VIRUSES
CHAPTER 10
1
What are Viruses?

Obligate intracellular parasites

Viral components
– Nucleic acids
– Capsid (protein)
– Envelope
(Lipid w/intg proteins)
2
4 overall types of viruses

1) bacteriophages - almost always DNA
with a protein capsid. Lytic and lysogenic
types
 2) DNA viruses of Eukaryotes - often have
phospholipid envelope outside of capsid
 3) RNA viruses - have RNA as genetic
material. Often hypermutable
 4) RNA retroviruses - have reverse
transcriptase. Many can integrate into
3
host chromosome
Viral Shapes and Sizes
Helical TMV, M13
 Icosahedral T4 and adenovirus

4
Some viruses
T-even
HIV
lambda
5
Infectious Properties

Viral Host range
Depends on target receptor

Viral specificity
Depends on target receptor

Viral Origins
Selfish DNA?
Transposable elements
6
Bacteriophage
Most diverse?
7
Bacteriophages

Plaque counts
8
Bacteriophages

Replication
9
Hershey-Chase Bacteriophage Experiment - 1953
Bacteriophage = Virus that
attacks bacteria and replicates
by invading a living cell and
using the cell’s molecular
machinery.
Structure of T2 phage
DNA & protein
10
Fig. 2.5: Life cycle of virulent T2 phage:
Lytic cyle
11
Fig. 2-6: Hershey-Chase Bacteriophage Experiment - 1953
1.
T2 bacteriophage is composed
of DNA and proteins:
2.
Set-up two replicates:
•
•
Label DNA with 32P
Label Protein with 35S
3.
Infected E. coli bacteria with
two types of labeled T2
4.
32P
is discovered within the
bacteria and progeny phages,
whereas 35S is not found within
the bacteria but released with
phage ghosts.
1969: Alfred Hershey
12
Composition and Structure

Composition

– Nucleic acid
• Genome size
• Modified
bases
– Protein
• Protection
• Infection
Structure (T4)
– Size
– Head or capsid
– Tail
Head/Capsid
Contractile
Sheath
Tail
Tail Fibers
Base Plate
13
Infection of Host Cells
• Adsorption
– LPS for T4
lamB for l

Irreversible attachment

Sheath Contraction

Nucleic acid injection
14
Bacteriophages
15

Eclipse
– Early genes
– Phage DNA
synthesis
– Late genes


Intracellular
accumulation
Lysis and Release
Number of Infectious Particles
Lytic Phage Multiplication Cycle
Total
Phage
Eclipse
Extracellular
Phage
Intracellular
accumulation
phase
Lysis
Time after Infection
16
Assay for Lytic Phage
Phage

Plaque assay
– Method
– Plaque forming unit
(pfu)
– Measures infectious
particles
Bacteria
+
Phage
17
Lytic vs Lysogenic Cycle?

Role of repressor
 Role of cro gene product
 Role of proteases
Lytic = copies and immediately lyses
Lysogenic = integrates into host chromosome
“Prophage” = the latent form of phage where viral genome
is incorporated into host genome
18
Bacteriophages Lysogenic

Lysogeny
19
Fig. 19-6
Gene designation
Function
Left end
Early
promoters 0.3
0.7
Transcribed
by host RNA
polymerase
Promoter
1.3
1.7
2
3
3.5
Promoter
Promoter
Transcribed
by T7 RNA
polymerase
1
1.1
Promoter
Inhibits host restriction
Protein kinase
Bacteriophage T7
T7 RNA polymerase
Unknown
Origin of DNA replication
DNA ligase
Nonessential
Inactivates host RNA polymerase
Endonuclease
Lysozyme
4
Helicase, primase
5
DNA polymerase
6
Exonuclease
7
8
Virion protein
Head protein
9
10
Head assembly protein
Major head protein
11
12
Tail protein
Tail protein
13
14
Virion protein
Head protein
15
Head protein
16
Head protein
17
Tail protein
18
DNA maturation
19
DNA maturation
1. Replication cycle requires 25 minutes
2. Genome is linear double-stranded DNA
of 39,737 bp
Proteins for
DNA replication
and host lysis
3. T7 encodes all of its own proteins for
DNA replication and transcription
4. Time to complete 100 T7 genome copies
from a single copy: 5 minutes
5. Burst size (Escherichia coli host): about
300 virions/cell
6. Head size, 45 nm
7. Forms large plaques
8. T7 promoters are unique and widely
used in biotechnology
Phage structural
components and
maturation proteins
Promoter
20
Events Leading to Lysogeny

