Nucleic acids are an important class of macromolecules found in all cells and viruses. Nucleic acids are responsible for the storage and expression of genetic information. The three main processes used by all cells to maintain and propagate their genetic information are replication, transcription, and translation.
3.1. Nucleic Acid Structure
There are two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Although their name suggests that their location is in the cell nucleus, some nucleic acids are also present in the cytoplasm and mitochondria. Nucleic acids are present in most living cells and represent hereditary determinants of all living organisms.
Nucleic acids are polymers of deoxyribonucleotides or ribonucleotides. All
nucleotides have a common structure: a phosphate group linked by a phosphodiester bond to a pentose that, in turn, is linked to a nitrogenous base (Fig. 3.1). The nitrogenous bases fall into two types. Purines have a double-ringed structures (Fig. 3.2). Pyrimidines have a single-ringed structures (Fig. 3.2).
The same two purines are present in both DNA and RNA. The pyrimidines in DNA are cytosine and thymine, whereas, in RNA, uracil is present instead of thymine. The only difference between thymine and uracil is the absence of a methyl group at position C5 (Fig. 3.2).
Two types of pentose sugars are found in nucleic acids (Fig. 3.3). In DNA, the pentose is deoxyribose. In RNA, the pentose is ribose. Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2d carbon atom (Fig. 3.3).