The Zombies of the Scientific Community Viruses

Transcription

1 The Zombies of the Scientific Community Viruses What are viruses and what do they look like? Viruses do not satisfy all of the characteristics of life. Often, viruses are called parasites because they need a host cell in order to continue on. Viruses cannot reproduce on their own! Remember, viruses are not out to get you! They replicate within the cell and as a consequence of their living, you get sick or worse. Viruses Viruses are small (about 17 nanometers in diameter). Some viruses are as large as 1000 nanometers, which is barely visible under light microscopy. Viruses occur in all organisms In every case, the basic structure is the same. They are segments of DNA or RNA wrapped in a protein coat called the capsid. Capsid Protective outer shell that surrounds viral nucleic acid Capsid spikes Composed of capsomer subunits Two types of capsids Helical Icosahedral Helical capsid Naked helical virus Ex. Tobacco mosaic virus Nucleocapsid is rigid and tightly wound into a cylinder-shaped package Enveloped helical virus Ex. Influenza, measles, rabies Nucleocapsid is more flexible Helical capsids have rod-shaped capsomers that form hollow discs that resemble a bracelet. Comparison between a naked helical plant virus and an enveloped helical human virus.

2 Icosahedron capsid Three-dimensional, 20-sided with 12 evenly spaced corners Variation in capsomer number Polio virus 32 capsomers Adenovirus 240 capsomers The structure and formation of an adenovirus. Icosahedral viruses can be naked or enveloped. Some viruses are enclosed by an envelope Most viruses infect only specific types of cells in one host Host range is determined by specific host attachment sites and cellular factors Envelope Lipid and proteins Envelope spikes During release of animal viruses, a part of the host membrane is taken Enable pleomorphic shape of the virus Spherical filamentous Function of the capsid/envelope Protect nucleic acid from the host s acid- and protein-digesting enzymes Assist in binding and penetrating host cell Stimulate the host s immune system Complex viruses Structure is more intricate than helical and icosahedral viruses Pox virus Several layers of lipoproteins Course surface fibrils Bacteriophage Polyhedral head Helical tail Fibers for attachment An example of a pox virus and T4 bacteriophage. Comparison of the morphology (helical, icosahedral, complex) of a naked virus, enveloped virus and a complex virus.

3 Negative staining, positive staining, and shadow casting are methods of viewing viruses. Nucleic acid Viruses contain either DNA or RNA Possess only the genes to invade and regulate the metabolic activity of host cells Ex. Hepatitis B (4 genes) and herpesviruses (100 genes) No viral metabolic genes, as the virus uses the host s metabolic resources Examples of medically important DNA viruses. Examples of medically important RNA viruses. Viral Taxonomy Order names end in -virales Family names end in -viridae Genus names end in -virus Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species Subspecies are designated by a number Herpesviridae Herpesvirus Human herpes virus 1, HHV 2, HHV 3 Retroviridae Lentivirus Human Immunodeficiency Virus 1, HIV 2 Three orders of viruses have been developed for classification. Classification of important human viruses. Virus Identification Cytopathic effects Serological tests Detect antibodies against viruses in a patient. Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot. Nucleic acids RFLPs PCR

4 In vivo methods Laboratory animals Embryonic bird tissues In vitro methods Cell or tissue culture Cultivation and Replication The early developing bird embryo contains a protective case, providing an ideal environment for viral propagation. A monolayer of monkey kidney cells is a cell culture enabling the propagating viruses. Animal and plants viruses may be grown in cell culture. Continuous cell lines may be maintained indefinitely. Adsorption Penetration Uncoating Synthesis Assembly Release Multiplication Adsorption to the host cell of a enveloped spike virus and naked capsid spike virus. Penetration of animal viruses occur by endocytosis or fusion between the viral envelope and the the host cell membrane. Uncoating and synthesis of viruses rely on the host s metabolic systems. A mature virus can obtain an envelope by budding off the host cell. Bacteriophage Bacterial virus Multiplication is similar to animal viruses except for the penetration (inject DNA), release (lyses) and prophage (lysogeny) stages Attachment Penetration Biosynthesis Multiplication of Bacteriophages (Lytic Cycle) Phage attaches by tail fibers to host cell Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell Production of phage DNA and proteins

5 Maturation Release Assembly of phage particles Phage lysozyme breaks cell wall T-even bacteriophage penetrate the host cell by specifically binding and injecting their DNA into the host cell. After viral multiplication inside the host cell, viral enzymes will weaken the host cell membrane, rupture the cell (lyses), and release numerous virions. Lysogeny is when the bacteriophage can insert its DNA into the bacterial host genome. Comparison of bacteriophage and animal virus multiplication. Lytic vs. Lysogenic Lytic cycle Phage causes lysis and death of host cell. Lysogenic cycle Prophage DNA is incorporated into the host cell.