MicroRNAs regulate the expression of genes, often large groups of genes. They are important in embryonic development, in cardiac function, in carcinogenesis and in many other processes. The recent discovery of miRNAs has altered our understanding of gene regulation, with many new reports of miRNA activity appearing in the literature. Experimental manipulation of miRNA activity will become a standard tool of molecular biologists, but currently these techniques are in development and still improving.

MicroRNAs mature through several steps. A primary miRNA is transcribed from DNA and folds into a stem-loop. The stem-loop is cleaved from the rest of the transcript by the nucleolytic enzyme Drosha, then the loop is cleaved from the stem by the nucleolytic enzyme Dicer. The double stranded stem interacts with the protein Argonaute, which cleaves and releases one of the strands, forming the miRISC complex with the other strand, now called the guide strand. It is the miRISC complex which interacts with mRNAs, altering their expression. miRNA complexes can also interact with DNA, changing gene expression by altering histone methylation and, by interacting with targets in promoter regions, by upregulating transcription; the specific actions of miRNA depend on the sort of organism.

To browse sequences of miRNAs and predicted secondary structures of their precursors, see the database maintained at the Sanger Institute, miRBase:
http://microrna.sanger.ac.uk/sequences/

As examples, try searching in miRBase for a few zebrafish (Danio rerio) sequences like dre-mir-1, dre-mir-10 etc.