A ddNTP (dideoxynucleotide triphosphate) is used in DNA sequencing to terminate the DNA strand during replication. When a ddNTP is incorporated into the growing DNA strand, it prevents further elongation, resulting in fragments of varying lengths. These fragments are then separated by size to determine the sequence of the original DNA strand.
The ratio of ddNTP to dNTP in the nucleotide mixture for Sanger sequencing is typically 1:10.
ddNTPs (dideoxynucleotide triphosphates) are used in DNA sequencing because they lack the 3'-OH group required for the formation of phosphodiester bonds with the next nucleotide, causing DNA polymerase to terminate the DNA strand synthesis upon ddNTP incorporation. This results in the production of a series of DNA fragments with varying lengths that can be separated by size to determine the sequence of the original DNA template.
They are ddNTPs (ddATP, ddGTP, ddTTP, ddCTP) that also lack a 3' hydroxyl (OH) group in their deoxyribose sugar. This allows them to terminate the complementary nucleotide sequence (forming antiparallel to the template strand) that starts with the oligonucleotide (primer) and continues with dNTPs (deoxynucleoside triphosphate) attached by DNA polymerase. ddNTPs terminate the sequence at different positions which results in sequences of different lengths that each end with a different ddNTP. As these sequences are ran through a capillary electrophoresis gel, they are sorted by length, with the shortest sequence exiting the capillary tube first, and so on. Since each ddNTP is dyed a different color and fluoresces differently, the sequencing machine reads the ddNTP at the end of each sequence, determines its nucleotide (A, G, T, or C), and writes out the sequence one nucleotide at a time. Below is a link to a wonderful interactive animation that teaches about this method of sequencing.
http://wiki.answers.com/Q/In_dideoxy_sequencing_of_dna_isn%27t_it_so_that_in_each_test_tube_containing_a_separate_ddNTP_apart_from_getting_fragments_ending_at_ddNTPs_you_shall_also_get_the_full_fragments_in_each_case" "http://wiki.answers.com/Q/In_dideoxy_sequencing_of_dna_isn%27t_it_so_that_in_each_test_tube_containing_a_separate_ddNTP_apart_from_getting_fragments_ending_at_ddNTPs_you_shall_also_get_the_full_fragments_in_each_case"