Final answer:
The purpose of dideoxynucleoside triphosphates in dideoxy sequencing is to terminate DNA synthesis and create DNA fragments of different lengths.
Step-by-step explanation:
The purpose of the dideoxynucleoside triphosphate (ddNTP) in the dideoxy sequencing reaction is to terminate DNA synthesis at specific bases. The ddNTPs lack a free 3' OH group, which is necessary for adding the next nucleotide in replication.
When a ddNTP is incorporated into a growing DNA strand, it acts as a chain terminator, preventing further elongation. By using a predetermined ratio of ddNTPs to regular deoxynucleotides (dNTPs), different DNA fragments of varying lengths can be created, representing the sequence of bases in the original DNA sample.