What is piRNAs and transposon silencing?

piRNAs are $26\text{ to }31$-nucleotide RNAs, made without Dicer from long single-stranded transcripts of genomic piRNA clusters — graveyards of defective transposon copies — and loaded into the PIWI subfamily of Argonaute proteins in the germ line. A piRNA antisense to a transposon guides cleavage of the transposon’s transcripts; the cleaved fragment becomes a new sense piRNA, which in turn cuts cluster transcripts to make more antisense piRNAs — the ping-pong cycle, an amplification that targets whatever transposon is currently active. Nuclear PIWI proteins also direct H3K9 methylation and DNA methylation onto the transposon’s genomic copies, connecting the RNA to the chromatin marks of Chapter 1. A mother deposits her piRNAs in the egg; a female whose lineage has never met a transposon has no piRNAs against it, and her offspring by a male carrying it are sterile — hybrid dysgenesis, first seen in Drosophila crosses between laboratory and wild strains.