2 THINK ABOUT ITFarmers began to lose their tobacco crop to a plant disease.What would you do next?How would you deal with the invisible?
3 Discovery of VirusesIn 1892, Dmitri Ivanovski--found in the liquid extracted from infected plants.In 1897, Martinus Beijerinck- named tiny particles in the juice viruses-“poison.”In 1935, Wendell Stanley- found crystals of tobacco mosaic virus. Since living organisms do not crystallize- viruses were not alive
4 Discovery of VirusesA virus is a nonliving particle made ofProteinsnucleic acidssometimes lipids.Viruses can reproduce only by infecting living cells.
5 Structure and Composition Viruses differ widely in terms of size & structure.Most viruses are so small they can be seen only with the aid of a powerful electron microscope.
6 Structure and Composition The protein coat surrounding a virus- capsid.May have an additional membrane that surrounds the capsid (flu virus)Contain a few genes to hundreds of genes
7 Structure and Composition Most viruses’ proteins on the surface membrane binds to proteins on the host cell.The proteins “trick” the cell to take the virus or some genetic materialOnce inside, genes are expressed & may destroy the cell.
8 Structure and Composition Most viruses infect only a very specific kind of cell.Plant viruses infect plant cellsAnimal viruses infect only certain related species of animalsViruses that infect bacteria -bacteriophages.RABIES!
9 Lytic Infections Lytic infection virus enters a bacterial cell makes copies of itselfcauses cell to burst- lyse.EX.) T4DNA core insideprotein capsid
10 Lytic Infections 1.) Attachment 2.) Injection: The virus injects its DNA into host cell.3.) Synthesis: Virus DNA uses host DNA to make more viruses4.) Assembly: Host cell creates more of the viruses5.) Releases: Finally the host cell lyses (ruptures)hundreds of virusesthat go on & infectother cells.
11 Lytic InfectionsA lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host.First, the outlaw eliminates the town’s existing authority.(In a lytic infection, the host cell’s DNA is chopped up)
12 Lytic InfectionsNext, the outlaw demands to be outfitted with new equipment from the local townspeople.(In a lytic infection, the viruses use the host cell to make viral DNA & viral proteins.)
13 Lytic InfectionsFinally, the outlaw forms a gang that leaves the town to attack new communities.(In a lytic infection, the host cell bursts, releasing hundreds of virus particles.)
14 Lysogenic InfectionSome Viruses cause a lysogenic infection.dormant stateProphase-DNA that is embedded in the host’s DNAThe prophage may remain part of the DNA of the host cell for many generations.
15 Lysogenic InfectionInfluences from the environment—radiation, heat, etc—trigger the prophage to become active.becomes an active lytic infection.
16 A Closer Look at Two RNA Viruses About 70 % of viruses contain RNA rather than DNA.In humans, RNA viruses cause a wide range of infectionsmild colds to severe HIV.Certain kinds of cancer also begin with an infection by viral RNA.HPVCommonCold
17 The Common ColdCold viruses attack with a very simple, fast-acting infection.A capsid settles on a celltypically in the nosebrought insideVirus makes many new copies of the viral RNA.
18 The Common ColdThe host cell’s ribosomes mistake the viral RNA for its own & makes other virus proteins.The new capsids assemble & within 8 hours, the host cell releases 100s of new virus particles to infect other cells.
19 HIVAcquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV).HIV belongs to a group of RNA viruses-retroviruses.The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.
20 HIVWhen a retrovirus infects a cell, it makes a DNA copy of its RNA.The copy inserts itself into the DNA of the host cell.
21 HIVRetroviral infections are similar to lysogenic infections of bacteria.like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particlesHIV damages the host’s immune system.
22 Viruses and CellsAll viruses are parasites.Parasites depend upon other living organisms for their existence +/-Viruses infect living cells in order to grow & reproducetaking advantage of the nutrients & cellular machinery.
23 Viruses and CellsViruses have many of the characteristics of living things.After infecting living cells, viruses can reproduce, regulate gene expression, & even evolve.Swine Flu Rotavirus
24 Viruses and CellsSome of the main differences between cells and viruses are summarized in this chart.