ddTTP

ddTTP, or dideoxythymidine triphosphate, is a nucleotide analog used as a chain-terminating substrate in Sanger DNA sequencing. It is the triphosphate form of dideoxythymidine, a sugar lacking both the 2' and 3' hydroxyl groups. The absence of the 3'-OH group prevents the formation of the next phosphodiester bond, so once ddTTP is incorporated by DNA polymerase, DNA synthesis stops.

In the Sanger method, a DNA polymerase reaction contains normal deoxynucleotides (dNTPs) and small amounts of one or more ddNTPs. When a ddNTP is incorporated, the growing DNA strand terminates at that position. Over many molecules, this creates a set of DNA fragments of varying lengths that all end with a ddNTP corresponding to the incorporated base. The fragments are then separated by size to reveal the sequence. In modern practice, fluorescently labeled ddNTPs allow the four terminating bases to be detected in a single reaction and read by automated instruments.

ddTTP is one of the four ddNTPs used in Sanger sequencing, alongside ddATP, ddCTP, and ddGTP. The method, developed by Frederick Sanger and colleagues in 1977, contributed to the accuracy and reliability of early DNA sequencing and remains in use for specific applications and educational purposes, although it has largely been supplanted by next-generation sequencing for high-throughput work.