Regulatory RNAs Flashcards
What are the major functional differences between siRNAs and miRNAs?
miRNAs are found in all major eukaryotic branches while siRNAs are found in most, but not all.
miRNAs regulate all kinds of genetic pathways. siRNAs are also regulators, but they are also functional as defenses against viruses.
miRNAs are always endogenous - encoded by organism’s own genome. siRNAs can be endogenous or exogenous.
What does the precursor of a miRNA look like?
Precursor is a hairpin structure derived from a ssRNA. Called pri-miRNA.
What does the precursor of a siRNA look like?
Precursor is a dsRNA.
What are the sources of a siRNA precursor, i.e how is it transcribed?
Convergent transcription: two close genes are being transcribed from opposite directions - regions that overlap and basepair.
Transcription of inverted transposons.
Bidirectional transcription - transcription of one gene from both directions
Founder gene and pseudogene each being transcribed - (not fully complementary)
How does the Dicer process the pre-miRNA and the precursor siRNA?
The Dicer cleaves away the hairpin knob from the pre-miRNA and produces 2 nt 3’-overhangs on both the siRNA and miRNA.
How does the process of loading the siRNA or miRNA onto the RISC work?
The Dicer processed small RNA is loaded onto the RISC/Ago complex, and one strand is chosen as a guide strand (that will guide the complex to the target RNA). The other strand (passenger strand) is degraded. Produces ssRNA = mature si/miRNA.
What are the two outcomes in terms of function when the mi/siRNA is loaded onto the RISC/Ago?
mRNA cleavage and degradation
Inhibition of translation
What are the requirements of the mi/siRNA and Ago complex in order to induce endonucleolytic cleavage at the target mRNA?
The complementary between Ago and small RNA has to be perfect or near perfect
Ago has to be cleavage competent - catalytic residues.
What is the outcome if there is an imperfect match between miRNA and the Ago protein?
If there is imperfect base pairing, Ago can recruit ribonucleases to the target site to inhibit translation, and sometimes degrade the mRNA.
How is the pre-miRNA precursor generated?
Pol II trancribes a precursor transcript in the nucleus, and a 5’-cap is added at the end. The pri-miRNA is generated by forming a hairpin formation. The hairpin binds to a drosha protein, which cleaves away the ends –> pre-miRNA.
What is one outcome of the RISC/miRNA complex if there is imperfect base pairing between miRNA and the Ago protein?
In animals: Instead of mRNA cleavage and degradation, there will likely be translational inhibition and (sometimes) mRNA degradation. The Ago will recruit a GW protein, which will induce decapping of mRNA - inhibiting translation and making it suspsceptible to degradation.
What are RdRPs? What is the main purpose of these?
RNA dependent RNA Polymerases.
Polymerases that use ssRNA as template to generate complementary small RNA strands.
They are used to amplify small RNAs from the RNA, instead of having to generate new ones from DNA.
What are the major purposes of siRNAs and miRNAs?
They regulate gene expression on several levels, most often post-transcriptional. They mainly repress expression by degrading mRNA or inhibiting translation.
