1. An Introduction to the Viruses Chapter 6 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display.

2. CellularAcellular ProkaryotesEukaryotesViruses Bacteria and archaea Fungi, protozoa, and helminthes Viruses and bacteriophages Types of Microbes Bacterial virus AIDS virus Envelope Capsid Nucleic acid Chromosome Ribosomes CellwallCell membrane Ribosomes Cell membrane Nucleus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

3. 6.1 The Search for the Elusive 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 1890s Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus Loeffler and Frosch discovered an animal virus that causes foot –and-mouth disease in cattle Many years of experimentation showed what we know today and by the 1950s virology had grown

4. 6.2 The Position of Viruses in the Biological Spectrum Can infect every type of cell Cannot exist independently from the host cell, so aren’t considered living things However, since they can direct life processes they are often considered more than lifeless molecules Referred to as infectious particles, either active or inactive 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. Properties of Viruses

6. 6.3 The General Structure of Viruses

7. Size Range Smallest infectious agents Most are so small, they can only be seen with an electron microscope Animal viruses Proviruses- around 20 nm in diameter Mimiviruses- up to 450 nm in length Viewing viruses Special stains and an electron microscope Negative staining outlines the shape Positive staining shows internal details Shadowcasting technique

8. The Scale of Viruses Too small for the light microscope Range in size from 20 to 200 nm (0.02-0.2 µm) 1. (1) (2) (3) (4) (6) (7) (8) (9) (10) 10. 2. 3. 4. 5. 6. 7. 8. 9. BACTERIA CELLS Rickettsia 0.3 µm Viruses Mimivirus 450nm Herpes simplex 150 nm Rabies 125 nm HIV 110 nm Influenza 100 nm T2 bacteriophage 65 nm Poliomyelitis 30 nm yellow fever 22 nm Protein Molecule Hemoglobin 15 nm molecule (5) E. coli 2 µm long Streptococcus 1 µm Adenovirus 75 nm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

9. Viral Components: Capsids, Nucleic Acids, and Envelopes Molecular structure- composed of regular, repeating subunits that give rise to their crystalline appearance Contain only those parts needed to invade and control a host cell Capsid Envelope- not found in all viruses Core- genetic material (DNA or RNA)

10. Basic Viral Structure External coating Capsid Envelope- in 13 of the 20 families of animal viruses If no envelope, called naked virus Core - Nucleic Acid 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- virion

11. Basic Viral Structure

12. Viral Capsid: The Protective Outer Shell Constructed from identical subunits called capsomers Made up of protein molecules Two different types Helical and Icosahedral

13. Helical Capsid

14. Icosahedral Capsid Three-dimensional, 20-sided figure with 12 evenly spaced corners Although they all display this symmetry, there are wide variations

15. Icosahedral Capsid

16. Complex Capsid Found in Bacteriophages Viruses that infect bacteria Shape is not symmetrical Contain tail fibers and capsid head

17. Concept Check What is the protein shell that surrounds a viral genome called? A.Envelope B.Capsid C.Core D.Cortex

18. The Viral Envelope Enveloped viruses take a bit of the host cell membrane in the form of an envelope In the envelope, some or all of the regular membrane proteins are replaced with viral proteins Some proteins form a binding layer between the envelope and the capsid Glycoproteins remain exposed as spikes - essential for attachment

19. Viral Envelope

20. 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 the host cells against future infections

21. Concept Check What is the lipid membrane that surround some viruses called? A.Capsid B.Envelope C.Outer membrane D.Capsule