What is a reversible terminator in DNA sequencing?

To identify the sequence in DNA, each base in a DNA fragment or DNA nanoball needs to be identified step-by-step. First, a primer binds to the adaptor in the DNA strand, and the primer will be extended using an artificial dNTP rather than a natural dNTP. The artificial dNTP usually prevents further extension, so one base is added at a time. That base could be identified as A, T, C, or G by detecting the unique fluorescent dye (unique for each base) on it, with a camera. Another technology, which is called coolMPS, detects the dye on the monoclonal antibodies. Each type of the monoclonal antibodies binds to a specific base. Anyway, a reversible terminator is often needed in these technologies (second-generation sequencing) to ensure that only one base is added at a time.

How is it done? DNA extension by DNA polymerase requires a 3’-OH group in the last base (3’-end). The group can be replaced with a different chemical group, preventing the extension. But the artificial 3’-O-blocking group can be reacted to specific reagent to be reverted into an OH group to allow extension again. The dye will also be cleaved away to remove the signal responsible for this base. The cycle repeats to identity the next bases.