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Virology. An Introduction to the Viruses Non-Living Etiologies . 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. Viruses.
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Virology An Introduction to the VirusesNon-Living Etiologies
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
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
The Size of Viruses Figure 6.1
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
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.
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.
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.
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
DNA Viruses • ssDNA • dsDNA • linear • circular
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
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
Modes of Viral Multiplication The host cell is absolutely necessary for viral multiplication
Viruses that Infect Bacteria • Bacteriophage • Most contain dsDNA • Often make the bacteria they infect more pathogenic for humans
Bacteriophage • Icosahedralcapsid head containing DNA • Central tube surrounded by a sheath • Neck • Base plate • Tail pins • Fibers
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
Lytic Cycle As A Model Penetration The virus "drills" a hole in the bacterial wall and the virusinjects its genome into the bacterial cytoplasm. Some phages accomplish this by contracting a sheath which drives a hollow tube into the bacterium.
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.
Lytic Cycle As A Model Viral Assembly Viral proteins are synthesized and self-assembled into viral components such as head, tail, and tail fibers.
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.
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.
