Nucleic acids are the polymers of nucleotides (polynucleotides) held by 3′ and 5′ phosphate bridges. In other words, nucleic acids are built up by the monomeric units—nucleotides (It may be recalled that protein is a polymer of amino acids).
The base pairing rules for DNA are governed by the complementary base pairs: adenine (A) with thymine (T) in an A-T pairing and cytosine (C) with guanine (G) in a C-G pairing. Conversely, thymine only binds with adenine in a T-A pairing and guanine only binds with cytosine in a G-C pairing.
DNA and RNA have great chemical similarities. In their primary structures both are linear polymers (multiple chemical units) composed of monomers (single chemical units), called nucleotides. Cellular RNAs range in length from less than one hundred to many thousands of nucleotides. Cellular DNA molecules can be as long as several hundred million nucleotides. These large DNA units in association with proteins can be stained with dyes and visualized in the light microscope as chromosomes.