DNA sequencing by the Sanger method take: advantage of which property of DNA synthesis to generate a sequencing ladder?
The Sanger method, also known as the chain termination method, uses dideoxynucleotides (ddNTPs). These ddNTPs lack a 3'-OH group, which is necessary for the formation of the phosphodiester bond during DNA synthesis. When a ddNTP is incorporated into the growing DNA strand, it stops further elongation because there's no hydroxyl group to form the next bond. This creates DNA fragments of varying lengths, each ending with a specific ddNTP. These fragments are then separated by size using gel electrophoresis to determine the sequence.
So the core concept here is the use of dideoxynucleotides to terminate DNA chain elongation. The correct answer should be related to chain termination due to the absence of the 3'-OH group.
Looking at possible distractors, common wrong options might include things like the use of DNA polymerase, the presence of RNA primers, or the use of restriction enzymes. But the key here is the specific property of DNA synthesis that's exploited: the chain termination caused by ddNTPs.
For the explanation, I need to detail why each wrong option is incorrect. For example, if an option mentions DNA polymerase, it's correct that the Sanger method uses it, but that's not the unique property being tested here. Similarly, if an option talks about gel electrophoresis, that's part of the process but not the mechanism of generating the ladder. The main point is the chain termination by ddNTPs.
Clinical pearl: Remember that ddNTPs are chain terminators because they lack the 3'-OH, which is essential for DNA polymerase to add the next nucleotide. This is a high-yield fact for exams and understanding the Sanger method.
**Core Concept**
The Sanger method of DNA sequencing relies on the **chain-terminating properties of dideoxynucleotides (ddNTPs)**. These molecules lack a 3'-hydroxyl (-OH) group required for phosphodiester bond formation, halting DNA synthesis when incorporated. This generates fragments of defined lengths, which are separated via gel electrophoresis to determine the DNA sequence.
**Why the Correct Answer is Right**
The Sanger method uses **ddNTPs** as chain terminators. During DNA synthesis, DNA polymerase randomly incorporates a ddNTP instead of a deoxyribonucleotide (dNTP). Since ddNTPs lack the 3'-OH group necessary for extending the DNA chain, synthesis stops at that position. This creates a set of nested fragments ending at each nucleotide in the template, forming a sequencing ladder. The fragments are size-separated by electrophoresis, revealing the sequence.
**Why Each Wrong Option is Incorrect**
**Option A:** If it mentions "DNA polymerase activity" — while DNA polymerase is essential for the method, the question asks about the *property of DNA synthesis* exploited, not the enzyme itself.
**Option B:** If it refers to "gel electrophoresis" — this is used