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Viruses and Prions and Bacteria, OH MY! Genetics of Viruses and Bacteria Introduction to the Genetics of Viruses Work on the tobacco mosaic virus (TMV) provided the first evidence for the presence of viruses, a pathogen smaller than bacteria. Structure of viruses Smallest viruses are 20 nm in diameter Largest can barely be seen by a light microscope Not a cell! Is it living? We can’t decide! Infectious particles consisting of at least a nucleic acid (RNA or DNA) enclosed in a protein coat Structure of Viruses Viral Genomes Can be : Double Stranded Single Stranded DNA X X RNA X X Structure of Viruses Capsids and Envelopes Capsid= protein shell enclosing the viral genome Rod-shaped, polyhedral, helical, or more complex Large number of protein subunits (but generally similar) The most complex are found in phages (bacteriophages) Viral envelopes Derived from host cell Help virus infect host Structure of Viruses Viral envelopes General Viral Reproductive Cycles Obligate intracellular parasites: Viruses lack metabolic enzymes, ribosomes, and other cellular machinery Each virus has a host range Can only reproduce in a host Limitations of what the virus can infect Due to the lock-and-key specificity of the virus and the cell’s receptor molecules Genome replication DNADNA RNA RNA RNADNARNA General Viral Reproductive Cycles Virus injects its genome into the host cell Viral genome reprograms the cell to perform reproductive work for the virus using the cell’s resources Assembly of viral proteins and genomes is often spontaneous after synthesis Viral progeny exit the cell Reproductive Cycles of Phages Lytic Cycle vs. OR Lysogenic Cycle Reproductive Cycles of Phages Lytic Cycle Phage infects a healthy cell Bacteria copies virus Progeny exit bacteria by lysing it Viruses in this cycle are considered virulent Bacterial defenses: Natural selection favors bacterial mutants with unrecognizable receptor sites Although, natural selection also favors phage mutants Phage DNA often recognized and dismantled by the cell’s restriction enzymes Reproductive Cycles of Phages Lysogenic Cycle Viral genome replicated without destroying host Phages that use both types of cycles are called temperate phages Viral DNA is incorporated into bacterial genome Bacteria reproduces normally, but also copies viral genome Viral DNA called a prophage at this point Prophages genes can regulate host cell Environmental signal triggers the switch from lysogenic cycle to lytic cycle Lytic and Lysogenic cycles Reproductive Cycles of Animal Viruses Many animal viruses with RNA genomes have a viral envelopes derived from the plasma membranes Enveloped viruses can exit the cell without destroying the cell: contrasts with lytic cycle of phages Some animal viruses with DNA genomes have viral envelopes derived from nuclear membrane Reproductive Cycles of Animal Viruses Attachment Entry Uncoating RNA and protein synthesis Assembly and exit Reproductive Cycles of Animal Viruses RNA Viruses Contains a viral enzyme that can be used for RNARNA synthesis In different viruses, original RNA genome can: Be used as mRNA Be used as a template for mRNA Reproductive Cycles of Animal Viruses Most complicated RNA animal viruses: Retroviruses (An example is HIV) Contain reverse transcriptase RNADNA New viral DNA integrates into cell DNA and stays in the cell’s DNA as a provirus (never leaves) Cell produces viral RNA to function both as mRNA and as genomic RNA Evolution of Viruses Not likely to have originated prior to the first cells Why? Because they are obligate intracellular parasites Believed to have originated from fragments of cellular nucleic acids High mutation rate in many viruses Vaccines Harmless variations of viruses used to elicit an initial immune response by the body The main way to fight viruses Why can’t we treat viral diseases? Drugs that would attack the virus would have to also attack the host cell Emerging Viruses Viruses that seem to appear suddenly Mutations of pre-existing viruses Jump to a new type of host RNA viruses particularly susceptible because they lack proofreading in replication e.g. Hantavirus Ebola, HIV, SARS, influenza Viruses and Cancer Some viruses have been shown to cause cancer e.g. Hepatitis B and liver cancer Viruses can trigger cancerous genetic conditions Oncogenes- a gene found in viruses or as a part of the normal genome that is involved in triggering cancerous conditions Proto-oncogenes- a gene that has be potential to cause cancer pending alteration into an oncogene Viroids and Prions Viroids Naked circular RNA molecules that infect plants Prions Infectious proteins e.g. scrapie in sheep, mad cow, Creutzfeldt-Jakob disease Slow acting No cures Cannot replicate itself, but can convert normal proteins Bacterial Genetics Bacterial Genome One, double stranded, circular DNA molecule. Referred to as a bacterial chromosome Supercoiled into a dense region called the nucleoid Some bacteria also have plasmids, smaller circles of DNA Bacterial Genome Bacterial cells divide by binary fission (asexual) Bacterial Genome Variation Due to asexual reproduction, much variation is due to spontaneous mutation Other variation can result from Transformation Transduction Conjugation Mechanisms for Gene Transfer/Genetic Recombination Transformation: the uptake of naked, foreign DNA from the surrounding environment which results in a change in the bacteria’s genotype and phenotype. e.g. Mechanisms for Gene Transfer/Genetic Recombination Transduction: the random transfer of DNA from one bacterial cell to another via virus (phage) transport Mechanisms for Gene Transfer/Genetic Recombination Conjugation: the direct, one way transfer of genetic material between two bacterial cells that are temporarily joined. Mechanisms for Gene Transfer/Genetic Recombination Conjugation continued: Donor (F+) cell extends a sex pilus to the recipient (F-) cell Ability to make sex pili due to the presence of an F factor F factor can be a plasmid, or part of the bacterial chromosome One strand of the F factor is donated to the F- cell then both cells synthesize (make) a second strand using the first as a template F Factor received by recipient cell may confer (give) advantages to the cell (because it increases genetic variation) Figure 18.18 page 350 Genetic Recombination Can also occur within a single cell Transferable elements Insertion sequences Contains a single gene that codes for enzyme called transposase (catalyzes movement within the genome) Transposons Contains insertion sequences and an additional gene Figure 18.19 page 352 Antibiotic Resistance Some bacteria are resistant to antibiotics The is the result of the presence of an R plasmid (R for Resistance) Contains genes that counteract the antibiotic When these and non-resistant cells are exposed to the antibiotic, non-resistant cells die, but resistant cells can still proliferate (grow/reproduce) Leads to a population of bacteria that is resistant to antibiotics A constant problem in the medical field Regulation of Gene Expression Operons!