Unit 3: Viruses!

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!

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: 20 -500nm in length (smallest infectious agent – except prions!) Not a cell

Relative sizes of Viruses Relative Size Tutorial

Viruses vs. Bacteria

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!

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

13 of the 20 families of animal viruses are enveloped

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)

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

Morphology of a Polyhedral Virus Examples: adenovirus poliovirus Figure 13.2

Polyhedral Viruses Figure 13.16a

Enveloped Viruses Figure 13.16b

Morphology of an Enveloped Virus Figure 13.3

Morphology of a Complex Virus Examples: Bacteriophages Figure 13.5

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

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

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

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

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!