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Lecture 17

2ee3 lecture 17 viruses.docx

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Jianping Xu

LECTURE 17  Virus o independently existing non-cellular organism that lives outside a host cell o plasmid cannot exist outside of a cell on its own because it doesn’t have the protective proteins that virus has o Viruses have structures that are useful for infection  effect stabilization on host cell and injection of genetic material (tube and feet)  Structure and morphology o protein capsid: encloses genetic material. Made up of capsomers  individual proteins. Capsomers in a virus belong to the same protein.  If they have a nucleic acid and capsomers they are called naked viruses. Small number of proteins involved in capsomer structure.  If they have a phospholipid envelope  enveloped viruses. Membrane proteins involved in recognition of host cell. o Icosahedral symmetry  depending on the subunits involved (twofold, five fold etc)  Genomes o can be RNA based or DNA based, single stranded or double stranded DNA/RNA, and + or – DNA RNA o mRNA used to translate protein  viruses classified into 7 categories  3 belong to DNA based viruses: utilize DNA polymerase for replication, DNA dependent rna polymerase for transcription (I). mRNA and reverse transcriptase used by (VII) to obtain a double stranded dna. (II) is the single stranded DNA. Positive strand refers to a single stranded DNA sequence identical to the RNA strand except it doesn’t contain U. We have not found – strand DNA virus.  (III) double stranded RNA based virus and 3 single stranded RNA stranded viruses which go through replication in different ways. (VI) is a retrovirus like HIV, + strand goes through reverse transcriptase, double stranded DNA which makes the mRNA. (IV) uses rna polymerase. (V) uses rna dependent rna polymerase to synthesize rna. o other ways to classify  linear/circular  one/multiple fragments of DNA. HIV has 1 fragment and influenza has 8 fragments.  capsid symmetry, envelope, size, host range  Bacteriophage diversity o Rna based  single/double stranded  some viruses (phi6) are very good at cheating  multiply very fast o single stranded dna  phiX174: first organism to be considered a virus o double stranded dna  Bacteriophage life cycles: 3 pathways o Lytic cycle: 1000s of copies of itself o Lysogeny: after bacteriophage affects host cell, it integrates its genome into host genome and co replicates with the host genome and spread itself in the process. However, these viruses can get released from the cell through lytic cycle. Almost all bacterial genomes have remnants of viral genome. o Slow release: virus doesn’t kill the host cell and slowly releases its viral particles to infect others  concequences of viral infections in animal cells  3 types o latent infection like lysogeny o latent infection: like slow release o lysis: like lytic cycle o transformation  integration of viral genome after infection activates the production of the host cell making it grow uncontrollably  cancer as a result of viral infections when they disrupt the genes that control cell cycle  viral quantification and growth o plaque assay  main method  for viruses that can lyse the host cell (lytic cycle)  once cell is lysed the viruses will form structures with different properties than intact cells aka plaque  evenly poured on petri dish, virus infects host cells and neighbouring cells  by counting plaques you can find out how many viral particles there were in the original culture  hollow in the plaque can be seen on the petri dish  influenza virus and the flu o 8 different segments that are single stranded each encoded differently o infection rate and death rate pretty high o NP: used to stabilize the single stranded RNA o end of each fragment has a cap and an enzyme  RNA polymerase so it doesn’t require the host cell RNA polymerase for replication and transcription o Matrix has NS2 which helps viral genomes pack nicely together and the outside has the matrix protein (M1) that is another protection layer, and further outside there is a phospholipid membrane containing 2 kinds of proteins (HA or NA), Ion channel protein (M2) is essential for the release of viral genome into the host cell. It allows the acidification of the viral genome once it is inside the host cell.  The genome of influenza A o 1,2,3 code for PB1, PA, PB2 o 4  HA, 5 NP, 6 NA, 7M1 and M2, 8Ns1 and Ns2 (involved in nuclear packaging)  Influenza entry o recognizes specific receptor on host cell that is attached to sialic acid linked through galactose o different hosts have diff connections between sialic acid and galactose. Humans have both kinds, predominantly 2-6 linkage for the upper respiratory tract and 2-3 linkage for lower respiratory tract.
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