The final step in a Sanger DNA sequencing reaction is to run the DNA fragments on a gel. What purpose does this serve?

• A. It adds ddNTP to the end of each DNA fragment.
• B. It changes the length of the DNA fragments.
• C. It separates DNA fragments based on their sequence.
• D. It separates DNA fragments generated during the sequencing reaction based on one nucleotide differences in their size.

Answer: (D)

Verified step by step guidance

1. Understand that Sanger sequencing involves the synthesis of DNA fragments of varying lengths, each terminating with a dideoxynucleotide (ddNTP).
2. Recognize that these DNA fragments differ by just one nucleotide in length due to the random incorporation of ddNTPs during the sequencing reaction.
3. The purpose of running these fragments on a gel is to separate them based on size. This is because gel electrophoresis allows for the resolution of DNA fragments that differ by as little as a single nucleotide.
4. During gel electrophoresis, an electric current is applied, causing the negatively charged DNA fragments to migrate towards the positive electrode. Smaller fragments move faster and travel further than larger ones.
5. By analyzing the pattern of bands on the gel, researchers can determine the sequence of the original DNA strand, as each band corresponds to a fragment ending with a specific ddNTP.