Nucleic acids constitute the genomes of living organisms, carry the information in the form of messengers, act as adapters, catalyze the biological reactions, and play regulatory and defense roles as well. Nucleic acids are the unbranched polymers of nucleotides; in a nucleic acid, each nucleotide, except the terminal ones, is linked to two nucleotides through phosphodiester bonds. Nucleic acids can be classified into two categories: ribonucleic acid (RNA; contains ribose sugar) and deoxyribonucleic acid (DNA; contains 2′-deoxyribose sugar). DNA can have A, G, C, and T as its bases while RNA can have A, G, C, and U. In nucleic acids, A can form hydrogen bonds with T and U while C can form hydrogen bonds with G. A-T, A-U, and G-C are said to constitute the complementary base pairs and play crucial roles in processes like replication, transcription, and translation. DNA is usually composed of two strands wrapped around each other in a double-helical fashion (dsDNA) (Figure 1.4C). Single stranded DNA (ssDNA), however, does exist in certain viruses. Similarly, RNA molecules are typically single stranded but certain viruses do have double stranded RNA (dsRNA). Furthermore, RNA molecules can also adopt local double-helical structures to adopt a 3-dimensional structure, e.g. tRNA (Figure 1.4D). In a double helical nucleic acid structure, the bases on one strand form hydrogen bonds with the complementary bases on the other. The bases lie roughly perpendicular to the nucleic acid backbone and the stacking interactions between them further stabilize the double helical structure.

Carbohydrates

Carbohydrates are the polyhydroxy aldehydes or ketones. Aldehyde sugars are often referred to as aldoses while ketone sugars are called ketoses. Monosaccharides such as glucose and fructose are the simplest carbohydrates (Figure 1.5). The monosaccharides can join covalently to give disaccharides, oligosaccharides, and polysaccharides. Unlike proteins and nucleic acid, polysachharides can be branched. Based on their functions, polysaccharides can be classified as structural polysaccharides (e.g. cellulose and chitin) and storage polysachharides (e.g. starch and glycogen).