Lecture 4

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University of Toronto St. George
Cell and Systems Biology
Mounir Abou Haidar

Replication of Viruses continued pg 7989Taking aRNA virus in this particular case TMV and looking at how they replicate look at diagram on pg 84It is a plant virusthe major difference between a plant virus and an animal virus is that plant cells have cell walls while animal cells dont So in order for the virus to be infectious it has to mechanically attach itself to the plant cell by stretching the cell wall But in nature insects suck the juice with their mouth and punch a hole and carry the virus In animals there is no need to break the cell wall as there is noneThe next step is virus attachment and entry Another major difference in plant viruses is that all plant viruses no exceptions must carry a gene for cell to cell movement a transport protein The virus itself does not move but its components like the RNA and protein go through the plasmodesmata which provides communication between the cells So all plant viruses have to carry an extra gene for cell to cell movement whereas animal viruses dont have this gene This is a distinct characteristic of plant virusesstAfter this uncoating occurs and the replication process begins The 1 step is translation As we knowRNA viruses are directly translatable by the ribosomes of the cell In this case you produce a polymerase gene which is a protein coded for by the virus at the 5 end It is a large protein which is combined not typical for animal viruses with 3 specific host proteins to make a complex of 4 subunits One of them is from the virus and the other 3 are hijacked by the polymerase of the virus from the plant cell These 4 subunits are then folded together to make a RNA dependant RNA polymerase in the plant system In human viruses the polymerase coded by the virus is sufficient for the replication processOnce the polymerase is translated it combines with host proteins to make a functional replicase which is a RNA dependant RNA polymerase but with a host component It starts copying theRNA of the virus intoRNA as seen on the diagram on page 84 At one stage theRNA has aand astrand complementary to each other This is called a replicated form double stranded RNA form NoteAny single stranded RNA virus that is being replicated will have a double stranded replicated formtogether at one stage This replicated form becomes a target for an enzyme in the cell which is part of a cellular defence mechanism found in all eukaryotic cells from plants to humans to animal cells The cells themselves have a cellular defence mechanism using an enzyme called DICER a ribonuclease which targets only dsRNA Why Because the cell doesnt normally have dsRNA it only has mRNA which is not double stranded So it is specifically designed to cut that dsRNA Once it cuts the dsRNA the virus cannot replicate anymore It is broken down into 2124 nucleotides of dsRNA This mechanism is called RNA SILENCING or GENE SILENCINGNoteHowever a lot of viruses develop repressor proteins that inhibit this DICER activity to be able to survive this defence mechanism
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