A new development in our understanding of metabolism as controlled by our genome is the finding of a large family of small non coding RNAs.
In a clear summary in Nature Grobhans and Filopwicz explain their biology
There arc three main types of well characterised RNA:
Messenger RNA, transfer RNA and ribosomal RNA.
Messenger mRNAs are translated into proteins, whereas transfer tRNAs and ribosomal rRNAs have housekeeping roles during mRNA translation
Small RNAs (20-30-nucleotides) are not translated into proteins, but regulate biological processes, often by interfering with mRNA translation.
Small RNAs include small interfering RNAs (siRNAsJ, microRNAs (miRNAs) and Piwi-associated RNAs (piRNAs).

A summary of RNAs

Messenger RNAs (mRNAs) act as templates for protein synthesis.

Ribosomal RNAs ( rRNAs ) are found in ribosomes, and mediate the decoding of mRNAs to the amino-acid sequences of proteins.
Transfer RNAs (tRNAs ) carry individual amino acids to the site of protein synthesis that recognize specific codons in -mRNA.
Non-coding regulatory RNA
Small interfering RNAs ( siRNAs)
Small RNAs (20-25 nucleotides in length)are formed by cleavage of long double-stranded RNA molecules. These are important in modulating of transposons activity and combating viral infection, and regulate protein-coding genes
.MicroRNAs (miRNAs) Small RNAs (20-25 nucleotides in length) are encoded by specific genes from long, single-stranded RNA sequences and fold into hairpin structure. The are important in repressing mRNA translation or mRNA degradation
Piwi-associated RNAs (piRNAs ) are small RNAs (25-3nudeotides in length) and are essential for the development of germ cells.
Longer non-coding RNAs of of 70 to thousands of nudeotides lengths which are involved in various cellular processes, including mRNA splicing and ribosome biogenesis.
Small RNAs are generally produced by fragmentation of longer precursors eg from double-stranded RNAs form ed by base-pairing of complementary RNAs by the enzyme Dicer into shorter double-stranded siRNAs some 20 base pairs long.
Each strand assembles into an effector complex known as an RNA-induced silencing complex (RISC), finds a mRNAs with a sequence perfectly complementary to the siRNA. RISC then cleaves the mRNA in the middle of the mRNA-siRNA duplex, and the resulting mRNA halves are degraded by other cellular enzymes
MicroRNAs are processed from specific genome-encoded precursors, in two steps, catalysed by the enzymes Drosha (in the nucleus) and Dicer (in the cytoplasm).
Piwi associated RNAs are generated from long, single-stranded precursors in a process independent of Drosha and Dicer. There are tens of thousands of these and are important in the development of the germ cell.
Grobhans and Filipowicz 2008 The expanding world of small RNAs vol 451, 414-416

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