TLS Online TPP Program

#Id: 6788

#Unit 3. Fundamental Processes

Both siRNAs and miRNAs are generated from longer RNA molecules by the enzyme Dicer, an RNase III–like enzyme that recognizes and digests longer dsRNAs or the stem-loop structures formed by miRNA precursors.  The siRNA and miRNA products are typically 21–23 nucleotides long

TLS Online TPP Program

#Id: 6789

#Unit 3. Fundamental Processes

The piRNAs which are 24–34 nucleotides long are derived in a manner that does not involve a dsRNA precursor

piRNAs are generated by processing long single-stranded transcripts covering so-called piRNA clusters found in the genome. This processing does not require Dicer.

TLS Online TPP Program

#Id: 6790

#Unit 3. Fundamental Processes

The miRNAs used in RNAi are produced as large RNA primary transcripts called pri-miRNA that are self-complementary and can automatically fold into a double-strand hairpin structure.

TLS Online TPP Program

#Id: 6791

#Unit 3. Fundamental Processes

pri-miRNA is processed in a two-step reaction

The first step is catalyzed by Drosha, an RNase III superfamily member endonuclease, in the nucleus. Drosha reduces the pri-RNA to about a 70-bp, hairpin-shaped precursor fragment, pre-miRNA

TLS Online TPP Program

#Id: 6792

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6793

#Unit 3. Fundamental Processes

The two cleavages thus generate the 65-nucleotide pre-miRNA composed of the 22 bp (two helical turns) of dsRNA and the top loop.