Evolution This page will provide you with information regarding virus structure and how viruses go about replicating. You can click on the bold words to get definitions from the vocabulary page. Viruses are submicroscopic intracellular parasites that consist of either RNA or DNA (never both) plus a protective coat of protein or of protein combined with lipid or carbohydrate components. The nucleic acid is usually a single molecule, either sing ly or doubly stranded. Some viruses, however, may have nucleic acid that is segmented into two or more pieces. The protective protein shell is known as the capsid, and the individual proteins of the capsomeres. Other viruses have another envelope that is usually formed as the capsid grows from the cell in which the virus is growing. The complete virus particle is called the nanometers in width. Making it impossible for even the largest viruses to be seen with the light microscopes which are used to study bacteria and other large microorganisms. Many of the viruses with helical internal structure have outer envelops composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometers in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. Complex viruses are the exceptions fo r most viruses are simple in shape. Viruses are not cells and do not contain the enzymes and metabolic precursors necessary for self-replication. They have to get these from the cells that they infect. The process of virus replication is one of separate synthesis of the components needed to create a new virus and the assembly of those components into the new virus particles. The process begins when a virus enters the cell. The virus coat (capsid) is removed by cellular enzymes, and the virus RNA or DNA comes into contact with ribosome’s inside the cell. Once there, the RNA or DNA of the virus directs the synthesis, by the cell, of proteins specified by the viral nucleic acid. The nucleic acid then replicates itself, and the protein sub units (capsomeres) constituting the viral coat (capsid) are synthesized. Thereafter the two components are assembled into a new virus. One virus can replicate into thousands of new viruses. Once more and more viruses are replicated some of them are released by destruction of the infected cell when it bursts from the infecting viruses. Others are released by pushing through cell membranes and do not kill the cell. In some instances infection causes no obvious cell damage. The RNA-containing viruses are unique among replicating systems because the RNA can replicate itself independently of DNA. The RNA can function as messenger RNA, replicating itself using the host cell. Also, RNA viruses can carry within the coat (capsid) an RNA-dependent enzyme that directs the synthesis of virus RNA. Other RNA viruses, which are known as retroviruses, can produce