Nucleic acids are polymers. This means they are strings of individual small molecule units joined together to make the larger nucleic acid molecules. In the case of nucleic acids, the individual units are called nucleotides. These have a consistent structure containing a pentose sugar, a phosphate, and a nucleobase which provides the variability between different sequences of DNA.
The part of the nucleotide that provides the variability that allows for different DNA or RNA sequences to exist is the nucleobase, which provides the "nucleic" part of the name nucleic acid. This is a nitrogen containing molecule, either a pyrimidine or a purine, that can integrate into the DNA or RNA molecule (Figure 1). These nucleobases are basic (with an alkaline pH) and can form hydrogen bonds with other bases on the basis of charge complementarity. The nucleobases adenine, cytosine and guanine are found in both DNA and RNA, whereas thymine is found only in DNA and uracil is found only in RNA.
The nucleobases bind to the nucleic acid backbone. The main part of this backbone is the pentose sugar, which is a saccharide molecule like glucose, only instead of having six carbon atoms it contains five (Figure 2). In RNA this pentose sugar is ribose, whereas in DNA the sugar is deoxyribose, which differs from ribose in that it has one fewer oxygen atoms, hence the name, deoxy-. The name of this pentose sugar lends the name ribo- or deoxyribo- in the full name of the nucleic acid.
The smallest part of the nucleotide unit is the phosphate group (Figure 3). Phosphate is a compound containing phosphorus (phosp-) and oxygen (-ate) together. Phosphate is also found in potassium phosphate (saltpetre, used to make gunpowder) and in phosphoric acid, an ingredient in most cola. As an acidic molecule, it gives rise to the "acid" part of the name of the nucleic acid.
The pentose sugar is the backbone of the nucleic acid, and it is to this that the nucleobase binds to the 1 carbon of the sugar, while the phosphate binds to the 5 carbon (Figure 4). These units can polymerize, as the phosphate group in the nucleotide is able to bind to another atom on a nearby pentose sugar molecule, namely the carbon 3 atom. This allows the units of phosphate and pentose sugar groups to repeat, with variance at the nucleobases, to give different DNA and RNA sequences. Furthermore, as the phosphate group is attached to the 5 carbon of one nucleotide and the 3 carbon of another, this gives rise to the description of the directionality of nucleic acid sequences, which are described as being read in a 5' (5 prime) to 3' (3 prime) direction, which is that the sequence is read from the end of the sequence with the free phosphate group attached at the carbon 5 to the end of the sequence with a free carbon 3 without a phosphate attached to it.
Nucleic acids exist in a variety of lengths, from short sequences of a few nucleotides that are used to initiate the polymerase chain reaction (PCR), to chromosomes, which are huge lengths of DNA which contain parts of an organism's genome.