Final answer:
Additional sequencing information is obtained by incorporating all four types of fluorescently labeled dideoxynucleotides (ddNTPs) into a DNA sample, resulting in fragments of different lengths. These fragments are separated using gel electrophoresis, and a laser scanner reads their sequences by detecting the fluorescent labels.
Step-by-step explanation:
If DNA fragments are terminated with a fluorescent dideoxycytidine nucleotide base after the first round of addition, additional sequence information for each fragment is obtained through a method called the dideoxy chain termination method. In this method, a mixture of deoxynucleotides and all four differently labeled dideoxynucleotides (ddNTPs) are added to a single tube containing the DNA template, a primer, and DNA polymerase. As DNA synthesis proceeds, elongation is randomly terminated each time a fluorescently labeled ddNTP is incorporated. Since ddNTPs lack a 3' OH group, no further nucleotides can be added, resulting in DNA fragments of varying lengths that terminate with different ddNTPs.
These DNA fragments are then separated by size using gel electrophoresis. The smallest fragments move fastest and thus farthest, while larger fragments move more slowly and travel a shorter distance through the gel. A laser scanner reads the fluorescent labels of the fragments as they pass by, which allows for the determination of the DNA sequence. This sequencing by synthesis approach allows researchers to determine the sequence of the entire DNA molecule by reading the terminal nucleotide on each fragment.