Microbiology Chapter 6 Notes: An Introduction to Viruses
Microbiology Chapter 6 Notes: An Introduction to Viruses BIO 2600
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This 5 page Class Notes was uploaded by Katelyn Farris on Monday September 12, 2016. The Class Notes belongs to BIO 2600 at William Carey University taught by Dr. Cunningham in Fall 2015. Since its upload, it has received 3 views. For similar materials see Microbiology in Biology at William Carey University.
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Date Created: 09/12/16
Chapter 6 notes The Search for the Elusive Virus -Louis Pasteur postulated that rabies was caused by a virus (1884) -Ivanovski and Beijerinck showed a disease in tobacco was caused by a virus (1890s) -1950s virology was a multifaceted discipline -Viruses: noncellular particles with a definite size, shape, and chemical composition General Structure of Viruses -Electron microscope is required Viral Components: Capsids, Nucleic Acids, and Envelopes -Viruses bear no resemblance to cells -Lack protein-synthesizing machinery -Viruses contain only the parts needed to invade and control a host cell *Virus profile pg. 162 General Structures of Viruses -Capsids -All viruses have capsids – protein coats that enclose and protect their nucleic acid - The capsids together with the nucleic acid are nucleocapsid -Some viruses have an external covering call envelope; those lacking an envelope are naked -Each capsid is constructed from identical subunits called capsomers made of protein -Two structural types: -Helical – continuous helix of capsomers forming a cylindrical nucleocapsid -Icosachedral – 20-sided with 12 corners -Vary in the number of capsomers -Each capsomer may be made of 1 or several proteins -Some are enveloped -Viral Envelope -Mostly animal viruses -Acquired when the virus leaves the host cell -Exposed proteins on the outside of the envelope, called spikes, essential for attachment of the virus to the host cell Functions of Capsid/Envelope -Protects the nucleic acid when the virus is outside the host cell -Helps the virus to bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell Chapter 6 notes General Structure of Viruses -Complex viruses: atypical viruses -Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins -Some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers* deceptacon* *Figure 6.9 *Figure 6.10 Nucleic Acids -Viral genome – either DNA or RNA but never both -Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses -Number of genes varies for each type of virus – few to hundreds -DNA viruses -Usually double stranded (ds) but may be single stranded (ss) -Circular or linear -RNA viruses -Usually single stranded, may be double stranded may be segmented into separate RNA pieces -ssRNA genomes ready for immediate translation are positive- sense RNA -ssRNA genomes that must be converted into proper form are negative-sense RNA General Structure -Pre-formed enzymes may be present -Polymerases – DNA or RNA -Replicases – copy RNA -Reverse transcriptase – synthesis of DNA from RNA (AIDS virus) Modes of Viral Multiplication -General phases in animal virus multiplication cycle: 1. Adsorption – binding of virus to specific molecule on host cell 2. Penetration – genome enters host cell 3. Uncoating – the viral nucleic acid is released from the capsid 4. Synthesis – viral components are produced 5. Assembly – new viral particles are constructed 6. Release – assembled viruses are released by budding (exocytosis) or cell lysis *Figure 6.11 pg. 170 Chapter 6 notes Adsorption and Host Range -Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane -Spectrum of cells a virus can infect – host range -Hepatitis B – human liver cells -Poliovirus – primate intestinal and nerve cells -Rabies – various cells of many mammals Penetration/Uncoating -Flexible cell membrane is penetrated by the whole virus or its nucleic acid by: -Endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle -Fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm *Figure 6.13 Replication and Protein Production -Varies depending on whether the virus is a DNA or RNA virus -DNA viruses generally are replicated and assembled in the nucleus -RNA viruses generally are replicated and assembled in the cytoplasm -Positive-sense RNA contain the message for translation -Negative-sense RNA must be converted into positive-sense message Release -Assembled viruses leave host cell in one of two ways: -Budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed -Lysis – nonenveloped and complex viruses released when cell dies and ruptures -Number of viruses released is variable -3,000 – 4,000 released by poxvirus - >100,000 released by poliovirus Damage to Host Cell -Cytopathic effects – virus-induced damage to cells 1. Changes in size and shape 2. Cytoplasmic inclusion bodies 3. Inclusion bodies 4. Cells fuse to form multinucleated cells (syncytia) 5. Cell lysis Chapter 6 notes 6. Alter DNA 7. Transform cells into cancerous cells -Some animal viruses enter host cell and permanently alter is genetic material resulting in cancer – transformation of the cell -Transformed cells have increased rate of growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors -Mammalian viruses capable of initiating tumors are called oncoviruses -Papillomavirus – cervical cancer -Epstein-Barr virus – Burkitt’s lymphoma Lysogeny: The Silent Virus Infection -Not all phages complete the lytic cycle -Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate -The viral genome inserts into bacterial genome and becomes an inactive prophage - the cell is not lysed -Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny -Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis *Figure 6.17 *Pg. 174-176 Lysogeny -Results in the spread of the virus without killing the host cell -Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology – lysogenic conversion -Corynebacterium diphtheria -Vibrio cholera -Clostridium botulinum Persistent Infections -Persistent infections – cell harbors the virus and is not immediately lysed -Can last weeks or host’s lifetime; several can periodically reactive – chronic latent state -Measles virus – may remain hidden in brain cells for many years -Herpes simplex virus – cold sores and genital herpes Chapter 6 notes -Herpes zoster virus – chickenpox and shingles Prions and Other Infectious Particles -Prions – misfolded proteins, contain no nucleic acid -Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases -Common in animals: -Scrapie in sheep and goats -Bovine spongiform encephalopathies (BSE), aka mad cow disease -Wasting disease in elk - Humans – Creutzfeldt-Jakob Syndrome (CJS) -Extremely resistant to usual sterilization techniques Kuru: a prion disease transmitted by ritual cannibalism Prions are not viruses
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