What is a Virus?
1 nanometer (nm) = one
billionth of a meter
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm
• Virus: A biological particle composed of
nucleic acid and protein
• Intracellular Parasites: organism that must
“live” inside a host
• Not made of cells or
organelles
• Can’t reproduce on
own
• Don’t metabolize
energy
• Don’t perform cellular
processes
• Reproduce
• Have nucleic acid
• Adapt to
surroundings
• Have
organization
• All Have:
– 1) Capsid: coat of
protein that
surrounds nucleic
acid
– 2) Nucleic Acid:
RNA or DNA
• Some Have:
– Tail Fibers: Used
for attachment
(not legs)
• Shape & size varies
Virus Video
• Bacteriophages
(viruses that
attack bacteria)
are often studied
• Replication is
similar with many
animal viruses
• Two “life” cycles:
Lytic & Lysogenic
Dozens of phage viruses attacking an
E. coli
bacteria
The
bacteria
cellcell
is destroyed
and new viruses escape
1st Step: Attachment
– Virus lands on cell
membrane
– Virus attaches to a
cell receptor
– Virus acts as key;
receptor acts as lock
• No attachment = No
infection
endocytosis
2nd Step: Entry
– Virus enzyme weakens cell membrane
– Genetic material (DNA or RNA) enters host
cell
3rd Step: Replication
– Virus DNA/RNA
uses ribosomes to
make virus proteins
– Virus proteins
created by
transcription/
translation
Viral DNA
transcription
Viral RNA
translation
Viral Proteins
4th Step: Assembly
–New virus
proteins are
assembled in the
cytoplasm
5th Step: Release
– Virus enzyme causes cell membrane lyse (burst)
– Viruses are released to find new host
– Cycle repeats
Attachment
All stages on
one slide
Entry
Replication
& Assembly
Release
1st step: Attachment =
Same as lytic cycle
2nd Step: Entry =
Same as lytic cycle
3rd step: Replication
– Provirus Created
Virus
DNA
• Virus DNA
combines with cell
DNA
– Infected cell
divides by mitosis
and copies the
provirus
– Each new cell will
contain the
provirus
Provirus
Procell
virus
DNA
Provirus
4th Step: Assembly:
Same, except many
new viruses are
assembled in many
cells
5th Step: Release
Same, except many
cells burst releasing
many more viruses
The viral DNA become active and starts making new viral proteins
The infected cells burst…releasing the new viruses
1. True or False: All viruses have a capsid, nucleic acid, and tail
fibers.
2. List 3 reasons why viruses may be living? Nonliving?
3. What is the virus capsid made from?
4. What type of organism do phages infect?
5. In order list the stages of the lytic/lysogenic cycles.
6. In which stage…
• does the host cell explode?
• are virus proteins built to make new viruses?
• is a provirus created?
• does a virus connect with the host cell’s receptors?
7. Which two objects make up a provirus?
8. Which virus “life” cycle is considered the most harmful? Why?