The arrival of a nerve impulse at a neuromuscular junction triggers an action potential in sarcolemma.

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.

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