RNA interference (RNAi) is a biological process in which RNA molecules inhibit gene expression, typically by causing the degradation of specific messenger RNA(mRNA) molecules. Two types of small RNA molecules – microRNA (miRNA) and small interfering RNA (siRNA) – are central to RNA interference. These small RNAs can bind to other specific mRNA molecules and decrease their activity by preventing an mRNA from producing a protein or by cleavage of the mRNA.
The RNAi pathway is found in many eukaryotes, including animals, and is initiated by the enzyme Dicer, which cleaves long double-stranded RNA (dsRNA) molecules into short double stranded fragments of ~20 nucleotide siRNAs. Each siRNA is unwound into two single-stranded RNAs (ssRNAs), the passenger strand and the guide strand. The passenger strand is degraded and the guide strand is incorporated into the RNA-induced silencing complex (RISC). The most well-studied outcome is post-transcriptional gene silencing, which occurs when the guide strand pairs with a complementary sequence in a messenger RNA molecule and induces cleavage by Argonaute, the catalytic component of the RISC complex.
RNAi is a valuable research tool, both in cell culture and in living organisms, because synthetic dsRNA introduced into cells can selectively and robustly induce suppression of specific genes of interest. RNAi may be used for large-scale screens that systematically shut down each gene in the cell, which can help to identify the components necessary for a particular cellular process or an event such as cell division. The pathway is also used as a practical tool in biotechnology and medicine.
Aptamers are single-stranded DNA or RNA molecules that can bind biological targets from small molecules to proteins with high affinity and specificity. Aptamers are generated using an unbiased selection process. A random ssDNA library is incubated in cancer cells and normal cells. ssDNA which bind to cancer cell are isolated, amplified by PCR and denatured to ssDNA. These steps are repeated for several rounds to enrich the aptamer pool for cancer cells. Contrary to the genetic material, their specificity and characteristics are not directly determined by their primary sequence, but instead by their tertiary structure. With their tendency to form helices and single-stranded loops, aptamers have potential use in sensor, diagnostic, and therapeutic tools.