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Figure 1. Induction of post-transcriptional gene silencing (PTGS) and transcriptional gene silencing (TGS) through hairpin structures (hp). The RNAi constructs by hairpin structures are made by cloning the target gene or part of it in the sense and antisense orientation separated by an intron. Upon transcription the hpRNA folds back to form double-stranded RNA (dsRNA). The dsRNA is recognized by one of the homologues of the Dicer enzyme, cleaving it into short interfering RNA (siRNA). These small RNAs serve as guide to recognize mRNA based on sequence homology and upon recognition are cleaved into siRNA. The siRNAs homologous to promoter sequences act as a guide for the RNA-induced transcriptional gene silencing complex (RITS), resulting in the methylation of the promoter sequences. Methylation of promoter sequences blocks gene transcription and results in TGS.