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Lecture 13

Lecture 13

Cell and Systems Biology
Course Code
Mounir Abou Haidar

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Lecture 13
We start seeing viroids and virosoids
-viroids are small circular RNA, have no coding capacity, but are capable of replication
-they use RNA polymerase, a DNA directed RNA polymerase in the nucleus for their
-viroids that have ribozymes, replicate in chloroplasts; small amount, not in nucleus;
majority are in nucleus
-usually they have a structure that is DS RNA, that has some mismatches; looks like DS,
this is diagram, shows lots of bubbles, they don’t match
-that 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 to
recognize an origin of replication
-20 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 mis-match base pairing
-this area has a lot of variability, this area if you change one nucleotide or another change
the disease symptoms of plant (refer to slide)
-they 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 time
-majority don’t have ribozyme, need to rely on ribo-nuclease, need Ribo nuclease to cut
them out, and ligase to join the ends and make the circle
-ligation is by RNA ligase
-now im going to go into virosoids
-only common things is that they look similar
-are special kind of satellite RNA, replicate with the help of a helper virus; ex is S-LCSD
-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 replication
-not only for that, they can package their c-RNA inside the capsid protein of the virus
-then inside the cells, the satellite , very small, will replicate with the polymerase and put
itself inside the capsid
-not only relying on Polymerase but also to package its circular satellite RNA
-all virosoids have a ribozyme, means they have a structre that can cut its own RNA by
itseld, do not need an Ribonuclease to cut it
-Hammer head ribozyme, not to memorize, will cut itself right in there
-+ and – sequence have ribozyme
-obviously they must have a region for the RNA polymerase of the virus, also to have a
sequence to have the RNA packaged into the capsid
-similarly, 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 plants.
-Hep 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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