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VIRUSES
Viruses Are Not Cells!
There are several structural and functional
differences between cells and viruses
The structural differences include:
No nucleus
No cell membrane
No organelles
The functional differences include:
Do not grow
Do not respire
Do not develop
Do not reproduce
Viral Structure
A typical virus is made
up of 2 parts:
Capsid—a protein
coat which enables
the virus to bind to a
cell
Nucleic acid—either
DNA or RNA
Some viruses have an
additional protective
coating called an
envelope
Envelope
Capsid
Nucleic
Acid
Replication vs. Reproduction
Viruses do not reproduce
Viruses replicate
Reproduction requires cell division
A virus requires a host for it to
replicate
A host is a living cell that provides
all the materials a virus needs to
replicate
Classification of Viruses
Shape: allows viruses to attach
to only a few kinds of cells; like
two pieces of a jigsaw puzzle
Filovirus—has no distinct shape
Ebola- Zaire Ebola Virus, has the
highest case-fatality rate, up to 90%.
Massive blood loss leads to death.
Polyhedral virus—multifaceted
geometric shape
Herpesvirus- (cold sores & shingles)
virus lays dormant along nerve fibers
 Binal virus—has a polyhedral
capsid and a helical tail
 Bacteriophage T4—virus
which infects E. Coli; used for
molecular biology research
 Helical virus—tightly coiled
DNA or RNA inside a long,
narrow capsid
 Tobacco Mosaic Virus—
infects members of nine plant
families
Classification of Viruses (cont.)
Host: another way to classify viruses is based
on the type of cell they infect
Plant virus
Animal virus
Bacterial virus—these are also called
bacteriophages
Infection Cycles
 There are two different pathways a virus
follows when it enters the host cell:
 Lytic Cycle: the virus enters the host,
makes copies of itself and then destroys
the cell
 Lysogenic Cycle: the virus integrates its
DNA into the DNA of the host cell so that
the virus’s genetic information is copied
along with the host cell; unlike the lytic
cycle, the virus doesn’t immediately
destroy the cell, but instead remains
inactive for a prolonged period of time
Steps in the Lytic Cycle
Bacterial Chromosome
Bacteriophage
Host Cell
Capsid
Nucleic
Acid
Lysis and
Release
Assembly
Attachment
LYTIC
CYCLE
Entry
Replication
Steps in the Lysogenic Cycle
Host Cell
Host Cells
Continue to
Reproduce
Attachment
Trigger—Viral
DNA Activates
Entry
LYSOGENIC
CYCLE
Prophage
Virus Now
Enters Lytic Cycle
Prophage Formation
Host Cell
Reproduction
Examples of Diseases
Caused by Viruses
Retrovirus
 Contains an RNA core that is replicated by 1st
transcribing its RNA into DNA
 Ex: HIV—the virus that infects white blood cells and
causes AIDS
1. RNA & reverse
transcriptase
enter the host
cell
4.Viral DNA
produces
new viruses
2. Enzymes
copy the viral
RNA into DNA
3. Viral DNA
attaches to
cell DNA
Viroids and Prions
 Viroid:
No capsid
Contains RNA
Cause plant diseases
 Prion:
Protein molecule with no RNA or DNA
Cause animal diseases (i.e. mad cow
disease)