©1999 Timothy G. Standish Exodus 15:26 26… If thou wilt diligently hearken to the voce of the LORD thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statutes, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the LORD that healeth thee.

©1999 Timothy G. Standish Genetics of Viruses Timothy G. Standish, Ph. D.

©1999 Timothy G. Standish 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

©1999 Timothy G. Standish Discovery of Viruses Mayer The first virus discovered was the tobacco mosaic virus (TMV) which causes tobacco leaves to be mottled and stunts growth 1883 A. Mayer (German) - Discovered that tobacco mosaic disease could be transmitted from one plant to another via the sap Mayer could not find bacteria in the sap He theorized that a bacterium too small to be seen with a microscope caused tobacco mosaic disease

©1999 Timothy G. Standish 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

©1999 Timothy G. Standish 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

©1999 Timothy G. Standish 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 bacterium was rejected and replaced with a new theory: Mosaic disease is caused by an agent much smaller and simpler than bacteria

©1999 Timothy G. Standish Discovery of Viruses Stanley 1935 Wendell Stanley (US) - Crystalized the Tobacco Mosaic Virus (TMV) After the advent of electron microscopy viruses could be viewed directly

©1999 Timothy G. Standish 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

©1999 Timothy G. Standish Phage Reproduction: The Lytic Cycle Destruction of the bacteria’s DNA Replication of the viral genome Production of viral parts Packaging Infection Lysis

©1999 Timothy G. Standish Many generations of bacteria Temperate phage Phage Reproduction: The Lysogenic Cycle Integration of phage DNA into the bacterial genome Circularization of phage DNA Infection On to the lytic cycle Exit of phage

©1999 Timothy G. Standish Are Viruses Alive? Viruses require cells to reproduce and do not have cells of their own, thus they do not fit the Cell Theory which states that being made out of cells is a characteristic of living things Are viruses “missing links” between living and non-living things? No, viruses require cells to reproduce, so cells had to come first, then viruses; thus viruses cannot serve as a “missing link” between living and non-living things

©1999 Timothy G. Standish