What is sequencing by synthesis approach in next-generation sequencing (NGS)?

Sequencing by synthesis is a DNA sequencing method in which DNA polymerases and dNTPs are used to replicate (synthesize) the strand to be sequenced. Nucleotides are introduced either on a single time or modified with identifying tags (e.g. fluorophore) so that the base type of the incorporated nucleotide can be recognized as the DNA molecule extends. Besides, these nucleotides are chemically blocked such that each incorporation is a unique event. After the signal of each incorporation step is collected, the blocked group will be removed to prepare the strand for the next incorporation event by DNA polymerase. By continuing this series of steps for a specific number of cycles, the strand of interest is replicated and sequenced.