DNA and RNA are key carriers of biological information. For example, DNA may store a gene coding for haemoglobin or insulin, which is then processed by RNA and ribosomes (which are sophisticated machines themselves made of RNA and proteins) to manufacture those proteins. Both DNA and RNA are nucleic acids (that’s the “NA” part of their acronym). They also contain a sugar group of a 5-carbon ring called a pentose. In DNA this is deoxyribose, while in RNA it’s ribose. This completes their respective names: deoxyribonucleic acid and ribonucleic acid. The monomers of these nucleic acid compounds are nucleotides. Aside from the nitrogen-containing base and the pentose, they contain a phosphate group.
As you can see, the nitrogenous base in the DNA nucleotide is one of four options: adenine, guanine, cytosine or thymine . RNA has thymine switched for uracil, making its base options adenine, guanine, cytosine or uracil . To form the DNA or RNA polymer, these nucleotide monomers join together via a condensation reaction which produces a phosphodiester bond .
Starting to look a bit familiar? This is just a small section of what would be a much longer DNA polynucleotide chain. Put two of them together in a double helical fashion, kept together by hydrogen bonds between the two strands, and voilà ! we have ourselves a DNA molecule.
As you can see, the base pairings are A-T and G-C. They follow this pairing rule and thus are known as complementary bases. Therefore, if the number of adenine bases were known, the number of thymine bases would be easy to deduce (equal to the number of adenine bases) as well as that of guanine/cytosine bases (total number of bases – adenine – thymine = guanine + cytosine; divided in half equals either the number of guanine bases or that of cytosine bases as they are equal).
RNA on the other hand does not follow the same complex overall polynucleotide structure as that of the DNA double helix, and is instead a relatively short, single strand of nucleotides – perhaps like the one depicted above with the phosphodiester bond labelled (a trimer)! Except of course, it would contain a uracil (U) instead of a thymine (T).
Because DNA is much more straightforward in structure compared to many proteins (for example it only contains the 4 variable bases while proteins are made of around 5 times more different kinds of their variable building blocks – amino acids), scientists dismissed it as a potential candidate for information storage in living things. Two lessons can be derived from this:
1. Don’t dismiss things in the absence of sufficient evidence
2. Maths. Maths is good. Had they realised that “just” 4 variables in a DNA sequence could yield as many as 4 to the power of 3,000,000,000 different sequences (which is an answer unobtainable if you try to calculate it and many, many times more than all the particles in the actual universe), they might’ve taken a second look at the humble molecule that is, after all, the code of life.