Ramifications of Nucleic Acid Complementarity

    In the realm of nucleic acids, those macromolecules called DNA and RNA are participants in the transfer of genetic information into RNA structure, and frequently then onwards into protein structure. This information transfer is commonly referred to as The Central Dogma, where the black and solid gray arrows indicate the information transfer, and the dotted arrows are various proteinaceous enzymes which facilitate those transfers:

    If we first look closely at the structure of double-stranded DNA (dsDNA), we see that each strand's bases are paired up with bases on the other strand in such a way that every A pairs with a T, and every C with a G. This is the essence of complementarity (being complementary to each other, ying and yang, male and female, not identical but complementary!). For use in the next steps, is a "gene" that is boxed in.

    On the molecular level, when the DNA information, or code, is read by an enzyme called transcriptase, a strand of RNA is made. More specifically, one strand of the dsDNA is read, and the RNA that is made is complementary to a section of that strand of DNA. Here, in red, is shown such a strand of RNA:

    You will notice that RNA has U's in it. They are substitutes for the T's in DNA. (Actually, T is a methylated U. In linguistics - for genetic code is a form of language, RNA and DNA are the two dialects of the nucleic acid language.)

    In the laboratory, we are able to go backwards to demonstrate that the above occurs. First we obtain some dsDNA and heat it in water to over 90°c, which causes the strands' H-bonds to break, and the strands come apart from each other. This is called "melting" the DNA.

    If we add back the RNA to the melted mix while all is hot, and then slowly cool the mix, the RNA will diffuse around until it finds a stretch of DNA with which it is complementary. Here we see that with one strand of the DNA, it cannot find such a stetch of complementarity:

    But with the other strand of DNA, such a stretch is found and the complementary bases form H-bonds and the RNA clings to the appropriate stretch of DNA forming a hybrid molecule - part DNA and part RNA - a hybrid!

    Micro-RNA's (μRNA) have been shown to have regulatory powers over genes. μgenes produce such μRNA's and these can hybridize with complementary stretches of DNA, and cause a lump in the gene making it impossible for transcriptase to easily read that stretch of DNA. The gene thus is poorly expressed, if at all.

    Evidence is fast accumulating that a few cancers - and perhaps many more will be found also - are caused by mutations in the μgenes resulting in μRNA's which do not hybridize where they normally should. Thus control is lost, and the cells run wild.