Lec13notescompletePart_1 - Bio 1A Lec 13 Complete Chapter...

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Bio 1A Lec 13 Complete 1 Chapter 18The Genetics of Viruses and Bacteria Overview: Microbial Model Systems • Viruses called bacteriophages can infect and take over the cell machinery of bacteria, such as Escherichia coli E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles Fig. 18-2 Viruses • A virus has a genome but can only reproduce within a host cell. • In the 1800s scientists could not see viruses but detected them indirectly. Researchers hypothesized that something smaller than bacteria caused tobacco mosaic disease. • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration. • In 1935 Wendell Stanley confirmed the existence of the small infectious particles when he crystallized the Tobacco Mosaic Virus. Structure of Viruses • Viruses are not cells • Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope. • A capsid is the protein shell that encloses the viral genome Viral genomes may consist of double- or single-stranded DNA, or double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus Fig. 18-4a, Fig. 18-4b Membranous envelopes • Some viruses have membranous envelopes that help them infect hosts • Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules • These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals Fig. 18-4c Phages Bacteriophages , also called phages , are viruses that infect bacteria • They have the most complex capsids found among viruses • Phages have an elongated capsid head that encloses their DNA • A protein tailpiece attaches the phage to the host and injects the phage DNA inside Fig. 18-4d General Features of Viral Reproductive Cycles • Viruses are obligate intracellular parasites , which means they can reproduce only within a host cell • Each virus has a host range, a limited number of host cells that it can infect • Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses
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Bio 1A Lec 13 Complete 2 Fig. 18-5 Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle The Lytic Cycle • The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell • The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses • A phage that reproduces only by the lytic cycle is called a virulent phage • Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Fig. 18-6 The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host
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Lec13notescompletePart_1 - Bio 1A Lec 13 Complete Chapter...

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