Practice question

Small interfering RNAs often guide heterochromatin formation in cis acting at locus producing homologous transcripts rather than diffusing. In fission yeast centromeric repeats transcribed into siRNA precursors processed into Ago1-loaded RITS remaining tethered to nascent transcript via base pairing, recruiting CLRC containing Clr4 methyltransferase to same domain. This cis-restricted recruitment ensures modification targeted exclusively to repeats producing dsRNA, not unrelated loci with limited similarity. In contrast cytoplasmic degradation by siRNAs acts in trans on distant transcripts. Dual modes explain epigenetic inheritance: transcriptional silencing cis, post-transcriptional trans turnover coordinated.