164 Heterocycles in Nature
DNA consists of two intertwining helices, each being a mixed polymer of nucleotides. Each of these polymeric intertwining
strands has a backbone on the outside, consisting of alternating sugars (deoxyribose) and phosphate (‘phosphodiester’)
bridges. From each sugar, pointing inwards, there is one of four heterocyclic bases: two purines (adenine (A) and guanine
(G)) and two pyrimidines (thymine (T) and cytosine (C)), linked from C-1' of the sugar to N-1 of the pyrimidine bases
or N-9 of the purine bases.
The close association of the two strands is based on very specific hydrogen bonding between an A residue of
one strand and a T residue in the precisely opposite section of the other strand, and between a C residue on one
strand and a G residue on the other. This pairing is absolutely specific: adenine cannot form multiple hydrogen
bonds with guanine or cytosine and cytosine cannot form multiple hydrogen bonds with thymine or adenine. It
is the sequence of the bases along the chain that carries the information – particular sets of three bases code for
a particular amino acid – the genetic information content comes down simply to heterocyclic chemistry! The
hydrogen bonding serves not only to hold the two strands together but also to transfer information since, when
the strands separate, these same types of hydrogen bond allow it to act as a template for both mRNA and self-
replication. Both the replication of DNA and transcription to mRNA are enzymatic processes using the nucleic
acid as a template.
The basic chemical differences between the two nucleic acids is the presence in RNA of the extra 2'-hydroxy in the sugar
and uracil (which occurs only in RNA) replacing the closely related thymine (which occurs only in DNA).
Although DNA and RNA are closely related and can bind with one another by the same type of hydrogen bonds, there
are signifi cant differences. DNA is a very large molecule, containing up to about a billion nucleotide units in animals.
The length of such a molecule, if extended, would be more than 30 cm; considerable folding is required to fi t into a cell!
On the other hand, RNA is much smaller, the smallest being tRNA with only around 80 nucleotides. Also, RNA exists
mainly as single strands, although substructures such as ‘hairpins’ are common, where the strand folds back along itself
with H-bonding between bases.
tRNA also contains hypoxanthine (inosine) residues, which are less selective in hydrogen bonding to specifi c comple-
mentary bases and thus can lead to some loss of selectivity in identifying codes for specifi c amino acids, a fl exibility
referred to as ‘wobble’.