ZEBRA FISH EMBRYOS THAT LACK MATURE

miRNAs are approximately 22-nucleotide small RNAs that act as posttranscriptional repressors by binding to the 3′UTR of target mRNAs (Lee et al. 1993; Reinhart et al. 2000; Bartel 2004; Kloosterman and Plasterk 2006). In animals, mature miRNAs are generated from a primary transcript (pri-miRNA) through sequential cleavage by nucleases belonging to the RNAse III family. Initially, Drosha cleaves the pri-miRNA and excises a stem-loop precursor (pre-miRNA), which is then cleaved by Dicer into an RNA duplex (Bernstein et al. 2001; Grishok et al. 2001; Hutvagner et al. 2001; Ketting et al. 2001; Knight and Bass 2001). One strand of the duplex constitutes the mature miRNA, which is incorporated into a silencing complex (RISC) and guides it to target mRNA