INTERFERING WITH GENE EXPRESSION
Triple helix-forming oligonucleotides, antisense, small interfering RNA (siRNA), microRNA (miRNA), tran-scription factor decoys, ribozymes, DNAzymes, and external guide sequences (EGS) are all members of the class of ONs that can knock down gene expression, albeit that they function at different stages of the gene expression process.
In eukaryotes, transcription takes place in the nucleus, and translation is located in the cytoplasm. To initiate transcription of a gene, promoters and transcription factors are required, whose action is further complemented by the action of enhancers, binding of specific proteins to regulatory DNA sequences, and methylation of CpG islands in the promoter region. RNA polymerase produces pre-mRNA molecules that are capped at their 50-ends and receive a poly(A) tail at their 30-ends. Nearly all mRNA precursors are spliced. Introns are excised and exons are joined to form the final mRNA sequence. Different splicing can form alternative sequences. Subsequently, mRNA binds to the ribosomes. Ribosomes are composed of rRNA and proteins and “read” the mRNA sequence. With the help of tRNA carrying the appropriate amino acids, the protein is formed. The process of transcription and translation is shown in Figure 2.