In E. coli the typical mRNA half-life is about 3 minutes, but half-lives of individual mRNAs may be as short as 20 seconds or as long as 90 minutes.  In budding yeast, mRNA half-lives range from 3 to 100 minutes, whereas in metazoans, half-lives range from minutes to hours, and in rare cases, even days.

#Id: 6803

#Unit 3. Fundamental Processes

The base pairing between miRNA and target RNA is initiated by interactions of so-called seed residues—typically the sequence between bases 2 and 9 of the 22-nucleotide miRNA

#Id: 6804

#Unit 3. Fundamental Processes

Humans express four Argonaute proteins. AGO2 is the human Argonaute protein in miRNA-containing RISC complexes.

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#Unit 3. Fundamental Processes

#Id: 6806

#Unit 3. Fundamental Processes

When the 5′ end of the siRNA of a RISC complex base-pairs precisely with a target mRNA over a distance of one turn of an RNA helix (10–12 base pairs), this domain of AGO2 cleaves the phosphodiester bond of the target RNA across from nucleotides 10 and 11 of the siRNA

#Id: 6807

#Unit 3. Fundamental Processes

The precursor RNA for the siRNA pathway is a longer dsRNA. As a consequence of this different initial substrate, Drosha is not needed for the generation of siRNAs. Cleavage by Dicer is still required, however, and again generates a suitable 21- to 22-nucleotide RNA for incorporation into RISC.

#Id: 6808

#Unit 3. Fundamental Processes

In plants, even miRNAs are generated by Dicer alone; it is not clear how they manage to forgo prior action of Drosha.