`klēətīd', ny`–), organic substance that serves as a monomer in forming nucleic acids nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis...... Click the link for more information. . Nucleotides consist of either a purine purine, type of organic base found in the nucleotides and nucleic acids of plant and animal tissue. The German chemist Emil Fischer did much of the basic work on purines and introduced the term into the chemical literature in the early 20th cent.
..... Click the link for more information. or a pyrimidine pyrimidine (pīrĭm`ĭdēn')
..... Click the link for more information. base, a ribose ribose (rī`bōs), monosaccharide carbohydrate of universal distribution in living tissue, found in ribonucleic acid (RNA; see nucleic
..... Click the link for more information. or deoxyribose, and a phosphate group. Adenosine triphosphate adenosine triphosphate (ATP) (ədĕn`əsēn trī'fŏs`fāt)
..... Click the link for more information. serves as the principle energy carrier for the cell's reactions. The most important nucleotides are those derived from the bases adenine adenine (ăd`ənĭn, –nīn, –nēn), organic base of the purine family.
..... Click the link for more information. , guanine guanine (gwä`nēn), organic base of the purine family.
..... Click the link for more information. , cytosine cytosine (sī`tōsēn'), organic base of the pyrimidine family.
..... Click the link for more information. , thymine thymine (thī`mēn), organic base of the pyrimidine family.
..... Click the link for more information. , and uracil uracil (y
r`əsĭl), organic base of the pyrimidine family...... Click the link for more information. .
nucleotide
Any of a class of organic compounds, including the structural units of nucleic acids. Each consists of a nucleoside and one or more phosphate groups. In nucleic acids, the phosphate of one nucleotide joins to the sugar of the next to form the backbone. Important nucleotides that are not part of nucleic acids include ATP, cyclic AMP (needed in glycogen breakdown), and certain coenzymes.
Nucleotide
A cellular constituent that is one of the building blocks of ribonucleic acids (RNA) and deoxyribonucleic acid (DNA). In biological systems, nucleotides are linked by enzymes in order to make long, chainlike polynucleotides of defined sequence. The order or sequence of the nucleotide units along a polynucleotide chain plays an important role in the storage and transfer of genetic information. Many nucleotides also perform other important functions in biological systems. Some, such as adenosine triphosphate (ATP), serve as energy sources that are used to fuel important biological reactions. Others, such as nicotinamide adenine dinucleotide (NAD) and coenzyme A (CoA), are important cofactors that are needed to complete a variety of enzymatic reactions. Cyclic nucleotides such as cyclic adenosine monophosphate (cAMP) are often used to regulate complex metabolic systems. Chemically modified nucleotides such as fluoro-deoxyridine monophosphate (Fl-dUMP) contain special chemical groups that are useful for inactivating the normal function of important enzymes. These and other such compounds are widely used as drugs and therapeutic agents to treat cancer and a variety of other serious illnesses. See Coenzyme, Cyclic nucleotides, Nicotinamide adenine dinucleotide (NAD)
Nucleotides are generally classified as either ribonucleotides or deoxyribonucleotides. Both classes consist of a phosphorylated pentose sugar that is linked via an N-glycosidic bond to a purine or pyrimidine base. The combination of the pentose sugar and the purine or pyrimidine base without the phosphate moiety is called a nucleoside. See Purine, Pyrimidine
Ribonucleosides contain the sugar d -ribose, whereas deoxyribonucleosides contain the sugar 2-deoxyribose. The four most common ribonucleosides are adenosine, guanosine, cytidine, and uridine. The purine ribonucleosides, adenosine and guanosine, contain the nitrogenous bases adenine and guanine, respectively. The pyrimidine ribonucleosides, cytidine and uridine, contain the bases cytosine and uracil, respectively. Similarly, the most common deoxyribonucleosides include deoxyadenosine, deoxyguanosine, deoxycytidine, and thymidine, which contains the pyrimidine base thymine. Phosphorylation of the ribonucleosides or deoxyribonucleosides yields the corresponding ribonucleotide or deoxyribonucleotide. See Deoxyribonucleic acid (DNA), Enzyme, Nucleic acid, Ribonucleic acid (RNA)
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