Presentation on theme: "Ch. 19 Viruses Objective: EK 3.C.3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts."— Presentation transcript:
1 Ch. 19 VirusesObjective:EK 3.C.3: Viral replication results in genetic variation,and viral infection can introduce genetic variation into the hosts.L.O The student is able to construct an explanation of how viruses introduce genetic variation in host organisms.
2 19.1 A Virus Consists of a Nucleic Acid Surrounded By a Protein Coat 1st discovered viruses – Tobacco Mosaic VirusFiltered plant sap still infected plants.Something smaller (viruses) was infected them.Extracted sap from tobacco plant with tobacco mosaic diseaseRESULTSPassed sap through a porcelain filter known to trap bacteriaHealthy plants became infectedRubbed filtered sap on healthy tobacco plants1234
3 Structure of VirusesNucleic Acid: typically 1 linear strand or circular moleculeDouble stranded DNASingle stranded DNADouble stranded RNASingle stranded RNAProtein shell (capsid) encloses nucleic acid. These come in many shapes/sizes.Some have accessory structures:Viral envelopesTail fibersCapsomere of capsidRNACapsomereDNAGlycoproteinGlycoproteinsMembranous envelopeCapsidHeadTail sheathTail fiber18 250 nm80 225 nm70–90 nm (diameter)80–200 nm (diameter)20 nm50 nm(a)Tobacco mosaic virus(b) Adenoviruses(c) Influenza viruses(d) Bacteriophage T4
4 19.2 Viruses Replicate Only in Host Cells Viruses do not have organelles to build proteins, replicate nucleic acids, metabolize, etc. Needs a host cell to do it.Viruses can only infect certain hosts (host range)Only certain speciesOnly specific cells/tissues within speciesEx: flu only infects cells of the respiratory system
5 General Features of Viral Replicative Cycles Infection begins with the insertion of the viral nucleic acid into the host cell:Use tail fibers to inject DNA into host cell (like a needle)Endocytosis of entire virusViral DNA commandeers cells to make more viruses using the cell’s own machinery.Kills cells when done.
6 The Lytic Cycle The virus injects DNA into the host cell New phage DNA and proteins are synthesized and assembled into phages.The cell lyses, releasing phages.PhagePhage DNAThe phage injects its DNA.Bacterial chromosomeLytic cyclelytic cycle is inducedorPhage DNA circularizes.Certain factors determine whetherlysogenic cycle is enteredThe virus injects DNA into the host cellHost cells starts copying viral DNA and making capsid and accessory parts.Once done, the cell lyses (bursts) releasing new viruses to infect other cells.
8 The Lysogenic Cycle Same beginning and end as lytic cycle Insert DNA …… make copies and cell lysesAfter the DNA is inserted, it is integrated into host cell’s DNA.Prophage; 1 gene codes for a protein that prevents transcription of the rest of its genes.Cell grows and replicates (binary fission) normally with viral DNA still “hiding” in host DNA.lytic cycle is inducedorPhage DNA circularizes.Certain factors determine whetherlysogenic cycle is enteredLysogenic cycleProphageDaughter cell with prophageOccasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.Cell divisions produce a population of bacteria infected with the prophage.The bacterium reproduces, copying the prophage and transmitting it to daughter cells.Phage DNA integrates into the bacterial chromosome, becoming a prophage.
10 Replicative Cycles of Animal Viruses Be aware of (but don’t memorize) that animal viruses are classified according to type of nucleic acid and presence of an envelope.
11 RNA as Viral Genetic Material Class IV – viral RNA serves as mRNA to immediately make proteins.Class V – viral RNA used to make many mRNAs.Class VI (retroviruses) –RNA + reverse transcriptase DNAViral DNA embeds in host’s DNAViral proteins made with host’s
12 Figure 19.8Membrane of white blood cellGlycoproteinViral envelopeHIVCapsidRNA (two identical strands)HOST CELLReverse transcriptaseHIVReverse transcriptaseViral RNARNA-DNA hybridDNA0.25 mHIV entering a cellNUCLEUSProvirusChromosomal DNARNA genome for the next viral generationFigure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.mRNANew virusNew HIV leaving a cell
14 Evolution of Viruses Not living but effects all living things. Evolved after life.Naked DNA infected injured cells at first, then evolved to have an envelope and could infect healthy cells.Viral nucleic acids are more similar to host’s nucleic acids but there are some viral genes that are similar for all viruses.