Virus Notes
Basic Definition
• Viruses: Submicroscopic,
parasitic, acellular entity
Submicroscopic parasitic
composed of a nucleic acid core surrounded by a
protein coat.
– Below the resolution of a microscope
– Relies on a host
– Does not have the properties of cellular life
– Viruses are measured in nanometers
• 0.000000001m or 10-9
Comparing the size of a virus,
a bacterium, and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Viral Shapes and structure
Capsomere
of capsid
RNA
Capsomere
DNA
Membranous
envelope
Head
Capsid Tail
sheath
RNA
• Although viruses can have several shapes, all DNA
have at least two parts:
Tail
fiber
– An outer capsid made of proteins.
Glycoprotein
Glycoprotein
– Genetic material
(DNA or80–200
RNAnm –(diameter)
never both)80  225 nm
18  250 mm
70–90 nm (diameter)
20 nm
(a) Tobacco mosaic virus
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
What is a bacteriophage?
T4 bacteriophages infecting
an
E.
coli
cell
Protein coat
• A virus that
invades
bacteria. It
consists of a
DNA core
and a protein
coat
DNA
Why aren’t viruses considered alive?
• Viruses are acellular (not cells)
• Viruses have no organelles to take in nutrients
or use energy.
• Viruses cannot make proteins.
• Viruses cannot move.
• Viruses cannot replicate on their own.
Virus Reproduction
Viruses reproduce by infecting other cells.
Two types of viral infections:
1. Lytic Infection
2. Lysogenic Infection
What are the steps of a lytic infection?
Step 5: New viruses “lyse” the host cell
Step 3: Replication of viral
and are released for further infection
and Synthesis
of to
Protein
Step 1:DNA
Attachment
of virus
host cell
Capsule using cellular
“machinery” – cellular
enzymes, ribosomes, etc.
Step 4: Assembly of new viruses inside
Step 2: Injection of viral DNA into cell
host cell
Characteristics of Lytic Infections
1. Fast acting
2. Symptoms emerge
within 24 – 48 hours
3. Examples –
influenza, west-nile
The Lysogenic Infection
Step 1: Virus attaches and
inserts its DNA inside host
Step 2: Viral DNA attaches to the
host DNA (prophage DNA)
Step 3: The viral DNA lies
“dormant” and the viral DNA
replicates each time the cell
divides
Step 4: Stress or other “factors”
causes the infection to progress
to the “lytic” phase
Characteristics of Lysogenic Infections
1. Slow Acting - Viral DNA can lie “dormant” for
many years, hidden within prophage DNA
2. The host are “symptom-free” during
dormancy
3. Infection is fast acting when the infection
progresses to the lytic phase
4. Example: HIV
What is a retrovirus?
• A retrovirus is a virus with RNA rather than
DNA for its genetic material.
• These viruses carry an enzyme to create DNA
from their RNA called reverse transcriptase.
• The viral DNA then integrates into a
chromosome.
(Just kidding… here’s a real retrovirus!)