A method of synthesising single or double-stranded polynucleotide comprising the steps: a) providing a polynucleotide starter strand comprising, in sequence from 5' to 3', a nucleotide strand selected from the group consisting: (i) 5' blocker - 5' end - segment 1- CUCM 1 - segment 2-3' end wherein the 3' end is either immobilised or blocked, and, optionally, a hybridised complementary strand comprising, in sequence from 5' to 3' 5' end - segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein the 5' end is either immobilised or blocked, (ii) 5' blocker-5' end - segment 1- CUCM1- segment 2 - HP1-segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein HP1 is optionally immobilised, and (iii) segment 1 - CUCM1 - segment 2 - HP1 - segment 2C - CUCM2 - segment 1C - HP2 wherein segment 1 is linked to HP 2 to form a double hairpin structure, wherein either HP1 or HP2 may optionally have a HP fluorophore attached and wherein, optionally, the other of HP1 and HP2 is immobilised, b) digesting CUCM1 with glycosylase enzyme 1 or CUCM2 with a glycosylase enzyme 2 and washing the starter strand to remove the remnant of the digest, c) where the method is employed to synthesise double stranded polynucleotide, digesting the other of CUCM1 with a first glycosylase enzyme 1 or CUCM2 with glycosylase enzyme 2 and washing to remove the remnant of the digest, d) ligating a first ligation strand comprising: 5' blocker - 5' end - segment 3 - CUCM3 - additional nucleotide 1-3' end, and/or a second ligation strand comprising: 5' end - additional nucleotide 2 - CUCM4 - segment 3C - 3' end - 3' blocker to the starter strand, and e) repeating steps b) to d) as many times as required to provide the desired synthetic polynucleotide. a) providing a polynucleotide starter strand comprising, in sequence from 5' to 3', a nucleotide strand selected from the group consisting: (i) 5' blocker - 5' end - segment 1- CUCM 1 - segment 2-3' end wherein the 3' end is either immobilised or blocked, and, optionally, a hybridised complementary strand comprising, in sequence from 5' to 3' 5' end - segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein the 5' end is either immobilised or blocked, (ii) 5' blocker-5' end - segment 1- CUCM1- segment 2 - HP1-segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein HP1 is optionally immobilised, and (iii) segment 1 - CUCM1 - segment 2 - HP1 - segment 2C - CUCM2 - segment 1C - HP2 wherein segment 1 is linked to HP 2 to form a double hairpin structure, wherein either HP1 or HP2 may optionally have a HP fluorophore attached and wherein, optionally, the other of HP1 and HP2 is immobilised, (i) 5' blocker - 5' end - segment 1- CUCM 1 - segment 2-3' end wherein the 3' end is either immobilised or blocked, and, optionally, a hybridised complementary strand comprising, in sequence from 5' to 3' 5' end - segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein the 5' end is either immobilised or blocked, (ii) 5' blocker-5' end - segment 1- CUCM1- segment 2 - HP1-segment 2C - CUCM2 - segment 1C - 3'end - 3' blocker wherein HP1 is optionally immobilised, and (iii) segment 1 - CUCM1 - segment 2 - HP1 - segment 2C - CUCM2 - segment 1C - HP2 wherein segment 1 is linked to HP 2 to form a double hairpin structure, wherein either HP1 or HP2 may optionally have a HP fluorophore attached and wherein, optionally, the other of HP1 and HP2 is immobilised, b) digesting CUCM1 with glycosylase enzyme 1 or CUCM2 with a glycosylase enzyme 2 and washing the starter strand to remove the remnant of the digest, c) where the method is employed to synthesise double stranded polynucleotide, digesting the other of CUCM1 with a first glycosylase enzyme 1 or CUCM2 with glycosylase enzyme 2 and washing to remove the remnant of the digest, d) ligating a first ligation strand comprising: 5' blocker - 5' end - segment 3 - CUCM3 - additional nucleotide 1-3' end, and/or a second ligation strand comprising: 5' end - additional nucleotide 2 - CUCM4 - segment 3C - 3' end - 3' blocker to the starter strand, and e) repeating steps b) to d) as many times as required to provide the desired synthetic polynucleotide.