A basic overview of viruses What is a virus History of virology Do not have any characteristics of living things unless they are inside of a host cell Viral diseases have plagued humans since before we even knew what they were smallpox in Egypt It began as a science in late 1800 s when infectious tobacco mosaic virus was isolated in a filtered bacteria free fluid by Ivanovski then Beijerinck showed in 1901 that a human disease yellow fever was caused by a virus A basic overview of viruses History of virology Walter Reed transmitted by mosquitos A basic overview of viruses Structure of viruses Viruses are Intracellular obligate parasites Must be inside of a host cell in order to do anything Typically between 10 and 100 nm Genomes typically between a few thousand to 200 000 nucleotides in length a virus of amoebas has a genome over 1 2 megabase pairs encodes 1 200 a virus of amoebas can be 400 nm in diameter with a 1 2 megabase pair A basic overview of viruses There ARE exceptions to the small size of viruses Megavirus proteins Mimivirus genome coding for 979 proteins A basic overview of viruses Structure of viruses Single or double stranded DNA or RNA capsid and genome together nucleocapsid Some have envelope plasma membrane around capsid A basic overview of viruses Structure of viruses Symmetry Icosahedral capsids poliovirus herpes virus Roughly spherical 20 equilateral triangles bacterium A basic overview of viruses Structure of viruses Symmetry Helical Filamentous M13 Ebola TMV Long tube of protein with genome inside Viruses must have a genome They have a much larger genome that other cellular organisms Protein covering capsid around genome composed of many capsomereproteins or faces that come together to look like a coccus shaped Tube made up of 100 s of identical protein subunits Tube length reflects size of viral genome A basic overview of viruses Structure of viruses Symmetry Complex capsids Mixture of icosahedral filamentous shapes Many bacteriophage viruses that infect bacteria Asymmetrical irregular shapes Tend to be larger viruses Poxviruses cow pox and small pox Vaccinia cowpox virus A basic overview of viruses Structure of viruses Viral Envelopes If a plasma membrane surrounds the nucleoplasmid the virus is enveloped many animal viruses are enveloped viruses They acquire envelope by budding from host and stealing host s plasma membrane If there is no plasma membrane the virus is naked Replication cycle BRIEFLY a virus must Stick to a host cell adhere Get into the cell penetrate and release its genome uncoat Express its genes to make proteins synthesis DNA must be transcribed into RNA and then proteins must be made Replicate its genome synthesis Put everything together assembly and get the new virus particles out exit Replication cycle Entry Mechanisms for entry vary depending on the host cell Animal viruses don t have to contend with a cell wall structure Plant fungal and bacterial Replication Cycle Entry into animal cells viruses do Endocytosis of a non enveloped virus The virus attaches to the cell receptor Endocytosis is initiated An endosome forms with the virus inside The nucleocapsid escapes to the cytoplasm to release the genome Membrane fusion of an enveloped virus most common method of entry into animal The virus attaches to the cell receptor A conformational change in the attachment protein and bound receptor initiates membrane fusion The viral envelope fuses with the plasma membrane The nucleoclaspid enters the cytoplasm and uncoats to release the genome cells Endocytosis of an enveloped virus The virus attaches to the cell receptor Endocytosis is initiated An endosome forms with the virus inside The low pH of the endosome initiates fusion of the viral envelope with the endosome membrane The nucleocapsids are released Replication cycle Entry into plant cells Often depends on some damage to the plant tissues to create an opening in the cell wall on plants damage Wind Insects feeding Hail rain Fire Human induced damage damage damage Replication cycle Entry into bacteria Tail fiers attach to receptors on the host cell Conformational change in tail fibers bring the base of the tail into contact with the host Rearrangement of tail proteins allows inner core tube proteins to extend down into the Contact with the plasma membrane initiates transfer of DNA through a pore formed in cell surface cell wall the lipid bilayer The capsid tail and tail fibers never enter a bacterial cell Viruses inject their genome directly into the bacteria Origins of viruses From what did viruses arise The evolutionary history of viruses isn t very clear but three hypotheses have emerged coevolution regressive Progressive Origins of viruses Coevolution hypothesis Viruses evolved along with their host cell This could explain the origin of many RNA viruses There is little supporting evidence as of yet Regressive hypothesis Viruses are cells that lost some of the replicative and metabolic traits over time making them lose their ability to live on their own Examples of this phenomenon exist in the biological world This doesn t adequately explain origins of RNA viruses because there is no evidence of a virus that has RNA as its genome Origins of viruses Progressive hypothesis Existing genetic elements gradually gained the ability to move from one cell to another or one location on a chromosome to another Evidence in the form of transposons for DNA viruses and retrotransposons RNA for retroviruses Reverse transcriptase Cultivation purification and quantification of viruses How can we grow and quantify viruses Viruses are much trickier to work with than bacteria They are very small They only replicate within appropriate host cells Ex If you want to grow an animal virus you mus have animal tissue Viral cultivation Two types of bacteriophages Lytic Phage will replicate in and burst open lyse the host cell Lysogenic Phage can integrate into the host cell genome becoming a prophage and be replicated each time the host cell replicates Some bacteriophage can only go through the lytic cycle and some have the choice of going through the lytic cycle or the lysogenic cycle Strictly lytic is easier to grow and isolate Cultivation purification quantification of viruses 1 a small volume of susceptible bacterial host cells are added to the phage sample 2 The mixture is added to molten nutrient agar and quickly mixed 3 The agar mixture is poured onto a nutrient agar base and allowed to solidify 4 Plaques