1. Unit 3: Viruses!

2. History of Viruses
Viruses have long been around – we just started studying them in the twentieth century.
1930s – term “virus” used (Latin: poison)
1935 – Wendell Stanley (American chemist) isolated the tobacco mosaic virus
1930s electron microscope invented!

3. General Characteristics of Viruses
How would you define a virus?
Obligatory intracellular parasites
Contain a single type of nucleic acid (DNA or RNA, not both!)
Protein coat
No ribosomes
Host range – most viruses infect only specific types of cells (determined by host attachment sites and cellular factors required for viral replication)
Size: nm in length (smallest infectious agent – except prions!)
Not a cell

4. Relative sizes of Viruses
Relative Size Tutorial

6. Viruses vs. Bacteria

7. Viral Structure 1. Nucleic Acids Double-stranded DNA
Double stranded RNA
Single strand of DNA
Single strand of RNA
Can be linear or circular strand
Can be broken up in several pieces
Capsid – Protein coat surrounding the nucleic acid of a virus
• some capsids are enclosed by an envelope (lipids, proteins & carbs)
The capsid is composed of subunits called CAPSOMERES (those can be just one type of protein or several types of proteins)
Spikes!

8. Virion Structure Nucleic acid Capsid DNA or RNA Capsomeres
Figure 13.2a

9. 13 of the 20 families of animal viruses are enveloped

10. Capsomer – a subunit of the virus capsid shaped as a triangle or disc
2 general shapes for capsids – helical and icosahedral.
ICOSAHEDRAL – regular geometric pattern having 20 surfaces that meet to form 12 corners. Examples: rotavirus (naked) and herpes simplex (enveloped)

11. Morphology of a Helical Virus
Long rods – rigid or flexible
Examples: Ebola hemorrhagic fever (enveloped helical) rabies, influenza
Figure 13.4

14. Morphology of a Polyhedral Virus
Examples:
adenovirus poliovirus
Figure 13.2

15. Polyhedral Viruses
Figure 13.16a

16. Enveloped Viruses
Figure 13.16b

17. Morphology of an Enveloped Virus
Figure 13.3

18. Morphology of a Complex Virus
Examples:
Bacteriophages
Figure 13.5

19. Figure 13.6 The complex shape of bacteriophage T4.
Bacteriophages (Phages)
Figure The complex shape of bacteriophage T4.
Head
Tail fibers
Tail
Base plate

20. RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath
Fig. 19-3
RNA
DNA
Membranous
envelope
Head
RNA
Capsomere
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 19.3 Viral structure
20 nm
50 nm
50 nm
50 nm
(a) Tobacco mosaic
virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

21. Morphology of Viruses Enveloped Viruses (spherical) Helical Viruses
Most common type
Influenza
Helical Viruses
Bacteriophages
Complex large viruses

22. Viral vs. Bacterial Genome
Partial genome
of E. coli
Viral
genome

23. Viral Structure & Infections
Host gets infected by virus -> host produces antibodies (proteins that react with the surface proteins of the virus)
-> virus becomes inactivated/ infection stopped!
So why can you get the flu more than once if you’ve already developed antibodies for the virus?
B/c of mutations in the genes that code for for the viruses’ surface proteins -> altered surface proteins ->antibodies no longer react against them.
Influenza frequently undergoes changes in its spikes!