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Nucleotide
A nucleotide is the basic building block of nucleic acid. Examples of a nucleic acid include DNA and RNA. Nucleic acids are one of the types of biomolecules (other biomolecules are carbohydrates, proteins, and amino acids). The major functions of nucleic acids are preservation, replication, and expression of hereditary information. They are also involved in the formation of nucleoside triphosphates. Nucleotides act as a secondary messenger in cell signaling in various cellular processes.

Characteristics of Nucleotide

A single nucleotide is made up of a nitrogenous base, five-carbon sugar, and a phosphate group. The RNA molecule is composed of ribose sugar whereas DNA consists of deoxyribose sugar. The sugar molecules of each nucleotide monomer are connected by a phosphate group, thus the phosphate group and sugar moieties are considered as a backbone of nucleic acid. The opposite orientation of two strands of DNA allows complementary base pairing between nucleobase constituents. Nucleotides can occur in long chains as well as in cyclic forms. When the phosphate group is linked twice to the sugar moiety, it forms cyclic nucleotides. Nucleotides also act as energy carriers. The cell requires chemical energy for various cellular activities that are carried by the nucleotides.

Nucleosides vs Nucleotides

Sometimes both the terms nucleosides and nucleotides are confused with each other, but there is a difference between them. The nucleoside lacks the phosphate group. The nucleotide is formed when the nucleoside is bound to a phosphate group. A nucleotide is also named nucleoside monophosphate (having a single phosphate group), nucleoside diphosphate (having two phosphate groups), or nucleoside triphosphate (consisting of three phosphate groups). A nucleoside is considered a ribonucleoside or deoxyribonucleoside depending upon the sugar component. The nucleoside having ribose sugar is named ribonucleoside whereas the nucleoside having deoxyribose is considered as deoxyribonucleoside. A ribonucleoside may be guanosine, cytidine, uridine, adenosine, or 5-methyluridine based upon the nucleoside component. Similarly, the deoyriboseribonucleooside may consist of deoxyadenosine, deoxyguanosine, deoxycytidine, deoxythymidine, or deoxyuridine. Based upon the nucleobase component, the nucleoside is of two types- purine and pyrimidine.

Classification of Nucleotides

Based on the structure of the nitrogenous base, the nucleotides are classified as purines and pyrimidines. Adenine and guanine bases are considered purines whereas pyrimidine bases include thymine and cytosine and uracil. The thymine base is replaced by uracil in RNA. The nucleic acids are distinguished as DNA and RNA based upon the nucleobases. In DNA, thymine pairs with adenine whereas in RNA instead of thymine, uracil pair up with adenine. The nucleobases pairs such as G-C and A-T or A-U are referred to as base complements.

Biological Functions of Nucleotides

The nucleotides are the precursors or monomeric units of nucleic acids. They are involved in cell signaling and metabolism as cofactors. Some common examples of cofactors are FAD, ATP, and NADP. The nucleoside triphosphates provide chemical energy for amino acid synthesis, protein synthesis, cell division, intercellular movements, etc.