(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 “anti­sense $RNA$ ”. The anti­sense $RNA$ and $mRNA$ 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 anti­sense $RNA$ 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.