Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 19 – Viruses Virus Obligate intracellular parasite No metabolic enzymes or ribosomes Structure o Protein coat (capsid) o Nuclei Acid core (linear or circular) DNA viruses – single-stranded or double-stranded RNA viruses – single-stranded or double-stranded Very small – 4 to 100s of genes Capsid o Gives shape to virus Spiral – tobacco mosaic, influenza Triangular plats in a polyhedron – adenovirus, bacteriophages o Contains proteins that assist in recognition of host cell o Can be surrounded by viral envelope Membrane from host + GPs & GLs from virus Reproduction Virus attaches to receptor site on host Nucleic Acid enters cell o Can enter w/o capsid o Enters w/ capsid & released inside Rest of reproduction depends on type of nucleic acid present DNA Viruses o e.g. bacteriophages (bacteria virus) o Viral DNA replicated by host o Viral DNA transcribed & translated by host making new viral proteins o New DNA & capsid assembled o New Viruses shed from cells o Lytic Cycle Phage causes host DNA to degrade e.g. T4 phage In last stage, virus directs an enzyme to damage cell wall Water floods in & cell lyses New viruses released to spread Virulent phages only use lytic cycle o Lysogenic Cycle Phage genome replicates w/o destroying host cell e.g. Lambda phage Viral DNA is inserted into host DNA (prophage) DNA synthesis yields daughter cells w/ prophage in genome Temperate phage uses both cycles RNA Viruses o ssRNA (class IV) – genome acts as mRNA & is translated directly o ssRNA (class V) – genome acts as template strand for mRNA & complementary RNA (gets copied again to make genome RNA) RNA replicase used to copy RNA o Retroviruses (Class VI) Deliver reverse transcriptase w/ RNA RNA read & cDNA made (no correcting) DNA then replicated = doubled stranded DNA spliced into host genome by integrase (forming a provirus) Viral DNA now transcribed & translated mRNA also serves as new genome Protease cleaves/activates polypeptides HIV leaves & picks up host membrane Viroids Smaller & simpler than viruses, consist of tiny molecules of naked circular RNA that infect plants Have several hundred nucleotides that don’t encode for proteins but can be replicated by host’s cellular enzymes RNA molecules can disrupt plant metabolism & stunt plant growth, perhaps by causing errors in regulatory systems that control plant growth Prions Infectious proteins in mammalian brains o Scrapie in sheep, mad cow disease, & Creutzfeldt-Jakob Slow-acting – up to 10 year incubation Virtually indestructible Misfolded protein that converts normal proteins into prion version Spread by eating meat w/ infected tissue Bacterial Plasmids Extra pieces of DNA, generally good o Very small (2-30 genes) Circular & self-replicating o Can replicate more or less often than bacterial chromosome Extra copies or no copies Some can enter & leave genome Conjugation o One way transfer of DNA b/w bacterial cells (Bacteria ‘sex’) o Sex pilus connects & pulls cells together o Fertility (F) Factor Part of DNA needed for sex pilus 25 genes in genome or plasmid F+ bacteria have it & F- don’t If F factor on plasmid, can be transferred via conjugation F+ can attach & donate to Fo R Plasmids Carry genes for different antibiotic resistances (R) Antibiotics kill bacteria w/o R plasmids (& survivors multiply) Conjugation can move R Transformation o Cell picks up & integrates foreign DNA from surroundings Transduction o Movement of genes b/w bacteria by bacteriophages o Generalized – accidental transfer of DNA b/w bacteria after lytic cycle o Specialized – prophage picks up part of bacteria DNA & moves it (across species?) after lytic cycle o Process Phage infects bacterium by injecting DNA DNA copied & broken up in pieces New phages made; A+ bacterial allele gets in one (accidental) Phage w/ A+ injects into a new bacteria (doesn’t have A+) A+ allele recombines into new bacterial genome