Nucleotides: Structure and Properties
Authors: Bowater Richard P , Gates Andrew J 
Published: February 2015

Nucleotides consist of a nitrogen‐containing base, a five‐carbon sugar and one or more phosphate groups. Cells contain many types of nucleotides, which are in constant flux between free and polymeric states. Nucleotides play central roles in many cellular processes, including metabolic regulation and the storage and utilisation of genetic information. The levels of nucleotides in cells are regulated efficiently due to the small pools of free nucleotides, with an exception being adenosine‐5′‐triphosphate, which is at higher concentrations due to its role as the universal currency of energy. Within cells, ribonucleotides are synthesised from simple building blocks or they are obtained via the recycling of preformed bases. Different types of cells have striking variations in their nucleotide composition as these are linked to the rate of cell growth; in mammals, the amounts of nucleotides and nucleic acids as a proportion of cell weight vary from approximately 1% in muscle to 15–40% in thymus gland and sperm cells.

Key Concepts

Nucleotides are relatively complex molecules that consist of three fundamental components: a nitrogenous base, a sugar and one (or more) phosphate groups.

A wide range of nucleotides are required for specific cellular functions, such as information transfer or signalling processes within and between cells; minor forms of nucleotides are particularly prevalent within stable RNAs (rRNA and tRNA) or as unwanted by‐products of reactions occurring within the cell.

Nucleotides exist as monomers and in polymeric forms, called nucleic acids, and there are two closely related types: ribonucleotides are present in ribonucleic acid (RNA) and deoxyribonucleotides, which are synthesised by the reduction of ribonucleotides, are present in deoxyribonucleic acid (DNA).

Nucleotides are in constant flux between their free and polymeric states, and their levels are regulated efficiently because cells have small pools of free nucleotides; an exception to this is adenosine‐5′‐triphosphate (ATP), which is more abundant due to its role as the universal currency of energy in biological systems, which arises from its high potential for phosphate transfer to other molecules.

The covalent structure of nucleotides are relatively stable, but they are involved in a variety of chemical reactions within the cell and can be broken down by acid‐catalysed hydrolysis, with purine nucleotides generally being more easily hydrolysed than pyrimidine nucleotides.

Within cells, ribonucleotides are synthesisedde novofrom simple building blocks or they are obtained via salvage pathways from the recycling of preformed bases, which is simpler and requires less energy than thede novoreactions.

Different types of cells have striking variations in their nucleotide composition because their amounts are linked to the rate of cell growth; nucleotides and nucleic acids are estimated to comprise 5–10% of the total weight of rapidly dividing bacteria, and in mammals, the amounts vary from approximately 1% in muscle to 15–40% in thymus gland and sperm cells.

The manner in which nucleotides interact with other molecules is determined predominantly by the manner by which charge is distributed across them, although a variety of atoms are available to interact with other molecules, particularly via hydrogen bond formation.

The major pyrimidine bases found in nucleotides are cytosine, thymine, uracil and orotate and, within cells, the majority of pyrimidine nucleotides exist in their polymeric form in DNA and RNA molecules; cytosine is found in both DNA and RNA, but uracil is only present within RNA and thymine is, in general, only found in DNA.

Purine nucleotides common to cellular metabolism contain the bases adenine, guanine, hypoxanthine and xanthine, and purine nucleotides that are synthesised for specific aspects of cellular metabolism include 3′,5′‐cyclic AMP, a range of nicotinamide–adenine dinucleotides (NAD+, NADH, NADP+, NADPH), flavin–adenine dinucleotide and coenzyme A (CoA).