Want to learn more about Viroids & Viruses. NEET Botany XI Biological Classification скачать в хорошем качестве

Want to learn more about Viroids & Viruses. NEET Botany XI Biological Classification

Want to learn about Viriods & Viruses. NEET Botany XI Biological Classification Viruses Viruses are the most primitive, acellular and non-cytoplasmic infectious agents. D.J. Ivanowski (1892) discovered Tobacco Mosaic virus (TMV). Pasteur coined the term virus Beijerinck called extract of diseased plant as Contagium vivum fluidum. W. M. Stanley (1935) gave the chemical nature of viruses. General Characteristics: • Viruses are acellular, non-cytoplasmic, obligate parasite and can multiply only within the living host cells. • They are smaller than bacteria, and thus can pass through bacteriological filter. • Viruses contain only a single type of nucleic acid, either DNA or RNA. • Viruses are host specific in that they infect only a single species and definite cells of the host organisms. • Viruses are effective in very small doses. Most of them are highly resistant to germicides and extremes of physical conditions. General Structure of Viruses: Shape and Size: Viruses are smaller than bacteria. They are so small that a hollow shell of a Staphylococcus can accommodate thousands of smallest viruses. The size ranges from 10 nm (virus of foot and mouth diseases of cattle) to 300 nm (smallpox virus: Variola). Plant viruses, in general, are smaller than bacterial or animal viruses. Largest virus is Beetle Yellow Virus. Chemical Structure and Composition: Viruses have a very simple structure. A virus is made of a nucleic acid and protein coat (capsid). A fully assemble particle, i.e., a virion, is capable of infecting the host. Nucleic Acid: A virion always contains only a single kind of nucleic acid, i.e., either DNA or RNA. The nucleic acid may occur as single or double strands. Plant viruses contain only single or double stranded DNA (ss or ds DNA) or single or double stranded RNA (ss or ds RNA). Bacterial viruses (bacteriophages) contain single or double stranded DNA (ds DNA) or single stranded RNA (ss RNA). The infectious property of a virion is due to its nucleic acid. A host cell can synthesize complete virion if only free viral nucleic acid is injected within the cytoplasm of a living host cell. Virus Type of nucleic acid Virus Type of nucleic acid Herpes Double stranded DNA Measles Single stranded RNA Chicken pox Double stranded DNA Mumps Single stranded RNA Hepatitis B Double stranded DNA Polio Single stranded RNA Cyanophages Double stranded DNA TMV Single stranded RNA Coliphase ϕ×174 Single stranded DNA Coliphase MS-2 Single stranded DNA Influenza virus Single stranded RNA Mycophages Double stranded RNA Rabies Single stranded RNA Reovirus Double stranded RNA HIV Single stranded RNA Wound tumour virus Double stranded RNA Capsid or Protein Coat: The protein coat is called capsid. It is made up of many identical protein sub units called capsomeres. The capsomeres are composed of either one or several types of proteins. In a virus particle, the capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. Some large virus particles (i.e., virions) have an additional covering outside the capsid. Such virions are called enveloped (e.g., influenza virus, mumps virus HIV, Rous Sarcoma virus) and those without this additional covering are referred to as naked (e.g., TMV). For more such resources go to https://goo.gl/Eh96EY Website: https://www.learnpedia.in/