class 14 RNAi

where do siRNAs come from

usually exogenous dsRNA

but, can also derive from double transcription of endogenous DNA sequences, transposons are common

siRNA processing

no DROSHA, just DICER.

cut into 20-25 bp pieces

forms si si* double stranded complexes (fully complementary) that are then similarly sorted by argonautes.

siRISC

fully complementary binding to mRNA

the RISC itself splices the mRNA

mechanism of action for siRNAs

direct mRNA cleavage

also heterochromatin formation

endo-siRNAs

endogenous

target transposable elements

transposons can have antisense transcripts/promoters that allows for bidirectional transcription.

cells use endosiRNAs to defend from transposons

can also have a stretch of DNA in series that have inverted repeates that when directly transcibed can form RNA moleculees that direcly base pair internally.

viral origin siRNAs

when the cell recognizes an RNA virus has invaded, it can develop its own siRNA against that virus to neutralize it.

why can miRISCs not cleave but siRISC can

there are for Agos, only 2 can do cleavage.

siRISCs are only made of Ago2, which miRISC can hae any of the 4

even if an miRISC has an Ago2, still can’t cleave bc. it is not an exact sequence match like in an siRISC

why is dsRNAi more potent than ssRNAi

because the dsRNA that you itnroduce is already a double stranded complemetn of your sequence of itnerest, can be directly DICEd and immediately done. apprently can be more efficient.

what region of mRNA do siRNAs bind to

anything

this is in contrast to miRNAs, only 3’ UTRs

how do siRNAs regulate transcriptional regulation

centrometere has dh and dg genes that can be transcribed to form dsRNAs that can bind to a nucelar version of RISC, which then base pair with complementary dh or dg target locus, to trigger heterochromatin formaiton at centromeres.