Nucleosides are the precursors of nucleotides; DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) consist of strings, or polymers, of nucleotides. A nucleoside has two parts, a heterocyclic amine, called a nitrogenous base, and a sugar molecule; the sugar molecule is either ribose or deoxyribose. When a phosphate group links up with a nucleoside, the nucleoside becomes a nucleotide.
Nucleosides for RNA
DNA encodes our heritable information. Messenger RNA (mRNA) molecules copy the information so that amino acids in protein can be specified. Other types of RNA include ribosomal (rRNA) and transfer (tRNA) RNA. These RNAs are involved in protein synthesis and other cellular processes and have more varied structures than mRNAs. The precursors of RNA are nucleosides that contain one of four bases: adenine, guanine, cytosine or uracil. Adenine and guanine belong to a class of compounds called purines, while cytosine and uracil are pyrimidines. Purines are distinguished by a double-ring structure, whereas pyrimidines have a single-ring molecular structure. In each nucleoside molecule, a base links to a ribose sugar molecule.
Nucleosides for DNA
The precursors of DNA nucleotides are nucleosides that also contain four bases, but one base differs between DNA and RNA. The bases in the DNA nucleoside precursors are adenine, guanine, cytosine and thymine; thymine substitutes for the base uracil found in RNA. Deoxyribose sugar molecules link to the bases in DNA nucleoside precursors. The deoxyribose sugar molecule differs from the ribose sugar molecule in RNA by having one fewer oxygen atom.
When a nucleoside acquires a phosphate group it becomes a nucleotide; one, two or three phosphate groups can attach to a nucleoside. Examples are adenine ribonucleoside monophosphate (AMP), adenine ribonucleoside diphosphate (ADP) and adenine ribonucleoside triphosphate (ATP). Nucleotides (phosphate-linked nucleosides) are not only the building blocks of RNA and DNA, they also serve as energy sources and transmitters of information in cells. For example, ATP serves as an energy source in many biochemical interactions in the cell, GTP (guanosine triphosphate) supplies energy for protein synthesis and cyclic AMP (cyclic adenosine monophosphate), a cyclic nucleotide, transduces signals in hormone responses and in the nervous system.
Nucleotides for RNA and DNA
Nucleotides with three phosphate groups, with the phosphate groups linked in tandem, serve as precursors of RNA and DNA. When the RNA or DNA strand forms, two of the three phosphate groups dissociate from the nucleotides, leaving the nucleotide within RNA and DNA with only one phosphate attached. Nucleotides are the basic building blocks of DNA and RNA, and the four specific nucleotides are repeated in variable order throughout the DNA or mRNA strand.