New developments in nucleoside analogues biosynthesis: A review

•Nucleoside analogues (NAs) are commonly used as antitumoral and antiviral compounds.•Chemoenzymatic synthesis of biologically important NAs is described.•The use of enzymes as biocatalysts improves chemo-, regio- and stereoselectivity.•Enzymatic modification of NAs improve the pharmacological properties of the existing drugs.•Biocatalyst immobilization favors separation, reuse, and bioprocess scale up.