Final answer:
2,3 dideoxy-cytosine tri-phosphate (ddCTP) is used in DNA sequencing to terminate the extension of a DNA molecule, which is crucial for the dideoxy chain termination method developed by Fred Sanger.
Step-by-step explanation:
In DNA sequencing, 2,3 dideoxy-cytosine tri-phosphate (ddCTP) is added to the reaction mixture to terminate DNA synthesis at specific points. This is because ddCTP lacks the essential 3' OH group needed to form a phosphodiester bond with the next nucleotide. Thus, when ddCTP is incorporated into the DNA chain, it prevents further extension of the DNA molecule, resulting in fragments of varying lengths. This is integral to the dideoxy chain termination method developed by Fred Sanger, which involves using a sparse amount of ddNTPs relative to the normal deoxynucleotides to ensure a mix of terminated fragments. After replication is complete, these fragments are separated by gel electrophoresis and the sequence is determined by reading the lengths of DNA from shortest to longest, usually detected by fluorescent markers linked to each ddNTP.