Nucleic Acids

Nucleic acids are the second great class of macromolecule found in the acid-insoluble fraction of a living tissue, sitting alongside polysaccharides and polypeptides. Together these three make up what is genuinely the macromolecular portion of any cell or tissue. Nucleic acids are polymers, and more precisely they are polynucleotides — long chains built by joining many repeating units called nucleotides.

The nucleotide — the building block

A single nucleotide is not one simple molecule but a combination of three chemically distinct parts:

• A nitrogenous base — a heterocyclic compound (a ring that contains atoms other than carbon, such as nitrogen).
• A sugar — a monosaccharide, specifically a pentose (five-carbon sugar).
• A phosphate — derived from phosphoric acid.

When these three pieces are combined and then linked in large numbers, the result is a polynucleotide.

The nitrogenous bases

Five nitrogenous bases occur in nucleic acids: adenine, guanine, uracil, cytosine, and thymine. They fall into two structural families based on the skeleton of their ring:

• Purines — these have the purine ring skeleton. Adenine and guanine are substituted purines.
• Pyrimidines — these have the pyrimidine ring skeleton. Cytosine, uracil, and thymine are substituted pyrimidines.

So the word "purine" or "pyrimidine" names the basic heterocyclic ring, and each actual base is a substituted version of that ring.

The sugar and the two kinds of nucleic acid

The pentose sugar in a polynucleotide is one of two closely related sugars:

• Ribose, a pentose monosaccharide.
• 2' deoxyribose, which is ribose lacking one oxygen at the 2' position. …