BIODIVERSITY CHAPTER 27 VIRUSES 10 6 focus on DNA v RNA viruses Common Structures of Viruses Figure 27 1 genetic material o DNA o RNA capsid outer protein covering that surrounds the genetic material o capsids have many shapes tobacco mosaic virus plant virus has a helical corkscrew shape containing RNA adenovirus animal virus has an icosahedral capsid that contains DNA bacteriophage bacteria virus has an icosahedral head and helical tail RNA envelope virus animal virus capsid surrounds RNA and is surrounded by an envelope structure around the capsid with antigens studded around it antigens determine cell type tricks cell type drives in and out o proteins are associated with the different shapes of capsids Bacteriophages DNA lytic cycle bacteria cell ruptures at the end releasing new virions viral replication cell death virion attaches to host cell after penetration viral DNA is injected into the cell attachment 1 2 penetration viral DNA hijacks cellular replication processes synthesis processes making synthesis 3 new proteins protein synthesis nucleic acids new viral pieces and parts are produced using the host s machinery assembly release virions are released into the system new virions are produced eventually the membrane lyses or ruptures and new involves spontaneous assembly of capsid and enzymes to insert DNA 4 5 lysogenic cycle cell does not die but viral DNA integrates itself into the host genome integration 1 2 propagation viral DNA has incorporated itself into the host genome leads to prophage prophage along with host genome host bacteria splits viral DNA is incorporated into the bacterial genome passed on through reproduction 3 cell stress limitations toxin etc pushes the cell into the lytic cycle changes in the environment relating to competition to resources food phage conversion bacterial host becomes infectious toxic following infection by bacteriophages o in the lysogenic cycle at this point o part of the viral DNA activates activation may have interactions with bacterial DNA leading to toxin production bacteria being infectious becoming a true disease causing organism could do nothing o cholera Vibrio cholera salmonella diphtheria these things undergo phage conversion and become more toxic ex Vibrio cholera when uninfected by bacteriophage is harmless no toxin production however when infected by a particular bacteriophage it becomes infectious toxic and reproduces rapidly produces cholera toxin that is encoded by the viral DNA bacteria colonize small intestine and lots of toxin is produced interaction leads to serious dehydration imbalance of electrolytes high death rate mostly occurs in areas with poor sewage systems poor water conditioning natural disasters warzones areas of conflict where balance gets disrupted Envelope Viruses envelope virus RNA flu influenza viruses has an outer envelope that contains a helical capsid with o envelope is studded with protein spikes antigens H antigen protein hemagglutin that is responsible for host recognition and N antigen protein neuraminidase used after viral replication is completed to get getting into the host cell new virions out of the host cell no cell death ex H1N1 virus swine flu has H1 subtype and N1 subtype H5N1 virus bird avian flu 15 16 H subtypes and 9 10 N subtypes o diversity could be problematic diversity is caused by the increased mutation rates in flu viruses because they are RNA viruses vaccine design is effected each new subtype new target immune system antibodies recognize fight diseases viruses and organisms and fights them better if they have previously encountered that disease virus or organism before new H or N antigens lead to a new viral strain o antigenic shifts why are flu viruses so prone to mutation o RNA viruses have very high error rates during replication extremely error prone ALL flu viruses are RNA viruses high rates of mutation increases the likelihood of new viral strains alters H or N antigen and leads to a new form subtype mutation can change the viral RNA segments w o changing the H or N o if a cell is invaded by more than one flu virus reassortment occurs reassortment ways and a form that didn t exist before now exists all four subtypes in the infected cell come together in four different o DNA viruses have very low error rates during replication ex pox herpes hep B bacteriophages o universal flu vaccine find and attack unvarying but functionally important structures on viruses efforts focused on the envelope and capsid antibodies in immune system would be in place to attack any flu examples of flu viruses o Avian flu H5N1 RNA virus not affected by antibiotics originally present in wild birds reservoir always an origination for the virus virus stays in bird populations cycles within populations occurred and viral RNA mutated and was then found in domestic birds host jump and cycles within that population second host jump occurred into humans but is not cycling in the human population o Swine H1N1 how it and other viruses replicate Swine is taken into the cell by lysosomes H antigen contacts receptor lysosome breaks down RNA goes into the nucleus and is replicated new virions are constructed outside of the nucleus N antigens allow new virions to bud out of the cell does not rupture after the RNA is replicated the cell continues to make new virions o HIV human immunodeficiency virus causes AIDS RNA envelope virus NOT a flu virus gp120 glycoproteins like H antigens that attach to receptors on white blood cells to gain entry into that cell capsid with RNA and reverse transcriptase makes them retroviruses RNA and reverse transcriptase use internal parts of host cell form DNA strands from the RNA template that are incorporated into the human genome occurs rapidly after infection HIV taking over a cell enters cell cellular receptors accept g20 glycoproteins o entry inhibitors prevents allows binding between cellular receptors and glycoproteins on the outside of the HIV virus o CCR5 to receptor type on white blood cells that glycoprotein binds Australian chap who had mutation for receptor CCR5 32 glycoproteins could NOT bind to this capsid breaks apart RNA transcriptase and RNA is releases into host cell RNA template is used to make a DNA RNA hybrid and then a DNA double strand injected into the host DNA RNA leaves nucleus leads to production of new virions using viral proteins virion buds off of cell more infection can occur ways to prevent HIV from reproducing taking over cell change
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