Figure 1.

Characteristic intermediates in microRNA processing. (a) A typical example of primary microRNA transcript before the Microprocessor cut distal to the stem loop. The 5' and 3' ends of the primary transcripts are not generally known; this example was obtained by folding hsa-mir-23a with 50 nucleotides flanking the Microprocessor site, as defined by the ends of the mature microRNA [41]. (b) The precursor microRNA as transported from the nucleus to the cytoplasm. (c) A mature duplex microRNA after Dicer processing, but before incorporation into RISC. Note that shRNAs can be similar to primary microRNA transcripts or precursors, whereas siRNAs are made similar to the mature duplex.