A virus is a non-living particle composed of genetic material and protein that can infect living cells. Viruses are tiny, obligate intracellular parasites that cannot reproduce on their own and lack metabolic processes. They contain either DNA or RNA as their hereditary material. Viruses are studied to develop vaccines and antiviral drugs, and used in molecular biology experiments. Viruses have a protein capsid that surrounds their nucleic acid and some have an additional phospholipid envelope. They utilize host cell machinery to replicate through lytic or lysogenic cycles.

1. What are Viruses
A virus is a non-cellular particle
made up of genetic material and
protein that can invade living cells.

2. Viruses – Are they Living Organisms?
• Tiny (50-100nm) “non-cellular particles”.
• Obligate, intracellular parasites:
– Cannot reproduce on their own.
– Cannot carry on metabolic processes.
• Do have hereditary material?

3. Studying Viruses
• Why?
– Produce vaccines
– Develop antiviral drugs
– Used as vectors for molecular biology
experiments – recombinant DNA studies.

4. Vaccines
• Viruses grown on chicken embryos are
attenuated vaccines
• Another type of vaccine is made by heat
killing the virus

5. Studying Viruses
• Morphology
– Genome –
– Capsid –
– Envelope – some have a phospholipid/
protein protective covering in addition to their
capsid. Part viral part host in composition.

6. The Structure Of a Virus
• Viruses are composed
of a core of nucleic
acid
• The Nucleic acid core
is surrounded by a
protein coat called a
capsid
• The Nucleic core is
either made up of
DNA or RNA but
never both

7. Nonenveloped virus
Genome (in this
case, DNA)
Capsid (protein)
Enveloped virus
Genome (in this
Viral protein case, RNA)
Host protein Capsid (protein)
Envelope
(phospholipid
bilayer)

8. T4 Bacteriophage

9. General Features of Viruses #1
• Obligate Intracellular Parasites
• Very small (ultramicroscopic)
• Non-cellular
• Not Alive
• Protein Coat Capsid
• Envelopes (some viruses..not all)

10. General Features of Viruses #2
• Utilization of Host Cell Machinery
• Host Range
• Genome
• DNA or RNA but NOT both
• Single stranded or double stranded
• Enzymes (Polymerases) (some viruses…not all)

11. Virus Reproduction
• General Replication cycle – requires a host
cell:

12. Virus Reproduction
• 2 types of replication cycles:
– Lytic – following entry, replication and
assembly, new virus particles exit the cell by
rupturing the host cell membrane, often
destroying the host cell,

13. Lytic
Cycle

14. Studying Viruses
• 2 Replication cycles cont’d
– Lysogenic
• Following entry, the viral genome is incorporated
into the host cell genome.
• As the host cell replicates its own genome the viral
genome also is replicated and is passed on to
daughter cells.
• This phase may last years.
• Usually some “trigger” will cause the host cell to
enter a lytic phase.

15. Lysogenic
cycle

16. How do Viruses Enter Cells?
• Viruses may enter through an injured spot
on a cell membrane/cell wall.
• Viruses with envelopes may enter by
endocytosis.
• Many viruses prefer a certain kind of cell.
– Recognize a specific membrane protein
(receptor) on the cell membrane and bind to
it.
– Example: Influenza viruses bind to receptors
in the respiratory tract only.

17. How do viruses enter cells?
• In general, viruses without envelopes
empty only their genome into cell while
viruses with envelopes incorporate
genome and capsid into cell (endocytosis).