Brain
It exists only among a single
tribe in Papua New Guinea.
The afflicted tribe - the Fore
Highlanders - describe it as
the "laughing death",
because it leads to loss of
coordination accompanied
by dementia.
Normal Brain
Kuru Brain
Genetics of Bacteria
The Bacterial Chromosome
One double-stranded, circular
molecule of DNA
 Located in nucleoid region,
so transcription and
translation can occur
simultaneously
 Many also contain
extrachromosomal DNA in plasmids

Binary Fission
Genetic Recombination
Produces New Bacterial Strain
Transformation
 Transduction
 Conjugation

Gene transfer
occurs separately
from bacterial
reproduction
Transformation

Alteration of bacterial cell’s genotype
by uptake of naked, foreign DNA
from the environment
Transformation

Biotech companies use this
technique to artificially introduce
foreign genes into bacterial genomes
(human insulin, human growth
hormone)
Transduction

Gene transfer
from one
bacterium to
another by a
bacteriophage
Plasmids
Short, circular DNA molecules
outside the chromosome
 Carry genes that are beneficial but
not essential
 Replicate independently of
chromosome
 Episomes—plasmids that can be
incorporated into chromosome

Conjugation

“male”
Direct transfer of genetic material
between bacterial cells that are
temporarily joined (bacterial sex)
“female”
F-
F+
Sex pili
“Maleness” results from
presence of F factor—
segment of DNA in
chromosome or in F plasmid
Conjugation
R Plasmids

Contain genes that confer antibiotic
resistance

Medical consequences:resistant
strains of pathogens due to overuse
of antibiotics
Transposition of Genetic
Elements


The DNA of a cell can also undergo
recombination due to movement of
transposable elements within the cell’s
genome
Transposable elements, often called
“jumping genes,” contribute to genetic
shuffling in bacteria
Insertion Sequences
Insertion sequence
5
3
3
5
Inverted
repeat


Transposase gene
Inverted
repeat
The simplest transposable elements, called
insertion sequences, exist only in bacteria
An insertion sequence has a single gene for
transposase, an enzyme catalyzing movement of
the insertion sequence from one site to another
within the genome
Transposons


Transposable elements
called transposons are
longer and more complex
than insertion sequences
 Discovered by Barbara
McClintock
In addition to DNA
required for transposition,
transposons have extra
genes that “go along for
the ride,” such as genes
for antibiotic resistance
LE 18-19b
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
3
3
5
Inverted repeat
Transposase gene
Individual bacteria respond to
environmental change by regulating
their gene expression
A bacterium can tune its metabolism
to the changing environment and food
sources
 This metabolic control occurs on two
levels:

Adjusting activity of metabolic enzymes
 Regulating genes that encode metabolic
enzymes

LE 18-20
Regulation of enzyme
activity
Precursor
Regulation of enzyme
production
Feedback
inhibition
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Regulation
of gene
expression
Enzyme 3
Gene 3
Enzyme 4
Gene 4
Enzyme 5
Tryptophan
Gene 5
Operons: The Basic Concept

In bacteria, genes are often clustered into operons,
composed of






Regulatory gene —makes repressor protein that blocks
RNA polymerase
Promoter region —DNA sequence that RNA polymerase
binds to start transcription
Operator region —an “on-off” switch; can block RNA
polymerase if region is blocked by repressor protein
Structural genes —DNA sequences that code for several
related metabolic enzymes that direct production of
some end product
An operon can be switched off by a protein called a
repressor
A corepressor is a small molecule that cooperates
with a repressor to switch an operon off
Repressible and Inducible Operons:
Two Types of Negative Gene
Regulation




A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
The trp operon is a repressible operon
An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
The classic example of an inducible operon is
the lac operon, which contains genes coding for
enzymes in hydrolysis and metabolism of
lactose
Inducible enzymes usually function in
catabolic pathways
 Repressible enzymes usually function
in anabolic pathways
 Regulation of the trp and lac operons
involves negative control of genes
because operons are switched off by
the active form of the repressor

Repressible Operon
LE 18-21a
trp operon
Promoter
Promoter
Genes of operon
DNA
Regulatory
gene
mRNA
trpE
trpR
3
trpC
trpB
trpA
C
B
A
Operator
Start codon Stop codon
RNA
polymerase
mRNA 5
5
E
Protein
trpD
Inactive
repressor
D
Polypeptides that make up
enzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
Inducible Operon
LE 18-22b
lac operon
DNA
lacZ
lacl
3
mRNA
5
lacA
Permease
Transacetylase
RNA
polymerase
mRNA 5
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
Lactose present, repressor inactive, operon on
Positive Gene Regulation



Some operons are also subject to positive
control through a stimulatory activator
protein, such as catabolite activator protein
(CAP)
When glucose (a preferred food source of E.
coli ) is scarce, the lac operon is activated
by the binding of CAP
When glucose levels increase, CAP detaches
from the lac operon, turning it off
LE 18-23a
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
and transcribe
Inactive lac
repressor
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized
LE 18-23b
Promoter
DNA
lacl
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase
can’t bind
Inactive lac
repressor
Lactose present, glucose present (cAMP level low): little lac
mRNA synthesized
Both plates of E.coli are transformed by pGLO plasmid as seen by
growth on LB with ampicillin. The upper plate also contains
arabinose, the inducer for the green fluorescent protein. This is
visualized under UV light. The lower plate does not glow even
though it has transformed cells because the media lacks arabinose.