PIWI-interacting RNAs (piRNAs) a very recent class of non-coding RNA was added as a key player to RNA interference in 2006. These small RNAs are 24-32 nucleotides in length. They associate with PIWI subfamily of members which belongs to Argonaute family of proteins. piRNAs have a bias for 5’ Uracil. Most piRNAs are derived from the repetitive regions of the genome. They have an affinity towards silencing the transposable regions. Till date the mechanism of biogenesis of piRNAs remains to be a mystery. However it has been suggested in literature that a ‘PING-PONG ’ cycle exists that accelerates the biogenesis of piRNAs.
Figure1. PING-PONG cycle for the biogenesis of piRNAs [adapted from Nature Cell Biology 10, 630 - 633 (2008)]
piRNAs are derived from clusters which are the ‘hotspots’ for biogenesis. Clusters can be uni-strand (monodirectional) or dual-strand (bidirectional) in nature. If piRNAs within a cluster are synthesized from a single strand, then it is said to be monodirectional. However if the piRNAs within a cluster are derived from both the strands then it is said to be bidirectional. Clusters are conserved in syntenic regions.
Figure2. Transcription of piRNA clustrs from the Genome
[adapted from Developmental Cell Volume 19, Issue 5, 16 November 2010, Pages 687–697 ]


This class of small-RNAs safeguards the genome from the ‘parasitic nucleic acids’ i.e. transposons. piRNAs are responsible for guiding vital processes of development, stem-cell self renewal, spermatogenesis and cancer epigenetics. piRNAs are proving to be a key player for the regulation of genome. 



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