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10/30/2014 Bacteria and Viruses Date __________ BACTERIA Bacterial Cells • Prokaryotic • No membrane-bound organelles • Cell wall with peptidoglycan • Much larger than viruses, but smaller than eukaryotes Bacterial Reproduction • Called binary fission • Two identical daughter cells formed, but not by mitosis Bacterial Genome • Double-stranded, circular DNA – 1/1000 as much as eukaryotes – 100x more than viruses • Densely packed in nucleoid region Bacterial Reproduction and Genetic Diversity • Bacteria change and evolve by… – Mutations: occur often due to high replication rate; if one mutation occurs, all future offspring will have it – Transformation: cell absorbs DNA from its environment that may have originated from another ruptured cell – Transduction: insertion of DNA by a virus – Conjugation: two live bacteria connect using external pili and pass a plasmid between the two of them 1 10/30/2014 Plasmid Treating Bacterial Infections • Small, circular, self-replicating segment of DNA in bacteria • Not necessary for survival, but can provide benefits like antibiotic resistance • May have originated as a virus • Two types • Much easier than viruses • Antibacterial agents can attack the bacteria without affecting our cells • Antibiotics may target bacterial… – Cell walls – Protein synthesis – DNA synthesis – Ribosomes – And so on… – R plasmid: antibiotic resistance – F plasmid: allows bacteria to pass DNA using pili What are Viruses? • • • • We’re not sure! Pieces of nucleic acid wrapped in a protein coat No ribosomes or organelles Cannot live independently – Have no machinery for reproduction or metabolism • They aren’t usually considered alive because… VIRUSES – They aren’t made of cells – They can’t reproduce on their own – They don’t grow or develop Viral Body Plans • Inner core: genetic material is DNA or RNA, various enzymes • Covering: coat is protein , some have envelope – Envelope is composed of glycoproteins that bind to specific receptors on the host cell surface – Sometimes the envelope is formed from the host’s membrane as the viral capsids exit Viral Genome Helical virus Polyhedral virus • May be DNA or RNA • Can be single or double-stranded • Usually short and circular Complex virus (bacteriophage) 2 10/30/2014 Examples of Viruses • • • • • • • • • Cold (rhino virus) Flu (influenza) West Nile HIV Chicken pox Smallpox Ebola Herpes HPV • • • • • • Rabies Polio Meningitis (viral) Measles Mumps Tobacco mosaic virus • Also affect plants, bacteria, and archaea – extremely widespread Viral Reproduction • • • • • Uses host to replicate Occurs extremely rapidly DNA viruses use DNA replication RNA viruses use transcription There are two “phases” of viral reproduction – Lytic cycle – Lysogenic cycle Lytic Cycle • Host copies and assembles new viruses • New viruses burst free from the cell • Makes a virus virulent Lysogenic Cycle • Virus incorporates itself into genome • Gets copied along with the host genome • Can alter the function of the host 3 10/30/2014 Retroviruses • RNA viruses like HIV must first make DNA from RNA to enter lysogenic cycle • Reverse transcriptase accomplishes this so DNA can be inserted • DNA inserts into host DNA; proteins are assembled, and virus is released • Hard to make vaccines because RNA mutates easily and each virus can be slightly different from the others • Drugs like AZT fight this action How do vaccines work? • Think of it like this: – You get sick with a virus and your body develops immunity – The virus isn’t GONE, but your immune system suppresses it • Vaccines are made by growing a weakened or killed form of the virus – This is injected into a person’s body, where it doesn’t make them sick – However, there are particles that the immune system can react to, and it does – but you don’t get sick because it’s not alive – You have immunity to it as if you were actually infected • SO…if you get a flu shot and get sick…guess what? It wasn’t with the type of flu you got the shot for. Where Did they Come From? • We don’t know! • 3 main hypotheses, each with some support: – Cells that lost other genes and structures necessary for independent survival via evolution – Evolved from plasmids or transposons (jumping genes) • Transposons, or “jumping genes”, can be copied/cut and pasted throughout the genome. They can change genes or introduce pattern repetition in the genome. – Co-evolved with the first forms of life Very Difficult to Fight Viruses • There’s nothing to attack! • Almost everything they use belongs to the host • Only mechanisms are vaccines to prevent viruses from entering cells and preventing reverse transcription Viral Evolution • Viral evolution occurs very quickly because they reproduce rapidly • Very little proofreading of errors (especially RNA viruses) • 2 viruses in the same host can combine • Where did they come from? – We believe that viruses are ancient, possibly as old as life itself – They likely don’t share a common ancestor – there are millions of them! Why Should We Care About Viruses? • Effects on human health – Contagious disease pathogens must directly contact a new host. They may cause… • Epidemics (incidence rate exceeds expected number), pandemics (worldwide epidemic), sporadic (“singular”) cases, and endemic (maintained in a population) cases – Viruses may be responsible for up to 8% of our normal DNA • Role in ecosystems – Viruses kill bacteria (estimated to kill about 20% of the biomass of the ocean every day) – Recycle nutrients and materials, prevent overgrowth of bacteria • Important role in genetics and evolution – Viruses carry genes from one individual to another and are very important in increasing genetic diversity 4 10/30/2014 Emerging Viruses • Viral strains can jump species, such as avian flu • Emerging pathogens include the Ebola virus, bird flu, and SARS • There are drug-resistant strains • These could be mutations of known viruses, exposed to new, developing areas with new hosts • Hosts and pathogens are always coevolving Not-Quite-Viruses • Viroids – Smaller than viruses – just strands or circles of RNA – Cause many plant diseases – Horizontal transmission via damaged cell walls or vertical transmission when inherited from parent • Prions (proteinaceous infectious particle) – Small proteins that are folded incorrectly; simple have to TOUCH another protein to convert it to a prion – Linked to human genetic diseases like Creutzfeldt-Jakob disease (CJD) and animal diseases like bovine spongiform encephalopathy (mad cow disease) 5