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Viruses Viruses • no cellular structure, no cytoplasm, organelles or cell membranes • no metabolism on their own, cannot grow or respire • Therefore are not classified as living things Viruses continued • Viruses can reproduce, a basic characteristic of life • They can take over control of a host cell • Can direct cell to make new virus particles • Mobile genes that attack cells Parts of a virus Most viruses consist of 4 parts 1) Core - located in the center of the virus and contains the virus’ DNA or RNA wrapped together with some proteins 2) Capsid - made of protein and forms a shell around the core, protects DNA from enzymes of host cell 3) Matrix - forms a layer between the capsid and the envelope 4) Envelope - consists of lipids stolen from the cell membrane of the host cell that the virus is growing in Note: some viruses do not have an envelope and others do not have a capsid Classifying Viruses Often viruses are named for: • Type of disease caused (poliovirus) • Those who discovered them (Epstein-barr virus) • Site of replication (rhinovirus or tobacco mosaic virus) • Where they were isolated (sendai-virus) Classification is now based on type of nucleic acid, shape, size etc. Shape and Size --> determined by the type and arrangement of proteins in the capsid • Cylindrical • Polyhedral • Spherical Site of Replication • Specific organism and cell • Wide variety of organisms e.g. Tobacco mosaic virus Type of disease they cause • 21 different groups of viruses infecting humans differ in set of genes (genome) and method of replication e.g. influenza virus Type of nucleic acid DNA viruses – Less virulent – follow the lytic cyle – Eg. Warts, shingles, chicken pox RNA viruses – Virulent – Follow lysogenic cycle – AIDS, rubella, rabies, measles How big are viruses? • Video Viral Replication • It depends on the metabolism of a prokaryotic or eukaryotic cell to replicate its DNA or RNA and to make protein coats foe each newly formed virus particle • Attaches to specific receptors on the plasma membrane of the host cell and can only enter particular cells with specific receptor sites • Enter host in one of two ways a) the virus injects its nucleic acid into host cell (lytic cycle) b) membrane of host engulfs virus creating a vacuole inside host • Host metabolism replicates the viral DNA or RNA and protein coat and new virus particles are assembled • New particles are released from the host cell in one of two ways: a) The host cell breaks open (Lysis) and releases the new viruses which then infect neighbouring cells; Host cell is destroyed in process a) The host cell releases new viruses without being destroyed Bacteriophage • Virus that infects a bacteria • Video • Same video different site T4 Bacteriophage • Specific to E.coli • Head capsid containing DNA • Sheath support and pump to move DNA • Base plate cut or bite a hole in cell • Tail fibres find site for attachment • Core pipeline to conduct DNA from head to cytoplasm of cell • Collar attach tail to head Viral Replication: Lytic and Lysogenic Cycles Video Viroids • Very small infectious pieces of RNA responsible for some serious plant diseases • Differ from viruses as RNA does not code for proteins • May interfere with the normal formation and functioning of RNA in host cell Eg. Viroid outbreak killed more than 10 million coconut plants in Philippines Prions • Abnormally shaped infectious protein responsible for some brain diseases in mammals, including humans • When tissues are eaten by another animal, prions enter blood stream and go to its brain • Prions interact with normal proteins causing them to become abnormal and infectious • E.g. Bovine spongiform encephalopathy (BSE) (Mad Cow Disease) Humans - Creutzfeldt-Jakob Disease (CJD)