2. Viruses have one major characteristic in common: they
are obligate intracellular parasites.
Virology; the study of viruses
Viruses are UNABLE to grow and reproduce outside
of a living cell. No virus is able to produce its own
energy (ATP) to drive macromolecular synthesis.
However, in many other respects, they are a
highly diverse group.
3. Definition of a Virus
Viruses are segments of nucleic acid enclosed
in a protein coat (virion / virus particle :
extracellular state)
Poliovirus
4. Viruses are genetic elements that can
replicate independently of a cell’s
chromosomes but not independently of cells
themselves (intracellular state)
Definition of a Virus
a host (a place for initiating the
intracellular state)
5. Properties of Viruses
Small size>range>0.02 - 0.3 micrometers
Picornavirus (“little RNA virus”) is
one of the smallest viruses, about
20 nanometers in diameter
Smallpox virus, one of the largest
viruses, about 300 nanometers, near
the resolution of the light microscope
• Size alone does not differentiate viruses & bacteria!
• smallest bacteria (e.g. Mycoplasma, Ralstonia pickettii)
are only 200-300 nm long.
10. Properties of Viruses
Do not grow in
size
Viruses grow by
independent
synthesis and
assembly of their
components
inside of a host
cell Human adenoviruses growing in the
nucleus of their host cell
14. Structure of Viruses
The viral genome is DNA or RNA
Most bacterial viruses contain double-stranded DNA
Many animal viruses contain ds DNA or ssRNA
15. Structure of Viruses
Most common morphologies are polyhedral
(icosahedral) and helical
Polyhedral virus
Helical virus
16. Structure of Viruses
Some viruses have additional structures:
animal viruses may have envelopes and
“spikes”
18. Classification of Viruses
Criteria: Baltimore
Type of nucleic acid
Size and morphology
Additional structures such as envelopes and tails
Host range > refers to the range of cells that can
be infected by the virus, most often expressed as
bacteria, plant and animal hosts
23. Virus Groups
Some members possess large DNA genomes encoding a range of
enzymes involved in nucleic acid synthesis.
Depending on virus group viruses show temporal regulation of protein
synthesis.
Small DNA genomes with limited coding capacity.
Some members of this group are dependant upon other viruses for their
replication.
1 dsDNA dsDNA mRNA Herpes simplex
virus
2 ssDNA Parvovirus
dsDNA mRNA
ssDNA
24. Virus Groups
Viruses possessing RNA genomes all encode an RNA-
dependant RNA polymerase.
RNA viruses show a higher mutation rate compared to DNA
viruses.
Segmented genomes.
Transcribes mRNA from the dsRNA genome without prior protein
synthesis using a virion associated RNA-polymerase
Early phase of mRNA synthesis is monocistronic mRNA molecules.
3 dsRNA dsRNA mRNA Reovirus
25. Virus Groups
“Positive” RNA viruses - Genome RNA is of the same sense as mRNA and
can be infectious.
First stage in replication is the translation of the genome RNA with the
production of the virus polymerase.
“Negative” RNA viruses – Genome RNA is complementary to mRNA.
Virion-associated RNA-polymerase and first stage in replication is
mRNA transcription.
4 +ve ssRNA dsRNA +ve ssRNA [Acts as mRNA] Enterovirus
5 -ve ssRNA Influenza A
virus
dsRNA -ve ssRNA mRNA
26. Virus Groups
Unique among RNA viruses in that they induce tumours.
Characteristic feature is their ability to produce a DNA copy of the
genome RNA using a virion associated ReverseTranscriptase.
DNA copy integrates into the cellular genome.
Circular DNA genome - double stranded with a nick in one strand.
The nick is repaired at an early stage in the virus replication cycle.
The virus encodes RNA polymerase with a reverse transcriptase
activity which produces a RNA intermediate from which the genome
DNA can be copied.
