Viruses
I. What are they (and what aren’t they)?
II. Virus structure and classification
Herpesvirus
III. Viral infection
Foot and Mouth Disease virus
Ebola virus
Characteristics of Viruses
Somewhere between life and non-life
Bacteriophage Phi X 174

Can’t reproduce or __________________ independently

But can direct living cells to do so for them

“___________________ intracellular parasites”
“Infectious particles” rather than “organisms”, “active” or
“inactive” rather than “alive” or “dead”.
General Structure
Nucleocapsid
•
Small -- _______________ nm
DNA or RNA
in diameter
•
All have nucleocapsid
composed of ______________
and nucleic acid
•
Capsid
Enveloped virus
Some have a phospholipid
bilayer membrane (envelope)
Envelope
Spike
Viral envelope
•
Common in animal and plant viruses
•
Membrane is required for ______________
•
Viral envelopes are actually portions of the
infected host cell _____________________
(viruses do not form their own)
•
Often replace ______________ proteins
with viral glycoproteins
Nucleocapsid architecture
Two most common shapes: helical &
icosahedral.
Both are regular, geometric shapes determined by
the structure and orientation of proteins in the
________________________
Helical structure
Virus _________
Capsomeres
(___________
building blocks)
Helical structure: Tobacco Mosaic Virus
TMV is
composed of
only_________
types of
molecules: a
single ________
molecule and
one type of
protein
Icosahedral structure
Composed of
only one or two
different proteins
+ nucleic acid
Note that in both of these architectures, only one or two types of
proteins used. Means that the virus needs only one or two
protein-coding ________________.
Adenoviruses -- non-enveloped icosahedral viruses which are
generally mild pathogens in humans, birds, etc. causing
___________________ (‘pink eye’) and _________________
illnesses.
Complex viruses
A number of types with somewhat more ___________
structures
One is typical bacteriophage, with icosahedral head, helical
tail, fibers for attachment
Bacteriophage
lambda
Nucleic acid content
Viral genomes are very small
 ___________
base pairs (enough to encode 3 or
4 proteins) to 200,000 bp
compared to bacteria (~106 bp) and
human (~2 X 109 bp).
 Small
Characterized by __________________ genes
Viral genomic DNA
mRNA transcripts
(overlapping)
Viral genomes. The genomes of viruses can be either _________ or
________ (or some use both at different stages in their life cycle)
E. coli bacteriophages
Viral classification
Viral classification (cont.)
Viral Infection -- Overview
1) Adsorption
2) ____________ (either of nucleocapsid or of
nucleic acid only)
3a) Viral _______________ (lytic phase) or
3b) Integration of viral genome into host cell
genome (lysogenic phase)
4) Exit from cell
Adsorption
Specific interaction of viral surface with cell
surface.
This is the reason viruses only infect certain cell
types, for example:

HIV only infects cells (such as T cells) with the CD4
glycoprotein on their surface.

Influenza virus (an orthomyxovirus) only infects
mucosal cells of upper ____________________ tract.

Bacteriophages are highly ___________________ for
certain hosts (e.g. coliphages only infect E. coli)
Entry
1) ______________ of nucleic acid (e.g. T4 phage)
2) Entry of ___________ nucleocapsid (most
common in animal and plant viruses)
Cell membrane
Endocytosis:
Virus in vescicle
Vesicle and capsid break down, releasing nucleic acid
Membrane ____________ (only in enveloped viruses)
Out
Cell membrane
In
Out
In
Out
In
Out
In
Viral replication
• Once inside, one of several things occurs,
depending on the virus.
• However, the end result is always ________
________________
• Involves:
1) Replication of the nucleic acid
2) Synthesis of capsid proteins
3) ________________ of nucleocapsids
General stages of (lytic) virus replication
One-step growth curve of virus replication
Lysogeny or ‘integration’ vs. lytic pathways
Exit
•
•
Cell lysis or
Budding
HIV movie
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.