The second parity rule applies more closely to introns than to exons.

TLS Online TPP Program

#Id: 6794

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6795

#Unit 3. Fundamental Processes

First cleavage reaction in nucleus leaves a 2-nucleotide overhang on the 3’ ends of the dsRNA product. This 3’ overhang is important for recognition of that RNA molecule by the next enzyme in the cytosolic pathway, Dicer.

TLS Online TPP Program

#Id: 6796

#Unit 3. Fundamental Processes

The pre-miRNA liberated by Drosha is exported to the cytoplasm, where the second RNA cleavage reaction, performed by Dicer, takes place. As with Drosha, 

Dicer selects its cleavage sites using a measuring, rather than sequence-specific, mechanism.

TLS Online TPP Program

#Id: 6797

#Unit 3. Fundamental Processes

Dicer is constructed of three modules: two RNase III domains and a dsRNA-binding domain called the PAZ domain (named for three proteins that contain this domain: Piwi, Argonaute, and Zwille).

TLS Online TPP Program

#Id: 6798

#Unit 3. Fundamental Processes

Dicer will act on any dsRNA, regardless of sequence, and will cleave this molecule 22 nucleotides from its end.  The PAZ domain anchors the 3’ terminus of the substrate RNA to position the active sites of the enzyme 22 nucleotides away in a ruler-like fashion

TLS Online TPP Program

#Id: 6799

#Unit 3. Fundamental Processes

The action of Dicer generates the short RNA molecule that will determine what target RNA is acted upon. The active form of the regulatory RNA is the single-strand form—at this stage called the guide RNA—incorporated into a RISC protein complex.