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.

less than 0.1 µm in diameter Hundreds of thousands can fit inside a typical human cell. Size of Viruses

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 10 th 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: 1) 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 2) 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: 1) Type of nucleic acid (DNA or RNA) 2) Single stranded or double stranded 3) 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 envelope made from the host cell’s cell membrane.

Diagrams of Viruses

Virus Lytic Cycle

Virus Lytic Cycle (also called the infectious cycle) There are 5 parts to the viral lytic cycle: 1) Attachment: the virus must attach to the cell wall or cell membrane of the host. 2) Penetration: the virus must enter the cell through cell membrane fusion or endocytosis. 3) Uncoating: the protein coat is worn away exposing the viral genome. 4) Replication and Assembly: the viral genome takes over the host cell’s machinery and creates more viruses. 5) Lysis: The new virus particles rupture from the host cell.

Lysogenic Cycle Lysogenic Cycle: enter a host cell but do not take over the activity by destroying the host DNA, but combines with and becomes part of the host DNA in a way that does not interfere with the host cell activity. When the host copies its own DNA, it copies the viral DNA as well for many generations in lysogeny Changing conditions cause the phage to enter the lytic cycle. Bacteria are not defenseless against phage infections. Some bacteria have enzymes called restriction enzymes that can recognize foreign DNA, and cut up the DNA, making it useless.

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Differences Between Lytic and Lysogenic Cycles In the Lytic Cycle: Viral DNA destroys Cell DNA, takes over cell functions and destroys the cell. The virus replicates and produces progeny phages. There are symptoms of viral infection. In the Lysogenic Cycle: Viral DNA merges with cell DNA and does not destroy the cell. The virus does not produce progeny. There are no symptoms of viral infection.

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.

RNA or DNA? Viruses with RNA (higher mutation rate) Human immunodeficiency virus (HIV) Influenza viruses Rabies Measles, mumps, pneunomia, polio, common cold SARS Viruses with DNA (usually stable/constant, vaccines effective ) Chickenpox, cold sores, genital herpes Mononucleosis Hepatitis Respiratory infections, tumours

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!

Viroids Small, infectious pieces of RNA that are smaller than viruses and don’t have a capsid. Plant pathogens that interfere with normal functioning of the host cells RNA.

Prions Abnormally shaped infectious protein responsible for some brain diseases of mammals. Found in the brain and nervous tissue EG: bovine spongiform encephalopathy (BSE) or “mad cow disease”

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End on a cuddlier, happier note…