[CENTER][I]Nucleic acids are macromolecules that as DNA and RNA are central to the storage and transmission of genetic information. Nucleotides are the subunits of DNA and RNA .They participate in numerous biological processes, convey energy, are part of essential coenzymes, and regulate numerous metabolic functions.[/I]
[I]Nucleotides are composed of three integral parts: a phosphate group,a sugar,and a purine or pyrimidine base.[/I]
A. Phosphate groups
Phosphate groups occur in nucleic acids and nucleotides as monophosphates (one P atom),diphosphates (2),or triphosphates (3).
B. Sugar residues
The carbohydrate residues in nucleotides are usually derived from either ribose as -D-ribose (in ribonucleic acid,RNA)or -D-deoxyribose (in deoxyribonucleic acid,DNA)(ribonucleoside or deoxyribonucleoside).
C. Nucleotide bases of pyrimidine
Cytosine (C),thymine (T),and uracil (U)are the three pyrimidine nucleotide bases. They differ from each other in their side chains (—NH2 on C4 of cytosine,—CH3 on C5 in thymine,and O on
C4 in uracil). In addition, cyotsine has a double bond between N3 and C4.
D. Nucleotide bases of purine
Adenine (A)and guanine (G)are the two nucleotide bases of purine.They differ in their side chains and in having a double bond between N1 and C6 (present in adenine,absent in guanine).
E. Nucleosides and nucleotides
A nucleoside is a compound of a sugar residue (ribose or deoxyribose) and a nucleotide base.
The bond is between the C atom in position 1 of the sugar and an N atom of the base (N-glycosidic bond).A nucleotide is a compound of a five-C-atom sugar residue (ribose or deoxyribose) attached to a nucleotide base (pyrimidine or purine base)and a phosphate group.
The nucleosides of the various bases are grouped as ribonucleosides or deoxyribonucleosides,e.g.,adenosine or deoxyadenosine, guanosine or deoxyguanosine,uridine (occurs
only as a ribonucleoside) ,cytidine or deoxycytidine. Thymidine occurs only as a deoxynucleoside.
Nucleotides are the sub units of nucleic acids. The nucleotides of the individual bases are referred to as follows : adenylate (AMP,adenosine monophosphate),guanylate (guanosine monophosphate,GMP),uridylate (UMP),and cytidylate (CMP)for the ribonucleotides (5 _ monophosphates)and deoxyadenylate (dAMP),deoxyguanylate (dGMP),deoxythymidylate (dTMP),and deoxycytidylate (dCMP)for the deoxyribonucleotides.
F. Nucleic acid
A nucleic acid consists of a series of nucleotides. A phosphodiester bridge between the 3 _ Ca tom of one nucleotide and the 5 _ C atom of the next joins two nucleotides.The linear sequence is usually given in the 5 _ to 3 _ direction with the abbreviations of the respective nucleotide bases.For instance,ATCG would signify the sequence adenine (A),thymine (T),cytosine (C), and guanine (G)in the 5 _ to 3 _ direction.
Examples of human hereditary disorders in purine and pyrimidine metabolism are: Hyperuricemia and gout:A group of disorders resulting from genetically determined excessive synthesis of purine precursors (MIM 240000).
Lesch –Nyhan syndrome:A variable,usually severe infantile X-chromosomal disease with marked neurological manifestations resulting from hypoxanthine –guanine phosphoribosyl transferase deficiency (MIM 308000).
Adenosine deaminase deficiency:A heterogeneous group of disorders resulting in severe infantile immunodeficiency.Different auto somal recessive and X-chromosomal types exist (MIM 102700).