Practice question

PIWI-interacting RNAs piRNAs are 24-31 nucleotide germline-enriched small RNAs essential for genome integrity protecting against transposable element mobilization. piRNAs associate with PIWI clade Argonaute proteins Piwi, Aubergine, Ago3 guiding recognition and silencing of transposon transcripts via post-transcriptional cleavage and transcriptional heterochromatin formation. Primary piRNA biogenesis originates from single-stranded piRNA cluster transcripts cleaved by mitochondrial endonuclease Zucchini generating antisense piRNAs that slice sense transposon mRNAs initiating ping-pong amplification loop producing secondary sense piRNAs amplifying response. Additionally nuclear Piwi recruits histone methyltransferase complex establishing H3K9me3 heterochromatin at transposon loci, transcriptionally silencing. Loss leads to DNA damage and sterility.