Viruses

What Are Viruses? Viruses are tiny ( nm) particles composed of a nucleic acid core (either DNA or RNA) surrounded by a protein coat (capsid) and sometimes a membrane envelope (derived from its host cell) Viruses are incapable of reproduction independent of host cells whose cellular machinery is taken over by the virus and used as a factory for producing new viruses

Discovery of Viruses Mayer The first virus discovered was the tobacco mosaic virus (TMV) which causes tobacco leaves to be mottled and stunts growth He theorized that a bacteria to small to be seen with a microscope caused tobacco mosaic disease

1890s D. Ivanowsky (Russia) filtered sap through ceramic filters designed to remove bacteria and found that the filtered sap still caused tobacco mosaic disease Ivanowsky theorized that the disease had two possible causes: 1 Bacteria small enough to pass through his filters 2 A toxin that passed through the filters Discovery of Viruses Ivanowsky

Discovery of Viruses Beijerinck 1897 Martinus Beijerinck (Holland)- Demonstrated that the mosaic disease agent could reproduce First sap was filtered, then placed on leaves When these leaves developed mosaic disease, their sap was used to cause disease in other plants for several generations At no point was the ability to cause disease diminished This eliminated the possibility of a toxin alone being the cause

Discovery of Viruses Beijerinck Beijerinck reasoned that if it was a small bacteria it could be cultured using nutrient media This proved to be impossible as the agent would only reproduce in the tobacco plant Bacteria should be killed by ethanol, but ethanol did not destroy the ability to cause mosaic disease Because of this, the idea of a very small bacteria was rejected and replaced with a new theory: Mosaic disease is caused by an agent much smaller and simpler than bacteria

Discovery of Viruses Stanley 1935 Wendell Stanley (US) - Crystallized the Tobacco Mosaic Virus (TMV) After the advent of electron microscope viruses could be viewed directly

Do viruses qualify as living things? They are much smaller and less complex than cells. They consist of either DNA or RNA enclosed in a protein coat called a capsid. Virons do not grow. They have no nucleus, cytoplasm, or membranes. They do not carry out cellular functions. Virons do not generate metabolic energy. They are obligate intracellular parasites, meaning that they require a host cell to reproduce.

Viral Structure Viruses are made of a maximum of three basic parts: 1 Genetic material: –DNA viruses - Single or double stranded DNA –RNA viruses - Single or double stranded RNA 2 Capsid - A protein coat made up of one or a few capsomere proteins 3 Envelope - Not all viruses have an envelope which is derived from the host cell’s membrane The envelope may contain proteins which assist in attachment and entry into new host cells

Characteristics used to classify viruses. Type of nucleic acid: viruses have either single-strand RNA or double-strand DNA, but never both. Physical structure: viruses are found in several general shapes. –The structure of viruses has been deciphered by electron microscopy and X-ray diffraction. These techniques identify two shapes –Envelope: some viruses use lipids from the cell to form a layer around the capsid during replication. –Type of host cell: most viruses are very specific about the type of cell they invade. There are specific receptor sites that viruses attach to on a host cell's surface.

Shapes of Viruses icosahedron (the capsid has 20 triangular faces) helix (the capsid resembles a coiled spring).

Common Viruses

Phage Reproduction: The Lytic Cycle Destruction of the bacteria’s DNA Replication of the viral genome Production of viral parts Packaging Infection Lysis

Size Of Viruses Viruses are very small, ranging in size from 20 nanometers to 250 nanometers. A nanometer is equal to inch (4X10-8 inch). The smallest of all bacteria is about the size of the largest virus. This picture shows round virus particles along with rod-shaped bacteria cells

Summary The sequence of events that occurs when you come down with the flu or a cold is a good demonstration of how a virus works: An infected person sneezes near you. You inhale the virus particle, and it attaches to cells lining the sinuses in your nose. The virus attacks the cells lining the sinuses and rapidly reproduces new viruses. The host cells break, and new viruses spread into your bloodstream and also into your lungs. Because you have lost cells lining your sinuses, fluid can flow into your nasal passages and give you a runny nose. bloodstreamlungs Viruses in the fluid that drips down your throat attack the cells lining your throat and give you a sore throat. Viruses in your bloodstream can attack muscle cells and cause you to have muscle aches.muscle