Nucleotide- Definition, Characteristics, Biosynthesis, Functions


Nucleotides are the basic building blocks of nucleic acids, such as DNA and RNA. Nucleic acids are polymers made of long chains of nucleotides. A nucleotide consists of three components: a sugar molecule, a phosphate group and a nitrogen-containing base .

The sugar molecule in a nucleotide can be either ribose in RNA or deoxyribose in DNA. The main difference between these two sugars is that ribose has an -OH group at the 2` position, while deoxyribose lacks it. The sugar molecules are present in their β-furanose form, which is a closed five-membered ring structure.

The nitrogen-containing base in a nucleotide can be one of four types: adenine (A), cytosine (C), guanine (G) or thymine (T) in DNA, or uracil (U) instead of thymine in RNA. These bases are derived from two parent compounds: purines and pyrimidines. Purines have a double-ring structure and include adenine and guanine. Pyrimidines have a single-ring structure and include cytosine, thymine and uracil.

The nitrogen-containing base is linked covalently to the sugar molecule by an N-glycosidic bond, which connects the N-1 position of pyrimidines or the N-9 position of purines to the C-1 position of the sugar. The phosphate group is attached to the 5` position of the sugar by an ester bond. A nucleotide can have one, two or three phosphate groups, forming a nucleoside monophosphate (NMP), diphosphate (NDP) or triphosphate (NTP), respectively.

Nucleotides are joined together by phosphodiester bonds, which link the 5` phosphate group of one nucleotide to the 3` hydroxyl group of another nucleotide. This forms a backbone of alternating sugar and phosphate units, with the bases projecting outwards. The sequence of bases along the nucleic acid chain determines the genetic information encoded by the molecule.

Nucleotides can also form hydrogen bonds with each other based on their complementary base pairing. In DNA, adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three hydrogen bonds. In RNA, uracil replaces thymine and pairs with adenine by two hydrogen bonds. These base pairs form the rungs of the double helix structure of DNA or the secondary structures of RNA.

Nucleotides have various functions in living cells. They are involved in energy storage and transfer (e.g., ATP), enzyme cofactors (e.g., NAD+), signaling molecules (e.g., cAMP), and regulation of gene expression (e.g., ppGpp). Nucleotides are synthesized by two pathways: de novo synthesis, which builds nucleotides from simple molecules, and salvage pathway, which recycles nucleotides from degraded nucleic acids. The biosynthesis of nucleotides is regulated by feedback inhibition and allosteric mechanisms.