Chapter 28: Nucleosides

Chapter 28: Nucleosides, Nucleotides and Nucleic Acids

Nucleosides

Nucleosides are N-glycosides in which the pyrimidine or purine N is bonded to the anomeric carbon in the carbohydrate by a b-N-glycosidic bond.

A schematic diagram of a generic nucleoside : two structural subunits : a nitrogen base and a carbohydrate.

In RNA, the carbohydrate is ribose (D-ribofuranose) while in DNA the carbohydrate is deoxyribose (2-deoxy-D-ribofuranose) which lacks the hydroxyl group at C2.



Ribose		Deoxyribose

In RNA the bases are adenine, cytosine guanine and uracil (i.e. A, C, G, U)
In DNA the bases are adenine, cytosine, guanine and thymine (i.e. A, C, G, T)
The base portion is cis to the -CH₂OH group hence they are b-glycosides.
The base is normally anti with respect carbohydrate so that the hydrogen bonding C=O and N-H groups in the base are away from the furanose (as shown below):

	pyrimidine base = adenine carbohydrate = D-ribofuranose or ribose
9-b-D-ribofuranosyladenine or adenosine