1. An Introduction to the Viruses Non-Living Etiologies
Virology
An Introduction to the Viruses Non-Living Etiologies

2. Virus Viruses were too small to be seen with the first microscopes
The cause of viral infections
was unknown for years
Louis Pasteur first proposed the
term virus
Viruses are considered filterable

3. Viruses
Can infect every type of cell Cannot exist independently from the host cell, so aren’t considered living things Referred to as infectious Particles Obligate intracellular parasites Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

5. The Size of Viruses
Figure 6.1

6. Visualizing Viruses Smallest infectious agents
Most are so small, they can only
be seen with an electron
Microscope
Viewing viruses
Special stains and an electron microscope
Negative staining outlines the shape
Positive staining shows internal details

7. Viral Components: Capsids, Nucleic Acids, and Envelopes
Contain only those parts needed to
infect and control a host cell
External coating
Capsid
Envelope- in 13 of the 20 families of animal viruses
If no envelope, called naked virus
Core
DNA
RNA
The capsid and the nucleic acid together are called the nucleocapsid.
Fully formed virus that is able to
establish an infection in a host cell is
termed a virion.

8. Figure 6.4

9. The Viral Envelope
Many viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein capsids. The envelopes typically are derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells. Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane. The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.

10. Figure 6.10

11. Figure 6.8

12. Functions of the Viral Capsid/Envelope
Protects nucleic acids
Help introduce the viral DNA or RNA
into a suitable host cell
Stimulate the immune system to
produce antibodies that can protect
he host cells against future infections
Note: The capsid surrounds the virus
and is composed of a finite number
of protein subunits known as
capsomeres, which usually associate
with, or are found close to, the virion
nucleic acid.

13. Nucleic Acids: At the Core of a Virus
Number of viral genes is small They only have the genes necessary to invade host cells and redirect their activity Two Types of Viruses: DNA RNA

14. DNA Viruses
ssDNA
dsDNA
linear
circular

15. RNA Viruses Mostly single-stranded
Positive-sense RNA: genomes that are ready for immediate translation into proteins
Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins
Segmented- individual genes exist on separate pieces of RNA

16. How Viruses are Classified and Named
Main criteria
Structure
Chemical composition
Similarities in genetic makeup
International Committee on the Taxonomy of Viruses, 2000
3 orders
63 famillies “-viridae”
263 genera “-virus”
Some virologists use a species naming system, but it is not an official designation

19. Modes of Viral Multiplication
The host cell is
absolutely necessary for
viral multiplication

20. Viruses that Infect Bacteria
Bacteriophage
Most contain dsDNA
Often make the bacteria they infect more pathogenic for humans

21. Bacteriophage Icosahedral capsid head containing DNA
Central tube surrounded by a sheath
Neck
Base plate
Tail pins
Fibers

22. Lytic Cycle As A Model Attachment Virus attaches to specific receptor
sites on the host bacterium .
The bacteriophage attach to the
bacterial cell wall.
Specific strains of bacteriophages
can only adsorb to specific strain of
host bacteria. This is known as viral
specificity .
Viruses are Host specific

23. Lytic Cycle As A Model Penetration
The virus "drills" a hole in the bacterial wall and the virus injects its genome into the bacterial cytoplasm.
Some phages accomplish this by contracting a sheath which drives a hollow tube into the bacterium.

24. Lytic Cycle As A Model Viral Component Replication
Enzymes coded by the phage genome shut down the bacterium's cellular activities.
The phage replicates its genome and uses the bacterium's metabolic machinery to synthesize phage enzymes and phage structural components (nucleic acids, proteins and enzymes.

25. Lytic Cycle As A Model Viral Assembly
Viral proteins are synthesized and self-assembled into viral components such as head, tail, and tail fibers.

26. Lytic Cycle As A Model Maturation
The phage parts assemble around the genomes.
As the viral components are assembled into new viral particals are made, an enzyme, lysozyme, is released weakening the cell wall of the host bacterium.

27. Lytic Cycle As A Model Release
Usually, a phage-coded lysozyme breaks down the bacterial peptidoglycan causing osmotic lysis and release of the intact bacteriophages .
The new viruses are released and are now able to infect a new host, therefore, starting the entire cycle all over again.

28. Figure 6.17

29. Figure 6.18

30. Figure 6.19