6 ssRNA mRNA
dsDNA
ssRNA Retrovirus
(e.g. HIV)
7 Nicked dsDNA
dsDNA Sobek
Hepatitis B
virus
nicked dsDNA intact dsDNA
dsDNA Utuh
mRNA
RNA
28. The (dsDNA) Virus Life
Cycle
1. Virus enters host cell (method
is variable, involves host
receptor molecule on cell
surface)
2. Viral DNA replicated using the
host's DNA polymerase,
nucleotides, etc.
3. DNA transcribed into mRNA
using host's RNA polymerase,
nucleotides
4. mRNA translated using host's
ribosomes, tRNAs, amino
acids, GTP, etc.
DNA
Protein
capsid
1
3
2
mRNA
DNA
capsid
proteins
4
29. The dsDNA Virus Life Cycle
5. New DNA and capsid proteins
assemble into new virus
particles, exit the cell (in
various ways)
DNA
Protein
capsid
1
3
2
mRNA
DNA
capsid
proteins
4
5
30. The ssRNA (type V) Virus Life
Cycle
1. Virus enters host cell
2. Capsid removed, RNA released
3. complementary RNA made from
genomic RNA by enzyme encoded
in viral genome
4. new genomic RNA made from
complementary strand
5. complementary strand is mRNA,
transcribed into viral proteins
6. Virus assembled, exits cell (by
various means)
1
2
5
4
3
6
RNA
cRNA
31. The Retrovirus Life
Cycle
1. Virus enters host cell
2. Reverse transcriptase (encoded in
viral genome) catalyzes synthesis
of DNA complementary to the
viral RNA (cDNA)
3. RTase catalyzes synthesis of 2nd
strand of DNA complementary to
the first
4. dsDNA incorporated into host
genome ("provirus")
provirus may remain unexpressed
for a period of latency
1
4
3
2
5
6
Host's DNA
RNA
cDNA
RTase
32. The Retrovirus Life
Cycle
5. Proviral genes are transcribed by
host's transcriptional machinery
into RNA
• RNA serves as mRNA for
translation into viral proteins and
as genomic RNA
6. New viruses are assembled
containing genomic RNA and
ReverseTranscriptase
7. Virus exits cell
1
4
3
2
5
6
Host's DNA
RNA
cDNA
RTase
33. Bacteriophages
Viruses that infect bacterial cells
Two types of infections:
1. Lytic infection: phage replicates its DNA and
lyses the host cell
2. Lysogenic infection: phage DNA is maintained
by the host cell, which is only rarely lysed
34. Virulent phages only
undergo a lytic cycle
Temperate phages can
follow both cycles
Prophage can
exist in a dormant
state for a long
time
It will undergo
the lytic cycle
Bacteriophage
36. Temperate phages can
follow both cycles
Prophage can
exist in a dormant
state for a long
time
It will undergo
the lytic cycle
Bacteriophage
37. Viruses are usually very host-specific:
one virus infects only one strain,
maybe not even other members of the
same species
Why?
Viruses enter cells via specific proteins in
the membrane
Phage’s host specificity
38. Lipid bilayer
(same in all
cells) cannot
be penetrated
Proteins differ,
even within a
species
39. Consequences of viruses attacking
specific proteins
1. A cell cannot be totally immune to all
viruses because it needs the membrane
proteins to communicate with outside
environment
Best example: lambda phage attacks
E.coli via the maltose transporter. No
transporter, no phage problem—but no
maltose (a sugar) also.
So, viruses can affect uptake, etc.
41. Electron microscopy:
Difficult, expensive
More definitive—you’re sure it’s a virus
More information from morphology
Epifluorescence microscopy
Easy, less expensive
Less definitive: “viral-like particles”
More quantitative
43. Virus counts with epifluorescence are higher
than with electron microscopy (TEM). Why?
1.Epifluorescence counts things that are
viruses.
2.TEM misses things that are viruses
3.Loss of viruses during preparation of
samples for TEM.
24
46. Uses for Bacteriophages
Phages as vectors in genetic engineering and
biotechnology designs
Phage lytic enzymes to control infections
Phage therapy in animals and other uses of
phage in agriculture
Bacteriophage therapy
Phages for detection of pathogenic bacteria
