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Viruses Big Idea 3: Living systems store, retrieve, transmit, and respond to info essential to life processes. Essential Knowledge • 3C3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts. Structure of Viruses • Viruses are not cells small infectious particles consisting of nucleic acid in a protein coat or a membranous envelope Viral Genomes Made of either: – Double- or single-stranded DNA, or – Double- or single-stranded RNA RNA Viruses Need Reverse Transcriptase: enzyme that makes double stranded DNA to match w/ host cell Lack replication error-checking mechanisms higher rates of mutation (HIV!) Herpes Virus-DNA Ebola Virus-RNA Capsids and Envelopes • A protein shell that encloses viral genome • Built from protein subunits called capsomeres Fig. 19-3 RNA DNA Capsomere Membranous envelope RNA Head DNA Capsid Tail sheath Capsomere of capsid Glycoproteins Glycoprotein 18 250 nm 70–90 nm (diameter) 80–200 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic (b) Adenoviruses virus 50 nm Tail fiber 80 225 nm 50 nm (c) Influenza viruses (d) Bacteriophage T4 Viral Replication • Highly efficient allow for rapid evolution and acquisition of new phenotypes! Bacteriophage • AKA: phages, viruses that infect bacteria • Have a long capsid head that encloses DNA • A protein tail attaches phage to host and injects DNA inside Fig. 19-1 0.5 µm Viruses Need Hosts! • Intracellular parasites that have a host range (a limited number of host cells that it can infect) Viral Reproductive Cycles Virus VC Fig. 19-4 VIRUS 1 Entry and DNA uncoating Capsid 3 Transcription and manufacture of capsid proteins 2 Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins 4 Self-assembly of new virus particles and their exit from the cell Reproductive Cycles of Phages • 1st Rep Cycle: lytic cycle Death of host cell • Virulent phage: virus that only reproduces via lytic cycle Fig. 19-5-1 1 Attachment Fig. 19-5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA Fig. 19-5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA 3 Synthesis of viral genomes and proteins Fig. 19-5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers Fig. 19-5-5 1 Attachment 2 Entry of phage 5 Release DNA and degradation of host DNA Phage assembly 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers Reproductive Cycles of Phages • 2nd reprod. Cycle: latent/lysogenic cycle • No destroying host cell viral DNA is mixed into host cell’s chromosome prophage • Gives host cell properties like increased pathogenicity in bacteria Temperate Phages Lambda Phage Attacks E. Coli • Phages that use both lytic and lysogenic cycles • Env. signal can trigger switch Fig. 19-6 Phage DNA Daughter cell with prophage The phage injects its DNA. Cell divisions produce population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle The bacterium reproduces, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced or New phage DNA and proteins are synthesized and assembled into phages. Lysogenic cycle is entered Prophage Phage DNA integrates into the bacterial chromosome, becoming a prophage. Evolution of Viruses • Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons, small mobile DNA segments • Mimivirus (double-stranded DNA) is 2nd largest virus discovered • Virus evolved before or after cells? Megavirus (2 strand DNA) Oct. 2011, largest virus Vaccines • Harmless derivatives of pathogenic microbes that stimulate immune system to mount defenses against actual pathogen • Viral infections cannot be treated by antibiotics With your partner, make a list of vaccines you have received! Epidemics vs Pandemics • Epidemic: New cases of a disease appear in a local area aka outbreak • If epidemic spreads to another locale pandemic Plant Viruses Most have an RNA genome – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent Viroids and Prions: Simple • Viroids: circular RNA molecules that infect plants and disrupt growth • Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals Ex: Prion Mad Cow Disease Ex: Prion Creutzfeldt Jakob Syndrome Understanding Viruses Movie http://www.schooltube.com/vide o/360cafa902fb60e55f61/Under standing-Viruses