Lecture 13

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

Lecture 13 We start seeing viroids and virosoidsviroids are small circular RNA have no coding capacity but are capable of replicationthey use RNA polymerase a DNA directed RNA polymerase in the nucleus for their replicationviroids that have ribozymes replicate in chloroplasts small amount not in nucleus majority are in nucleususually they have a structure that is DS RNA that has some mismatches looks like DS this is diagram shows lots of bubbles they dont matchthat structure that looks like DS is very important for the life of those viroids they have to have an origin of replication DNA dep RNA polymerase of the cell has torecognize an origin of replication20 types of viroids did a study most have this conserved sequence nothing to memorize in the middle then have conserved domains which are circular but then makes this kind of odd shape due to mismatch base pairingthis area has a lot of variability this area if you change one nucleotide or another change the disease symptoms of plant refer to slidethey replicate with the RNA polymerase of the cell if they have a ribozyme they use it for catalytic breaking breaking RNA from the rolling circle model to create a circle end ligation at the same timemajority dont have ribozyme need to rely on ribonuclease need Ribo nuclease to cut them out and ligase to join the ends and make the circleligation is by RNA ligasenow im going to go into virosoidsonly common things is that they look similarare special kind of satellite RNA replicate with the help of a helper virus ex is SLCSD those are called specifically called virosoids which means special kind of satellite RNA but replicate with the help of a helper virus they suck from the virus the RNA dependent RNA polymerase which is what the virus coded for its own replicationnot only for that they can package their cRNA inside the capsid protein of the virusthen inside the cells the satellite very small will replicate with the polymerase and put itself inside the capsidnot only relying on Polymerase but also to package its circular satellite RNAall virosoids have a ribozyme means they have a structre that can cut its own RNA by itseld do not need an Ribonuclease to cut itHammer head ribozyme not to memorize will cut itself right in there andsequence have ribozymeobviously they must have a region for the RNA polymerase of the virus also to have a sequence to have the RNA packaged into the capsidsimilarly hepatitis delta or D is also satellite RNA much bigger but codes for a protein the D antigen 1600 nucleotides is a satellite not a virosoid or a viroid virosoid and viroids are not found in animals only plantsHep D is circular satellite RNA used Hep B virus not for replication because it is circular RNA will replicate with DNA dep RNA of the cell like viroid has a region which is viroid like
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