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VIRAL CHARACTERISTICS 1.Extremely small (20-14,000 nm in length) 2.Obligate intracellular parasites (only reproduce within a living cell) 3.Not cells 4.Not sensitive to antibiotics 5.No independent metabolism --> few or no enzymes 6. Host range •Spectrum of host cells that virus can infect •Virus recognizes host cell by “lock & key” fit between protein on outside of virus & specific receptor sites on cell surface •Some have broad host range ex. Rabies numerous mammalian species •Some have narrow host range a) Infect only one species ex. Phages of E. coli b) Infects only a single tissue type of one species ex. Rhinovirus (cold) upper respiratory tract cells ex. AIDS specific receptors on certain white blood cells VIRAL STRUCTURE 1.Nucleic Acid Contain either DNA or RNA May be double-stranded or single-stranded Smallest viruses have only 4 genes - largest have several hundred Nucleic acid is circular or linear Some viruses have several separate segments ex. Influenza 2.Capsid Protein coat that surrounds nucleic acid of virus Composed of subunits called capsomeres Structure may be rod-shaped, helical, polyhedral, or complex Most complex capsids found among bacteriophages Have icosohedral (polyhedral with 20 triangular facets) head which encloses the nucleic acid helical tail sheath with tail fibers 3.Envelope Membrane that covers the capsid in some viruses Some viral envelopes are covered by spikes Help virus to infect their host Lytic & Lysogenic Cycles Diagram Lytic Cycle Viral replication that results in death or lyse of the host cell Virulent bacteriophages reproduce only by a lytic replication cycle Virulent Phages •Virus that lyse (& therefore kill) their host cells Steps of the Lytic Cycle 1.Attachment Phage attaches to host cell Attachment site on virus attaches to receptor site on host cell Chemical interaction in which weak bonds are formed between the attachment & receptors sites 2. Penetration Phage penetrates host cell & injects its DNA into host cell Phage’s tail releases lysozyme (enzyme) which breaks down bacterial cell wall Tail sheath of phage contracts & tail core is driven thru cell wall acts like a syringe Capsid remains outside the host cell 3. Biosynthesis Phage DNA directs synthesis of viral components by the host cell Host protein synthesis is stopped by viral enzyme produced by the host, which degrades the host DNA 4. Maturation Viral components are assembled into viruses Occurs spontaneously 5. Release Host cell lyses & new viruses are released Lysozyme, which digests cell wall, is made within the cell Osmosis causes cell swell & finally burst Released phages then infect other host cell **Lytic Cycle Only Takes 20-30 Minutes** Lysogenic cycle viral replication cycle that involves the incorporation of the viral DNA into the host DNA Temperate Virus Virus that can integrate their genome into a host chromosome & remain latent until they initiate a lytic cycle Example: phage (lambda) Steps of the Lysogenic Cycle 1. Phage attaches to host cell &injects DNA 2. Phage DNA forms circle & either begins a lytic or Lysogenic cycle 3. During Lysogenic cycle, phage DNA integrates within the bacterial chromosome by recombination prophage *most prophage genes are inactive* 4. Lysogenic bacterium reproduces normally also replicates the prophage 5. Prophage may excise (“pop out”) from the bacterial chromosome rare spontaneously event, radiation, chemicals This initiates the lytic cycle. Lysogenic Cycle Host cell carrying a prophage in its chromosome