Circularization of the phage chromosome
– Cohesive ends
Cohesive Ends
Lygase
Linear Double Stranded
Opened Circle
Closed Circle
21
Events Leading to Lysogeny

Site-specific
recombination
– Phage coded
enzyme
gal
• Repression of the
phage genome
– Repressor protein
– Specific
– Immunity to
superinfection
bio
gal
bio
gal
bio
22
Termination of Lysogeny

Induction
– Adverse
conditions

Role of proteases
bio
gal
– recA protein
– Destruction of
repressor
• Gene expression
• Excision
• Lytic growth
bio
gal
gal
bio
gal
bio
23
Significance of Lysogeny

Model for animal virus transformation
 Lysogenic or phage conversion
– Definition: A change in the phenotype of a
bacterial cell as a consequence of lysogeny
• Modification of Salmonella O antigen
• Toxin production by Corynebacterium
diphtheriae
24
Types of Bacteriophage

Lysogenic or temperate phage: Phage
that can either multiply via the lytic cycle
or enter a quiescent state in the bacterial
cell. (e.g., )
– Expression of most phage genes repressed
– Prophage
– Lysogen
25
Viruses part II - Animals and Plants
Unique challenges. Must evade immune systems and must
cross 2 lipid bilayer barriers. (ie cross into nucleus)
26
RNA Viruses

Chromosomal Arrangements
–
+ strand
–
– strand
–
Double strand
(directly transcribed)
27
RNA Virus Families

11 RNA virus families
–
–
–
–
Picornaviridae (fmdv, polio)
Togaviridae (rubella)
Flaviviridae (hep C, west nile, yellow fever)
Orthomyxoviridae (flu)
RNA viruses more prone to mutation
28
Fig. 19-18
29
RNA Virus Families (cont.)



Retroviridae (hep B, htlv)-retrovirus
reverse transcriptase
Paramyxoviridae (measles, mumps,
pneumonia) - ss strand
30
RNA Virus Families (cont.)

Rhabdoviridae (rabies)
31
RNA Virus Families (cont.)

Orthomyxoviridae (all influenza)
32
RNA Virus Families (cont.)

Filoviridae

Bunyaviridae

Arenaviridae

Reoviridae
33
DNA Virus Families

Adenoviridae

Herpesviridae

Poxviridae
34
DNA Virus Families (cont.)

Papovaviridae

Hepadnaviridae
35
DNA Virus Families (cont.)

Parvoviridae

Emerging
viruses
36
Viral Replication

Activities
– Adsorption
– Penetration (virus or chromosome)
– Synthesis
– Maturation
– Release
37
Animal Viruses

DNA viruses
Envelope derives from
cells own plasma membrane
38
Animal Viruses

RNA
viruses

Latent
viruses
retroviruses
39
Culturing Animal Viruses

Live animals

Eggs
40
Culturing Animal Viruses

Cell Culture
– Primary
– Continuous
41
Viral Cytopathic Effects

Cytopathy
Damage to cells

Teratogenic effects
Japanese word for “little monsters” mutations that affect
tissue growth
42
Viruslike Agents

Satellites

Viroids
PLANTS
43
Viruslike Agents

Prions
Kuru
Creutzfeld-Jacob
BSE
Scrapie
44
Alpha helix
B-pleated sheet
Viruses and Cancer

Mechanism of
cancer
causation

HPV
45
Viruses and Cancer

Oncogenes/proto-oncogenes
 V-myc V-ras
Rous Sarcoma Virus RSV
Kaposi’s sarcoma - appears when immune system depressed
probably by herpes virus 8
46
Viruses to know something about
•HPV (DNA)
• HIV (RNA)
• Flu (RNA)
• Adenovirus(DNA)
• Herpes(DNA)
47
Herpes Simplex
After initial infection, the viruses move to
sensory nerves, where they reside as life-long,
latent viruses.
48
HPV
human papilloma virus
• Causes warts and some strains cause cervical cancer
• teratogenic
49
HIV human immunodeficiency virus
• RNA retrovirus
• T-cell host (CD4+ T-killer cells)
• needs protease to replicate
• binds to CCR5 and CD4 receptors
50
Adenovirus
• Common cold
• Usually affects respiratory tract.
• sometimes engineered for gene therapy
• DS DNA virus
51
Influenza
H = hemaglutinin
N = neuraminidase
• RNA virus
• mutates rapidly
• animal reservoirs
• can cross species lines
52