The Viruses

• The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell. 
• Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host. 
• Viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell. 
• Viruses are obligate parasites.In addition to proteins viruses also contain genetic material, that could be either RNA or DNA. 
• No virus contains both RNA and DNA. 
• A virus is a nucleoprotein and the genetic material is infectious. 
• In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA. 
• Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses. 
• The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms. 
• Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus. 
• In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.