## **Core Concept**
The Sanger sequencing method, also known as the chain termination method, relies on the **specificity of DNA polymerase** for DNA synthesis and the **incorporation of dideoxynucleotides** (ddNTPs) to terminate the DNA strand elongation. This technique generates a collection of DNA fragments of varying lengths, which are then separated by size to create a sequencing ladder.

## **Why the Correct Answer is Right**
The correct answer, **D. chain termination by dideoxynucleotides**, is right because the Sanger sequencing method uses ddNTPs, which lack the 3' hydroxyl group necessary for phosphodiester bond formation. When a ddNTP is incorporated into a growing DNA strand, it **terminates the strand elongation** because no further nucleotides can be added. By including a mixture of deoxynucleotides (dNTPs) and ddNTPs in the reaction, the DNA polymerase generates a collection of fragments of varying lengths, each ending with a ddNTP. These fragments are then separated by size using gel electrophoresis, creating a sequencing ladder.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *template-dependent nucleotide incorporation* is a fundamental property of DNA polymerase but does not specifically describe the mechanism by which the Sanger sequencing method generates a sequencing ladder.
- **Option B:** *processivity of DNA polymerase* refers to the ability of DNA polymerase to add multiple nucleotides without releasing the template, which is not directly related to the generation of a sequencing ladder in Sanger sequencing.
- **Option C:** *exonuclease activity* refers to the ability of some DNA polymerases to remove nucleotides from the ends of DNA strands, which is not relevant to the Sanger sequencing method.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Sanger sequencing is limited to reading approximately 1000 nucleotides per reaction**, making it less suitable for sequencing large genomes. This limitation led to the development of newer, high-throughput sequencing technologies.

## **Correct Answer:** D. chain termination by dideoxynucleotides