Dideoxynucleotide triphosphates (ddNTPs) are used in manual Sanger sequencing and in many automated methods of sequencing. What is the purpose of using dideoxynucleotides in DNA sequencing? Select ALL that apply. The figure below may be helpful.

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll evaluate what each option says about how nucleotides are added during DNA synthesis and how ddNTPs function.
Option 1: 'Nucleotides are added at the 3' hydroxyl group (–OH) of the growing chain during DNA synthesis.' This is a true general principle of DNA replication and sequencing chemistry: the 3' end provides a new site for forming the phosphodiester bond with the incoming nucleotide triphosphate. While this statement describes DNA synthesis in general and not ddNTPs alone, it is still factually correct and relevant background.
Option 2: 'ddNTPs lack a free hydroxyl (–OH) group at the 2' carbon, thus preventing additional nucleotides from being added to the chain when a ddNTP is incorporated.' This is inaccurate. In standard deoxynucleotides, the 2' carbon lacks a hydroxyl anyway, and the critical feature for termination is the absence of the 3' –OH, not the 2' position. Therefore, this misidentifies the key group involved in termination and is not correct.
Option 3: 'ddNTPs lack a free hydroxyl (–OH) group at the 3' carbon, thus preventing additional nucleotides from being added to the chain when a ddNTP is incorporated.' This is the core reason ddNTPs cause chain termination: without the 3' –OH, a phosphodiester bond cannot form with the next nucleotide. This option accurately states the essential chemical property of ddNTPs that halts elongation.
Option 4: 'Nucleotides are added at the 5' hydroxyl group (–OH) of the growing chain during DNA synthesis.' This statement is incorrect. DNA synthesis proceeds by forming a new bond at the 3' end of the primer, not at the 5' end; the 5' –OH is not the site of nucleotide addition. This misstates the directionality and chemistry of chain elongation.
In summary, option 1 is correct as a general description of nucleotide addition at the 3' end, option 2 is incorrect due to misidentifying the critical hydroxyl involved in termination, option 3 is correct because the lack of a 3' –OH in ddNTPs leads to chain termination, and option 4 is incorrect for the same reason that elongation occurs at the 3' end rather than the 5' end.

Dideoxynucleotide triphosphates (ddNTPs) are used in manual Sanger sequencing and in many automated methods of sequencing. What is the purpose of using dideoxynucleotides in DNA sequencing? Select ALL that apply. The figure below may be helpful.多项选择题

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Dideoxynucleotide triphosphates (ddNTPs) are used in manual Sanger sequencing and in many automated methods of sequencing. What is the purpose of using dideoxynucleotides in DNA sequencing? Select ALL that apply. The figure below may be helpful.

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