Chain Termination Method

The missing 3′-hydroxyl group on a dideoxynucleotide creates the chemical feature that produces a termination event. Once DNA polymerase incorporates that labeled nucleotide, the growing strand cannot be extended further, so each incorporated dideoxynucleotide marks a specific stopping position. This converts nucleotide incorporation during synthesis into fragments that support sequence reconstruction.

Normal deoxynucleotides allow DNA polymerase to continue extending the strand, whereas dideoxynucleotides interrupt extension when incorporated. Including both types in the synthesis reaction produces a population of fragments ending at different positions. Their varied lengths preserve information about successive nucleotide locations while the labeled terminating bases identify which nucleotide occurs at each position.

Fragment length indicates the position at which synthesis stopped along the DNA strand. After capillary electrophoresis separates the products, the ordered fluorescent signals reveal the succession of termination sites. Shorter and longer fragments therefore contribute different positions to the read, allowing the sequence to be reconstructed from the separated pattern rather than from one undifferentiated product.

Fluorescent labeling identifies the nucleotide present at each termination site. Because each fragment carries a signal associated with its terminal dideoxynucleotide, the detection pattern reports both the fragment’s position in the separation and the base that ended it. This combination allows researchers to distinguish the nucleotide sequence represented by the collection of terminated products.

A typical workflow combines a DNA template with synthesis reagents containing normal deoxynucleotides and fluorescently labeled dideoxynucleotides. DNA polymerase extends the strand, generating products that stop at different incorporation sites. The products then pass through capillary electrophoresis, where separation and fluorescence detection create the signal pattern used to read the sequence.

Sequence recovery depends on considering two signals together: fragment mobility and terminal fluorescence. Electrophoresis separates products according to their lengths, while the fluorescent label identifies the dideoxynucleotide at each endpoint. Reading these signals in their separated order reveals the succession of bases represented by the termination fragments and produces the sequence of the analyzed strand.

The method remains valuable because it provides highly accurate sequence data for focused verification. Researchers can use it to verify cloned DNA, identify mutations, or confirm results produced by high-throughput sequencing. This complementary role makes it useful when the objective is to check a particular sequence result rather than rely only on a broader sequencing dataset.