Nucleotide

Nucleotides are organic molecules that serve as the monomer units for forming the nucleic acid polymers deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth. Nucleotides are the building blocks of nucleic acids; they are composed of three sub unit molecules: a nitrogenous base (also known as nucleobase), a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. Nucleotides are organic molecules that serve as the monomer units for forming the nucleic acid polymers deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth. Nucleotides are the building blocks of nucleic acids; they are composed of three sub unit molecules: a nitrogenous base (also known as nucleobase), a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. A nucleoside is a nitrogenous base and a 5-carbon sugar. Thus a nucleoside plus a phosphate group yields a nucleotide. Nucleotides also play a central role in metabolism at a fundamental, cellular level. They carry packets of chemical energy—in the form of the nucleoside triphosphates Adenosine triphosphate (ATP), Guanosine triphosphate (GTP), Cytidine triphosphate (CTP) and Uridine triphosphate (UTP)—throughout the cell to the many cellular functions that demand energy, which include: synthesizing amino acids, proteins and cell membranes and parts, moving the cell and moving cell parts (both internally and intercellularly), dividing the cell, etc. In addition, nucleotides participate in cell signaling (cyclic guanosine monophosphate or cGMP and cyclic adenosine monophosphate or cAMP), and are incorporated into important cofactors of enzymatic reactions (e.g. coenzyme A, FAD, FMN, NAD, and NADP+). In experimental biochemistry, nucleotides can be radiolabeled with radionuclides to yield radionucleotides. A nucleotide is composed of three distinctive chemical sub-units: a five-carbon sugar molecule, a nitrogenous base—which two together are called a nucleoside—and one phosphate group. With all three joined, a nucleotide is also termed a 'nucleoside monophosphate'. The chemistry sources ACS Style Guide and IUPAC Gold Book prescribe that a nucleotide should contain only one phosphate group, but common usage in molecular biology textbooks often extends the definition to include molecules with two, or with three, phosphates. Thus, the terms 'nucleoside diphosphate' or 'nucleoside triphosphate' may also indicate nucleotides. Nucleotides contain either a purine or a pyrimidine base—i.e., the nitrogenous base molecule, also known as a nucleobase—and are termed ribonucleotides if the sugar is ribose, or deoxyribonucleotides if the sugar is deoxyribose. Individual phosphate molecules repetitively connect the sugar-ring molecules in two adjacent nucleotide monomers, thereby connecting the nucleotide monomers of a nucleic acid end-to-end into a long chain. These chain-joins of sugar and phosphate molecules create a 'backbone' strand for a single- or double helix. In any one strand, the chemical orientation (directionality) of the chain-joins runs from the 5'-end to the 3'-end (read: 5 prime-end to 3 prime-end)—referring to the five carbon sites on sugar molecules in adjacent nucleotides. In a double helix, the two strands are oriented in opposite directions, which permits base pairing and complementarity between the base-pairs, all which is essential for replicating or transcribing the encoded information found in DNA. Unlike in nucleic acid nucleotides, singular cyclic nucleotides are formed when the phosphate group is bound twice to the same sugar molecule, i.e., at the corners of the sugar hydroxyl groups. These individual nucleotides function in cell metabolism rather than the nucleic acid structures of long-chain molecules. Nucleic acids then are polymeric macromolecules assembled from nucleotides, the monomer-units of nucleic acids. The purine bases adenine and guanine and pyrimidine base cytosine occur in both DNA and RNA, while the pyrimidine bases thymine (in DNA) and uracil (in RNA) occur in just one. Adenine forms a base pair with thymine with two hydrogen bonds, while guanine pairs with cytosine with three hydrogen bonds. Nucleotides can be synthesized by a variety of means both in vitro and in vivo.