Question

Short RNAs that are encoded by genes much like activators or repressors for the purpose of regulating eukaryotic gene expression are known as _____.
a) snRNAs
b) RNAi’s
c) snoRNAs
d) miRNAs
e) siRNAs

Which of the following apply to BOTH siRNA and miRNA?
a) initially consists of two strands of RNA
b) initially consists of a single strand of RNA, folded on itself
c) processed in two steps by endonucleases
d) enters the RISC complex, where one RNA chain is discarded
e) all of the above

Answer 1

Answer 1) The correct options are (d) and (e).

option (a) is incorrect: snRNA (small nuclear RNA) is also a small RNA and play essential roles in RNA processing. snRNA forms snRNPs (small nuclear ribonucleoproteins) by combing with different proteins and form a complex called Spliceosome which guide the RNA splicing process.

option (b) is incorrect: RNAi (RNA interference) is a mechanism of inhibiting gene expression or translation by using other small RNAs (like miRNA and siRNA) together with many other proteins.

option (c) is incorrect :snoRNA (small nucleolar RNA) is also a small RNA and their role is to guide the post transcriptional modifications in tRNA, rRNA and snRNA through a series of processes.

option (d) and (e) are correct: the process of RNAi (RNA interference) is a mechanism of regulation eukaryotic gene expression by either degrading the target mRNA or by blocking it. Th etwo small RNAs that play central role in RNAi are siRNA and miRNA.

Answer 2) Option (a) is correct.

Both miRNA and siRNA are small RNAs having role in regulation of gene expression by largly blocking the translation. Although both perform similar function, they differ in quite a few points of their biogenesis, processing and mechanism of blocking the translation.

Option (a) is correct. We can say that both miRNA and siRNA are formed by precursors consisting of two strands of RNA. Here the minor difference is that the siRNA precursors are long double stranded RNA molecules however miRNA precursors are stem-loop structure with incomplete double stranded nature. But incomplete or complete, both precursors have two strands of RNA.

Option (b) is incorrect: siRNA are formed from long dsRNA with full complementarity. miRNA precursors are not true dsRNA molecule and forms two strands by folding one long RNA molecule in to a hair-pin loop structure.

Option (c) is incorrect: miRNA and siRNA differ in their processing mechanisms due to different origin. miRNA are endogenous RNA encoded by cell's own genome whereas siRNAs are exogenous in origin (from viruses or transposons). miRNA are first processed in two major steps: first inside the nucleus by enzyme Drosha (a nuclear endonuclease) and after being exported into the cytoplasm by Dicer (a cytoplasmic endonuclease). However siRNA are exogenous in origin and  enters the cell from outside and is processed in cytoplasm by Dicer with some other proteins.

Option (d) is incorrect: Loading on to the RISC complex is different for miRNA and siRNA. siRNAs are loaded on to RISC complex as double stranded RNA and one of the strand is degraded in to the RISC complex. However miRNAs are loaded as sigle strand molecules onto the RISC complex.

Option (e) is incorrect: because b, c and d options are not correct.