Practice question

In Sanger sequencing reaction, DNA polymerase extends primer and randomly incorporates either canonical deoxyribonucleotide or dideoxy analog such as ddATP opposite template thymine. Incorporation of normal dATP allows continued elongation, but incorporation of ddATP terminates strand because its ribose lacks 3' hydroxyl required for phosphodiester linkage with next nucleotide. The terminated chain remains stable product whose length marks position of adenine in synthesized strand. Accumulation of such fragments across many templates generates ladder where termination sites indicate base identity, not chain breakage in middle or no effect.