Final answer:
The biochemical structure crucial to the chain termination method of sequencing is Dideoxynucleotide (ddNTP).
Step-by-step explanation:
The biochemical structure crucial to the chain termination method of sequencing is Dideoxynucleotide (ddNTP). The chain termination method, also known as the Sanger sequencing method, uses ddNTPs that lack a free 3' OH group necessary for further extension of the DNA chain. By incorporating a small proportion of fluorescently labeled ddNTPs into the DNA synthesis reaction, the sequencing process generates DNA fragments of different sizes that can be visualized and analyzed.