Encoded by the genome of most eukaryotes examined so far,
microRNAs (miRNAs) are small ~21-nucleotide (nt) noncoding
RNAs (ncRNAs) derived from a biosynthetic cascade involving
sequential processing steps executed by the ribonucleases
(RNases) III Drosha and Dicer. Following their recent
identification, miRNAs have rapidly taken the center stage as key
regulators of gene expression. In this review, we will summarize
our current knowledge of the miRNA biosynthetic pathway and its
protein components, as well as the processes it regulates via
miRNAs, which are known to exert a variety of biological functions
in eukaryotes. Although the relative importance of miRNAs remains
to be fully appreciated, deregulated protein expression resulting
from either dysfunctional miRNA biogenesis or abnormal miRNA-based
gene regulation may represent a key etiologic factor in several,
as yet unidentified, diseases. Hence is our need to better understand
the complexity of the basic mechanisms underlying miRNA biogenesis
and function