Chemical nature of DNA

The polymeric structure of DNA may be described in terms of monomeric units of increasing complexity. In the top shaded box of the following illustration, the three relatively simple components mentioned earlier are shown. Below that on the left , formulas for phosphoric acid and a nucleoside are drawn. Condensation polymerization of these leads to the DNA formulation outlined above. Finally, a 5'- monophosphate ester, called a nucleotide may be drawn as a single monomer unit, shown in the shaded box to the right. Since a monophosphate ester of this kind is a strong acid (pK_a of 1.0), it will be fully ionized at the usual physiological pH (ca.7.4). Names for these DNA components are given in the table to the right of the diagram. Isomeric 3'-monophospate nucleotides are also known, and both isomers are found in cells. They may be obtained by selective hydrolysis of DNA through the action of nuclease enzymes. Anhydride-like di- and tri-phosphate nucleotides have been identified as important energy carriers in biochemical reactions, the most common being ATP (adenosine 5'-triphosphate).

Names of DNA Base Derivatives Base Nucleoside 5'-Nucleotide Adenine 2'-Deoxyadenosine 2'-Deoxyadenosine-5'-monophosphate Cytosine 2'-Deoxycytidine 2'-Deoxycytidine-5'-monophosphate Guanine 2'-Deoxyguanosine 2'-Deoxyguanosine-5'-monophosphate Thymine 2'-Deoxythymidine 2'-Deoxythymidine-5'-monophosphate

A complete structural representation of a segment of the DNA polymer formed from 5'-nucleotides may be viewed by clicking on the above diagram. Several important characteristics of this formula should be noted.

• First, the remaining P-OH function is quite acidic and is completely ionized in biological systems.
• Second, the polymer chain is structurally directed. One end (5') is different from the other (3').
• Third, although this appears to be a relatively simple polymer, the possible permutations of the four nucleosides in the chain become very large as the chain lengthens.
• Fourth, the DNA polymer is much larger than originally believed. Molecular weights for the DNA from multicellular organisms are commonly 10⁹ or greater.