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Discuss reasons for studying viruses.
Understand the history of virus discovery
Define the term ‘Virus’
Describe the definitive properties/characters of viruses
Explain whether are viruses living or nonliving entities.
Describe the basic structure and components of viruses.
Types of Viruses and their Impact.
DEFINITION
A virus is a ultramicrosopic, acellular (non-cellular) parasite of cells. Its genome, which is composed of either DNA or RNA, is enclosed in a protein coat (capsid). |
REASONS FOR STUDYING VIRUSES
- Ubiquitous Presence
- Human Diseases Causation
- Infect all Living things
- Potential Cross-species Transmission
- Can be Beneficial
- Unique Tools for Biology |
DISCOVERY
1886 |
Adolph Mayer |
Described light and dark green patterns on tobacco leaves infected with mosaic virus. |
1892 |
Dmitri Ivanovsky |
Tobacco mosaic disease (ssRNA Plant Virus) transmitted through porcelain Pasteur-Chamberland filters, excluding bacteria. |
1898 |
Martinus Beijerinck |
Replicates Dmitri Ivanovsky's filtration experiments and identified a "virus" responsible for tobacco mosaic disease. a “virus” (Latin for Poison), which he describes as a “contagium vivum fluidum” or “contagious living fluid”. Beijerinck and Ivanovsky are considered to be founders of virology. |
1935 |
Wendell Stanley |
Produces the first crystals of tobacco mosaic virus and shows that the virus remains active after crystallization. Crystallization of the virus was the first step toward proving that the virus is particulate. |
VIRUSES: LIVING OR NON-LIVING?
The debate on viruses' living or nonliving status depends on definition of life. |
Living characteristics.. |
Possess genetic material. |
Ability to replicate. |
Undergo evolution. |
Adapt to hosts. |
Engage in biotic interactions. |
Occupy ecological niches. |
Non-living characteristics.. |
Lack of metabolism |
Requires host cell to replicate. |
Exists in an inactive state (Outside of the host cell) |
Inability to divide. |
Absence of cell organelles |
DEFINITIVE FEATURES OF VIRUSES
Smaller than Bacteria |
Non-Cellular Infectious Entities |
Obligate Intracellular Parasites |
Single type of nucleic acid genomes (DNA /or RNA) |
Nucleic Acid Genomes Shielded by Protein coat called "Capsids" |
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VIRUS STRUCTURE
All particles have
Genome (Nucleic acids either DNA or RNA)
Protein Coat (Capsid)
Enzymes
Some particles have
Envelope (Lipid bilayer)
Envelope proteins |
COMPONENTS OF VIRUS
1. Nucleic Acid Core |
DNA or RNA (not both!)
RNA viruses are either positive (+) sense or negative (-) sense. Positive sense RNA can function directly as mRNA
Single stranded vs. double stranded
Linear vs. circular
Continuous vs. segmented |
Functions: Contains hereditary information Codes for
synthesis of structural proteins (capsid)
non-structural proteins (enzymes necessary for the replication of the genome)
Genome replicates itself within a living system to pass on genetic information to the viral progeny. |
2. Capsid/Shell |
Consists of protein subunits, capsomeres |
Functions: Contains hereditary information Codes for
synthesis of structural proteins (capsid)
non-structural proteins (enzymes necessary for the replication of the genome)
Genome replicates itself within a living system to pass on genetic information to the viral progeny. |
3. Envelopes / Membranes |
Enveloped vs. naked viruses .
Lipid bilayer Acquired from host cell when budding through plasma membrane (paramyxoviruses), golgi apparatus and endoplasmic reticulum (hepatitis B) or nuclear membrane (herpes simplex virus)
Damaged by detergents, alcohols, etc. |
4. Spikes/Envelope Proteins |
Situated in the envelope of enveloped viruses |
Functions:
attachment to host-cell receptor for viral entry
Major antigenic determinant(s) of virion.
Host immune responses directed towards these 'spikes'. |
5. Enzymes |
Many Viruses package their own enzymes needed for replication |
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SHAPES OF VIRUS
Helical |
Capsid consists of a ribbonlike protein that forms a spiral around the nucleic acid. |
Tobacco Mosaic Virus |
Polyhedral |
Polyhedral (many-sided) capsid shapes is the icosahedron; icosahedral viruses have 20 triangular faces. |
Adenovirus |
Complex |
A combination of helical and icosahedral shapes, |
Bacteriophages |
TYPES OF VIRUSES
Based on Genetic material
RNA Virus: Contain RNA (Tobacco mosaic virus)
DNA Virus: Contain DNA (T-Phage) |
IMPACTS OF VIRUSES
Negative |
Viruses have caused extensive disease and suffering for Humans Domesticated plants and animals |
Positive |
Viruses are so important in terms of recycling.
Phage Therapy
Prevention & treatment of Human infectious disease
Prevention & tratment of animal infectious diseases
Control of bacterial plant diseases
Sanitizing & disinfection of Food
Oncolytic viruses: are ones that selectively infect and destroy cancer cells, are also increasingly being explored as a less toxic and more efficient cancer treatment. |
IMPORTANT TERMS
Capsid |
The protein coat that encloses the nucleic acid of a virus. |
Capsomere |
Small, protein subunits that make up the protein coat (capsid). |
Prion |
A protein-based infectious particle that causes disease. |
Virion |
A complete virus, with all the components needed for host cell infection. |
Envelope |
A lipid bilayer and associated protein forming the outer component of an enveloped virion. |
Naked Virus |
A virus that does not have an envelope. |
Nucleocapsid |
The virus genome enclosed in a protein capsid. |
Bacteriophage (phage) |
A type of virus that infects bacteria. |
Endemic |
A disease that is constantly present or commonly present in a geographical area. |
Epidemic |
A rapid increase in the number of cases of a disease that spreads over a larger geographical area. |
Pandemic |
A disease outbreak throughout the world. |
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