Nucleotide analogs that can sustain the enzymatic synthesis of double-stranded nucleic acid from a nucleic template are described. The nucleotide analogs include: (i) a base selected from the group consisting of adenine, guanine, cytosine, thymine, uracil and their analogs; (ii) a label attached to the base or analog of the base via a cleavable linker; (iii) a deoxyribose; and (iv) one or more phosphate groups. The linker and/or the label inhibits template directed polymerase incorporation of a further nucleotide substrate onto an extended primer strand. In addition, cleavage of the linker leaves a residue attached to the base which is not present in the natural nucleotide and which does not inhibit extension of the primer strand. The nucleotide analogs can therefore be used as reversible terminators in sequencing by synthesis methods without blocking the 3' hydroxyl group. Methods of sequencing DNA using the substrates are also described.