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An Introduction to the Viruses Chapter 6 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display. Types of Microbes Cellular Prokaryotes Acellular Eukaryotes Nucleus Chromosome Ribosomes Viruses Envelope Capsid Ribosomes Nucleic acid AIDS virus Cellwall Cell membrane Cell membrane Bacteria and archaea Fungi, protozoa, and helminthes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bacterial virus Viruses and bacteriophages 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 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 Properties of Viruses 6.3 The General Structure of Viruses 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 The Scale of Viruses • Too small for the light microscope • Range in size from 20 to 200 nm (0.02-0.2 µm) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. BACTERIA CELLS Rickettsia 0.3 µm Viruses 450nm 1. Mimivirus 2. Herpes simplex 150 nm 3. Rabies 125 nm 4. HIV 110 nm 100 nm 5. Influenza 6. Adenovirus 75 nm 7. T2 bacteriophage 65 nm 8. Poliomyelitis 30 nm 9. yellow fever 22 nm Protein Molecule 15 nm 10. Hemoglobin molecule Streptococcus 1 µm (1) (2) E. coli 2 µm long (10) (9) (8) (7) (3) (6) (4) (5) 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) 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 Basic Viral Structure Viral Capsid: The Protective Outer Shell • Constructed from identical subunits called capsomers • Made up of protein molecules • Two different types • Helical and Icosahedral Helical Capsid Icosahedral Capsid • Three-dimensional, 20-sided figure with 12 evenly spaced corners • Although they all display this symmetry, there are wide variations Icosahedral Capsid Complex Capsid • Found in Bacteriophages • Viruses that infect bacteria • Shape is not symmetrical • Contain tail fibers and capsid head Concept Check What is the protein shell that surrounds a viral genome called? A. Envelope B. Capsid C. Core D. Cortex 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 Viral Envelope 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 Concept Check What is the lipid membrane that surround some viruses called? A. Capsid B. Envelope C. Outer membrane D. Capsule
