Nucleosides and
nucleotides
Nucleosides
are molecules formed by attaching a nucleobase to a ribose or deoxyribose ring.
Examples of these include cytidine (C), uridine (U), adenosine (A), guanosine
(G), thymidine (T) and inosine (I). Nucleosides can be phosphorylated by
specific kinases in the cell, producing nucleotides. Both DNA and RNA are
polymers, consisting of long, linear molecules assembled by polymerase enzymes
from repeating structural units, or monomers, of mononucleotides. DNA uses the
deoxynucleotides C, G, A, and T, while RNA uses the ribonucleotides (which have
an extra hydroxyl (OH) group on the pentose ring) C, G, A, and U. Modified
bases are fairly common (such as with methyl groups on the base ring), as found
in ribosomal RNA or transfer RNAs or for discriminating the new from old
strands of DNA after replication. Each nucleotide is made of an acyclic
nitrogenous base, a pentose and one to three phosphate groups. They contain
carbon, nitrogen, oxygen, hydrogen and phosphorus. They serve as sources of
chemical energy (adenosine triphosphate and guanosine triphosphate),
participate in cellular signaling (cyclic guanosine monophosphate and cyclic
adenosine monophosphate), and are incorporated into important cofactors
ofenzymatic reactions (coenzyme A, flavin adenine dinucleotide, flavin
mononucleotide, and nicotinamide adenine dinucleotide phosphate).
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