Outline on Viruses
A. Is a Virus Alive? 1. Viruses are segments of nucleic acids contained in a protein coat. 2. Pathogens are agents that cause disease. 3. Viruses do not grow, do not have homeostasis, and do not metabolize, therefore scientists don’t consider them to be living. 4. Discovery of Viruses i. Scientists filtered bacteria from the sap of infected plants, and were surprised to find that the filtered sap could still cause uninfected plants to become infected. ii. In 1935, Wendell Stanley of the Rockefeller Institute purified tobacco mosaic virus. The crystallized the purified virus, and concluded that TMV is a chemical rather than an organism. B. Viral Structure 1. The virus protein coat, or capsid, may contain either RNA or DNA, but not both. 2. Many viruses, such as the influenza virus, have a membrane, or envelope, surrounding the capsid. 3. The envelope consists of proteins, lipids, and glycoproteins, which are proteins with attached carbohydrate molecules. 4. Viruses exist in a variety of shapes, from rods, spheres, polyhedral figures, etc. 5. Viruses that infect bacteria are called bacteriophages. C. Viral Reproduction 1. Lytic Cycle i. In bacterial viruses, the cycle of viral infection, replication, and cell destruction is called the lytic cycle. 2. Lysogenic Cycle i. In the lysogenic cycle, the viral genome replicates without destroying the host cell. ii. During an infection, some viruses stay inside the cells but do not make new viruses. Instead of producing virus particles, the viral gene is inserted into the host chromosomes and is called a provirus. 3. Host Cell Specificity i. Viruses are often restricted to certain kinds of cells. For example, TMV can infect tobacco and related plants, but not animals. Scientists hypothesize this specificity may be due to the viruses’ origin. 4. Structure of HIV-an Enveloped Virus i. Many viruses that infect only animals, such as influenza, have an exterior viral envelope. ii. In many cases, the viral envelope is composed of a