(a) : Many nematodes live in plants and animals including human beings. A nematode Meloidogyne incognita infests the roots of tobacco plants and causes a great reduction in yield. A novel strategy was adopted to prevent this infection that was based on the process of RNA interference (RNAi). RNA interference (RNAi) is the phenomenon of inhibiting activity of a gene by synthesis of RNA molecules complementary to the mRNA. The normal (in vivo synthesized) mRNA of a gene is said to be "sense" because it carries the codons that are "read" during translation. Normally, the complement to the mRNA "sense" strand will not contain a sequence of codons that can be translated to produce a functional protein; thus, this complementary strand is called "anti-sense RNA". The anti-sense RNA and m RNA molecules will anneal to form duplex RNA molecules (or double stranded RNA) and the duplex RNA molecules can not be translated. Thus, the presence of anti-sense RNA will block translation of the mRNA of the affected gene. In fact, recent evidence indicates that these RNA duplexes are often rapidly degraded in vivo.