Nucleic Acid Structure

The genetic information of the cells is stored in nucleic acids, usually deoxyribonucleic acid (abbreviated as DNA). The information stored in the sequence is transformed into a functional protein molecule through the translation process, and the ribonucleic acid (abbreviated as RNA) molecule is used as a messenger to transmit information between the DNA and the translation machine. RNA molecules can also store information, such as in some viruses, in addition, RNA plays an important role in other processes. The core metabolic processes associated with nucleic acids are replication, transcription, and regulation, all of which rely on the recognition of nucleic acid sequences by proteins. Therefore, it is very important to understand the structural information of DNA and RNA to gain insight into the biological problems associated with the interaction of nucleic acids and proteins.

Figure 1. Diagram of DNA and RNA.

The Building Blocks of Nucleic Acid: Nucleotide

Both DNA and RNA are called nucleic acids. They are given this name because they consist of building blocks called nucleotides. The nucleotide has a ribose or 2'-deoxyribose as the backbone, a phosphate group is attached to the 5'-hydroxyl group of the pentose, and a base is bonded to a 1'-carbon atom. In a nucleic acid polymer, the phosphate group of one nucleotide is linked to the 3'-hydroxyl group on the pentose of the next nucleotide, therefore, the nucleic acid molecule has a 5' end and a 3' end.

• Sugars

Ribose is present in the RNA, while 2'-deoxyribose is present in the DNA. The carbon atoms of the pentose are numbered in the order of 1'-5'. The only difference between ribose and 2'-deoxyribose is that only the 2'- hydroxyl group does not appear in the latter. The 2'-hydroxyl group may contribute to the catalytic properties of the RNA molecules.

• Bases

There are two main types of purines in nucleic acids, namely adenine (A) and guanine (G), but many isomers exist. DNA has two types of pyrimidines, cytosine (C) and thymine (T). In RNA, thymine is usually replaced by a very similar base, uracil (U), which lacks a methyl group compared to the former. Mammalian DNA has been found to contain a methylated base, 5-methylcytosine (m5C). In addition, bacterial DNA contains two additional methylated bases, N6-methyladenine (m6A) and N4-methylcytosine (m4C).

• Phosphates

The phosphate group is made up of central phosphorus surrounded by four oxygens. The Phosphorus combines with three oxygens via three single bonds and binds to the fourth via a double bond. Because of the nature of the chemical bonds, there is a negative charge on each oxygen that has only one bond coming off of it. This negative charge accounts for the total negative charge on the phosphate backbone of the nucleic acid molecule.

Table 1. The comparison of DNA and RNA.

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