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The fourth type of organic compound important to human structure and function are the nucleic acids, the subunits of which are nucleotides. The two classes of nucleic acid molecules are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
A nucleotide is composed of three subunits:
one or more phosphate groups
a pentose sugar: either deoxyribose or ribose
a nitrogen-containing base: adenine, cytosine, guanine, thymine, or uracil
Nucleotides can be assembled into nucleic acids (DNA or RNA) or the energy compound adenosine triphosphate.
Nucleic Acids
The nucleic acids differ in their type of pentose sugar. Deoxyribonucleic acid (DNA) is nucleic acid that stores genetic information. DNA contains deoxyribose (so-called because it has one less atom of oxygen than ribose) plus one phosphate group and one nitrogen-containing base. The “choices” of base for DNA are adenine, cytosine, guanine, and thymine. Ribonucleic acid (RNA) is a ribose-containing nucleic acid that helps manifest the genetic code as protein. RNA contains ribose, one phosphate group, and one nitrogen-containing base, but the “choices” of base for RNA are adenine, cytosine, guanine, and uracil.
The nitrogen-containing bases adenine and guanine are classified as purines. A purine is a nitrogen-containing molecule with a double ring structure, which accommodates several nitrogen atoms. The bases cytosine, thymine (found in DNA only) and uracil (found in RNA only) are pyramidines. A pyramidine is a nitrogen-containing base with a single ring structure
Bonds formed by dehydration synthesis between the pentose sugar of one nucleic acid monomer and the phosphate group of another form a “backbone,” from which the components’ nitrogen-containing bases protrude. In DNA, two such backbones attach at their protruding bases via hydrogen bonds. These twist to form a shape known as a double helix. The sequence of nitrogen-containing bases within a strand of DNA form the genes that act as a molecular code instructing cells in the assembly of amino acids into proteins. Humans have almost 22,000 genes in their DNA, locked up in the 46 chromosomes inside the nucleus of each cell (except red blood cells which lose their nuclei during development). These genes carry the genetic code to build one’s body, and are unique for each individual except identical twins.
In contrast, RNA consists of a single strand of sugar-phosphate backbone studded with bases. Messenger RNA (mRNA) is created during protein synthesis to carry the genetic instructions from the DNA to the cell’s protein manufacturing plants in the cytoplasm, the ribosomes.
Anatomy and Physiology by Rice University is licensed under a Creative Commons Attribution 4.0 International License
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