RNA, or ribonucleic acid, is a molecule that is integral to all forms of life. Organisms with DNA genomes make copies of their genes in an RNA format. The organism reads these accurate copies, which make "sense," and forms the correct proteins. Antisense RNA is a sequence that is the opposite of the "sense" RNA, and by sticking to the "sense" RNA, it can block the correct formation of proteins. While not widely occurring in nature, antisense RNA has applications in areas of science such as medicine and genetically modified organisms.
The regular process of protein production begins with the DNA of a particular gene being copied into messenger RNA (mRNA). All mRNA is single stranded. Ribosomes and transfer RNAs (tRNA) then read the mRNA and build the protein the gene codes for.
The sequence of the mRNA is essential for the production of the right protein. In addition, tRNA and ribosomes only read single strands, not double strands. Antisense RNA is itself a single strand but has a sequence of bases that is complementary to the sequence of bases in a specific mRNA.
Uracil (U), adenine (A), cytosine (C), and guanine (G) make up RNA's different bases. Uracil binds to adenine, and cytosine binds to guanine. For example, a portion of an mRNA that codes CAU has a complementary antisense sequence of GUA. The antisense sequence binds to the mRNA to form a double-stranded complex.
Genetic engineers have found this concept useful in creating modified organisms. One such example is that of the tomato known as a Flavr-Savr. Tomatoes produce an enzyme called polygalacturonase (PG) that softens the fruit during ripening. PG is coded for by the tomato genome. Farmers of regular tomatoes must pick them before they are fully ripe so that the PG does not turn the fruit soft before it gets to the supermarket shelf.
Flavr-Savr tomatoes have an extra gene placed there by the genetic engineers, which produces an antisense version of the PG mRNA. This antisense strand sticks to the majority of the PG mRNA the tomato produces and thereby blocks the production of the PG enzyme. This keeps the tomatoes from going soft during ripening so that farmers can grow tomatoes that taste and look ripe but are not soft.
Antisense RNA may also have applications in medicine. Some diseases, such as Huntington's disease, are caused by the genes producing defective or undesirable proteins. People cannot be bred to have an altered genome like tomatoes, but scientists can somehow deliver antisense RNA, or a gene to code for antisense RNA, into the cells that produce an unwanted protein.
Using a virus as a carrier of the antisense gene or injecting the RNA directly into the area are possible delivery methods. One problem with the science, however, is that optimizing the delivery methods is complex. Another disadvantage is that the RNA may not be specific enough to only target the unwanted mRNA, a situation that could be dangerous for the patient. Examples of antisense RNA in nature are uncommon. One such occurrence happens in humans and in mice, where the gene for insulin-like growth factor two receptor, inherited from the mother's side, is blocked by antisense RNA produced from the father's version of the gene.