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Viruses. Virus Introduction. A virus is a sub-microscopic infectious agent only capable of reproducing within a host cell. Virus is Latin for toxin or poison. Virus Introduction. Biologists debate whether or not viruses are living.
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Virus Introduction • A virus is a sub-microscopic infectious agent only capable of reproducing within a host cell. • Virus is Latin for toxin or poison.
Virus Introduction • Biologists debate whether or not viruses are living. • Some say they are not because they are not composed of cells or fit any of the other criteria of life. • Others would argue that since they have been shaped by evolution through natural selection, they must be living. • A viral infection in a higher organism usually invokes an immune response. Viruses cause disease or illness. • Antibiotics do not work against viral infection, only antivirals or vaccines are able to keep a virus at bay.
Viruses in History • Viruses have been written about since the 10th century. • Many Muslim doctors of the Middle Ages described viral diseases and how to best keep them from spreading.
Viruses in History • The first virus identified was in tobacco plants, the tobacco mosaic virus (TMV). • The first human virus found was that which caused Yellow Fever. • Scientists then began to grow and study viruses to better understand them.
The Origin of the Virus • The origin of the modern virus is unclear. • Two hypotheses exist: • They could be runaway stretches of nucleic acid from a larger organism that detached and became active, therefore new viruses are forming frequently and many do not have ancestors • Viruses once lived outside of host cells, but over time due to their parasitic lifestyle, they lost the genes necessary to live outside the host
Classification of Viruses • The Linnean classification system is modified for viruses since they do not fit into any of the current three Domains. • There has been talk of a new Domain, Domain Acytota, meaning ‘without cells’ for the viruses. • Currently they have Phylum down to Species. • Nobel Prize winner, David Baltimore has developed his own classification system for viruses, the Baltimore Classification System.
Baltimore Classification System • Classifies viruses based on: • Type of nucleic acid (DNA or RNA) • Single stranded or double stranded • Whether or not they use reverse transcriptase. (work backwards going from RNA to DNA)
Structure of a Virus • Viruses are composed of genetic material, either RNA or DNA, and a protective protein layer known as a capsid. • Viruses can have a lipid envelop made from the host cell’s cell membrane.
Virus Lytic Cycle • There are 5 parts to the viral lytic cycle: • Attachment: the virus must attach to the cell wall or cell membrane of the host. • Penetration: the virus must enter the cell through cell membrane fusion or endocytosis. • Uncoating: the protein coat is worn away exposing the viral genome. • Replication and Assembly: the viral genome takes over the host cell’s machinery and creates more viruses. • Lysis: The new virus particles rupture forth from the host cell.
DNA Viruses • These usually infect a host through a chance encounter with the virus. • These include some of the more well known viruses such as the herpes virus and any of the pox viruses such as smallpox or chicken pox.
RNA Viruses • Use RNA as their genetic material or use an RNA intermediate to replicate. • These include the Rubella virus which causes the German Measles.
Reverse transcripting viruses • These convert RNA into DNA and incorporate it into a host cell. • Often called retroviruses because they work backwards. • HIV is the most well known of these types of viruses.
Viruses and Disease • Viruses cause some of the most common illnesses such as the common cold, the flu, cold sores, etc… • Viruses also cause some of the deadliest diseases known to man, Ebola, Avian Flu, AIDS, etc… • The virulence of a virus is its ability to cause disease.
Applications of Viruses • Viruses have been used to study the basic mechanisms of molecular biology such as DNA replication, protein synthesis, etc… • Viruses have also been used by geneticists to genetically modify organisms. • Virotherapy uses viruses to treat bacterial diseases and some forms of cancer. • Viruses are also being used as biological insecticides.
Applications of Viruses • Viruses are currently being used in nanotechnology acting as tools to stop or start sequences needed in host cells. • Viruses have also been explored as a viable biological weapon. Scary thought!
