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Viruses Bacteria vs. viruses--Bacteria: prokaryotic cell, most are free-living (some parasitic), relatively large size. Antibiotics used to kill bacteria.--Viruses: not a living cell (genes packaged in protein shell), intracellular parasite, 1/1000 the size of bacteria, vaccines used to prevent viral infection, anti-viral treatments The Virus-very small (<ribosomes)-components = nucleic acid + capsid --nucleic acid: DNA or RNA (double or single-stranded) --capsid: protein shell --some viruses also have viral envelopes that surround capsid-limited host range (eg. human cold virus infects upper respiratory tract)-reproduce within host cells-ex: tobacco mosaic virus, adenoviruses, influenza viruses, bacteriophages Classes of Animal Viruses-I Double-stranded DNA (dsDNA), ex: herpesvirus-II single-stranded DNA (ssDNA), ex: parvovirus-III double-stranded RNA (dsRNA), ex: reovirus-IV single-stranded RNA (ssRNA);serves as mRNA, ex: picornavirus-V ssRNA; template for mRNA synthesis, ex: orthomixovirus-Vi ssRNA; template for DNA synthesis, ex: retrovirus Simplified viral replicative cycle1. entry and uncoating2. replication3. transcription and manufacture of capsid proteins4. self-assembly of new virus particles and their exit from the cell Bacteriophage = virus that infects bacterial cells.-ex: bacteriophage T4-lytic cycle of T4 phage: attachment à entry of phage DNA and degredation of host DNA à synthesis of viral genomes and proteins à assembly à release Bacteriophage Reproduction-Lytic cycle: use host machinery to make copies of virus, death of host cell by rupturing it (lysis), virulent phages replicate this way-Lysogenic cycle: phage DNA incorporated into host DNA and replicated along with it. Phage DNA = prophage. Temperate phage – uses both methods of replication.-certain factors determine whether lytic or lysogenic cycle is intered Animal viruses have a membranous envelope-host membrane forms around existing virus-difficult for host immune system to detect virus Retrovirus-RNA virus that uses reverse transcriptase (RNA à DNA)-newly made viral DNA inserted into chromosome of host-host transcribes viral DNA (=provirus) to make new virus parts-ex: HIV (human immunodeficiency virus) -infects white blood cells -HIV+ = provirus (DNA inserted) -AIDS = active viral reproduction Other human viruses:-herpes simplex virus I (HSVI): oral herpes-herpes simplex virus II (HSVII): genital herpes-smallpox: eradicated in 1979 due to worldwide vaccination campaigns-ebola: transmission – contact with contaminated blood or bodily fluids. Symptoms fatigue, fever, severe headache, vomiting, diarrhea, rash, bleeding. Appears 2-21d post-exposure. Treatments include IV to balance electrolytes, experimental treatments like antiviral drugs, plasma transfusions from survivors, antibodies. No vaccine yet Vaccines – weakened virus or part of pathogen that triggers immune system response Emerging virus = mutations of existing virus – brand new virus (ex: H1N1) Viroids – small circular RNA molecules that infect plants-cause errors inregulatory systems that control plant growth Prions – misfolded infectious proteins that cause misfolding of normal proteins-ex: mad cow disease (BSE), Creutzfeldt-Jakob disease (humans), scrapie (sheep)-diseases caused by prions-prions act slowly – incubation period of at least 10y before symptoms develop -prions are virtually indestructible (cannot be denatured by heating) -no known cure for prion disease Bacterial control of gene expression:Operon = cluster of related genes with on/off switch-three parts: 1. promoter – where RNA polymerase attaches 2. operator – “on/off” – controls access of RNA polymerase 3. genes – code for related enzymes in a pathway SO…Operon – genes grouped together with related functionsPromoter – RNApol binding site. Single promoter controls transcription of all genes in an operon. Transcribed as one unit and a single mRNA is madeOperator – DNA binding site of the repressor protein Regulatory gene – produces repressor protein that binds to operator to block RNA polymerase Repressible Operon: ON à OFF-default – genes expressed (normally ON)-anabolic (build organic molecules)-organic molecule product acts as corepressor à binds to repressor to activate it-operon is turned OFF-corepressor molecule binds to and activates the repressor molecule à activated repressor binds to operator, blocks RNApol à genes not expressed-ex: trp operon – tryptophan absent, repressor inactive, operon on, makes tryptophan, tryptophan present, repressor active, operon off. Trp is an allosteric regulator of the repressor protein. Inducible Operon: OFF à ON -default – genes not expressed (normally OFF) -catabolic (break down food for energy) -repressor is active à inducer binds to and inactivates repressor -operon is turned ON-inducer molecule binds to and inactivates the repressor molecule à RNApol no longer blocked à genes expressed-ex: lac operon – lactose absent, repressor active, operon off, lactose present, binds to lac repressor protein and triggers repressor to release DNA repressor inactive, operon on. Lac causes conformational change in repressor protein so it can no longer bind. Allosteric regulator of repressor.
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