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Biology Notes On Viruses. Essay

1309 words - 5 pages

Notespg. 455-461Biologists first suspected the existence of viruses near the end of the nineteenth century. At that time, European scientists were seeking to identify the infectious agent responsible for tobacco mosaic disease, a disease that stunts the growth of tobacco plants and makes their leaves blotchy. One experiment used fine-pored porcelain filters to strain bacteria from sap extracted from infected tobacco plants. The pores in these filters were so small that bacteria could not pass through them. However, the infectious agent passed through the filters without difficulty. Scientists concluded that the infectious agent must be smaller than a typical bacterial cell. After further investigation, the scientists found that the agent could reproduce only inside the living cells it infected. They called the agent a virus, a Latin word meaning "poison."The true nature of viruses was discovered in 1933 when the biologist Wendell Stanley of the Rockefeller Institute tried to purify an extract of the tobacco mosaic virus (TMV). To his great surprise, the purified TMV extract formed crystals, a property of chemicals. Stanley concluded that TMV is chemical matter rather than a living organism.Within a few years of Stanley's findings, scientists were able to disassemble TMV and confirm his conclusion. TMV is a chemical, not a cell. Each particle of TMV is made of only two kinds of molecules: the nucleic acid RNA and protein.TMV consists of a core of RNA surrounded by a coat of protein. Later scientists were able to separate the RNA from the protein. When they reassembled the two components, the reconstructed TMV particles were fully able to infect healthy tobacco plants. The chemicals were the virus itself, not merely derivatives of it. Through this experiment and others, a general picture of virus structure and function has emerged. A virus is a strand of nucleic acid encased in a protein coat that can infect cells and replicate within them. Biologists do not consider viruses to be living organisms.Most viruses have a protein sheath, or capsid, surrounding a core of nucleic acid. Many plant viruses, as well as some animal viruses such as the human immunodeficiency virus (HIV) that causes AIDS, contain RNA. However, the nucleic acid found in animals, such as the influenza virus, have a membranous envelope surrounding the capsid. The envelope helps the virus gain entry into cells. It contains proteins, lipids, and glycoproteins (proteins with carbohydrate molecules attached) derived from the host cell.Viruses differ greatly in appearance. Simple viruses consist of a single molecule of a nucleic acid and a capsid made of a single protein or a few different protein molecules repeated several times. The more complex viruses may consist of several different segments of DNA or RNA contained within a capsid made of several different kinds of protein. Most viruses have an overall structure that is either helical or polyhedral. A helical virus, like the...

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