Viruses are intracellular parasites composed of nucleic acid and protein that can only reproduce inside host cells. They enter the host cell and use the cell's machinery to produce new virus particles, which causes the host cell to rupture and release the new viruses to then infect other cells. The viral replication cycle involves the virus attaching to and entering the host cell, producing new viral components using the host cell, assembling new virus particles, and then causing the host cell to burst open, releasing the new viruses.

1. Virus
Replication

2. What is a Virus?
• Virus: A biological particle composed of
nucleic acid and protein
• Intracellular Parasites: organism that must
“live” inside a host

3. Is Fire Alive?
• 1) Made of cells? no
• 2) Organization? no
• 3) Require energy? yes
• yes
4) Respond to Environment?
yes
• 5) Grow?
yes
• 6) Reproduce?
no
• 7) Adapt?

4. Are Viruses Alive?
No Yes
• They are not made of cells • Reproduce
or organelles • They have DNA or
• They cannot reproduce RNA
without a host • They can adapt to
• They don’t have both surroundings
DNA and RNA (RNA is a single strand of • The have
DNA that has been copied. RNA is used to tell your body what
proteins and enzymes to make, along with which genetic codes to
activate.) organization
• They do not metabolize
energy

5. Virus Parts • All Have:
– 1) Capsid: coat of
protein that surrounds
DNA or RNA
– 2) Nucleic Acid: RNA
or DNA
• Some Have:
– Tail Fibers: Used for
attachment (not legs)
• Shapes vary

6. Viral Replication
• Cannot reproduce w/o a host cell
– Virulent: cause disease
• Bacteriophages (infect bacteria) are often studied
– Replication is similar with many animal
viruses

7. The Lytic Cycle
• 1st Step: Attachment
– Virus lands on cell
membrane
– Virus attaches to a
cell receptor
– No attachment = No
infection
– Virus acts as a “key”
to the receptor

8. The Lytic Cycle
endocytosis
• 2nd Step: Entry
– Virus enzyme weakens cell membrane
– Genetic material (DNA or RNA) enters host cell

9. The Lytic Cycle
• 3rd Step: Replication
– Virus DNA/RNA uses
ribosomes to make
virus proteins
– Proteins created by
transcription/translatio
n
– Virus proteins make
new virus parts

10. The Lytic Cycle
• 4th Step: Assembly
(formation)
– New virus parts
are assembled in
the cytoplasm

11. The Lytic Cycle
• 5th Step: Lysis & Release
– Virus enzyme causes cell membrane to “lyse”
– New viruses are released to find new host
– Cycle repeats