30. In DNA sequencing reactions usmg the chain termination method, the ratio of ddNTPs to dNTPs
should be
(A) 0 (B) <1
(C) 1 (D) >1
Correct Answer: (B) <1
In the chain termination method (Sanger sequencing), ddNTPs (dideoxynucleotide triphosphates) act as chain terminators by lacking a 3′-OH group, stopping DNA polymerase extension when incorporated. The ratio of ddNTPs to dNTPs must be precisely controlled to generate fragments of all lengths for accurate sequencing.
Option Analysis
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(A) 0: Incorrect. Zero ddNTPs means no termination occurs, producing only full-length products and no sequence ladder.
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(B) <1: Correct. ddNTPs are used at lower concentrations (e.g., 1:10 to 1:300 ratio) than dNTPs to ensure probabilistic, random incorporation, creating fragments terminated at every position.
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(C) 1: Incorrect. Equal ratios cause excessive early terminations, yielding mostly short fragments and poor coverage of longer sequences.
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(D) >1: Incorrect. Excess ddNTPs leads to immediate terminations after the first matching base, producing only tiny fragments and failing to read the full sequence.
In chain termination DNA sequencing reactions—also known as Sanger sequencing—the ddNTPs to dNTPs ratio plays a critical role in generating the full spectrum of DNA fragments needed for readable sequence ladders. This method relies on dideoxynucleotides (ddNTPs) competing with deoxynucleotides (dNTPs) during polymerase extension. Keeping the ddNTPs to dNTPs ratio <1 ensures controlled, random chain termination at every nucleotide position.
Why ddNTPs to dNTPs Ratio <1 is Essential
DNA polymerase preferentially incorporates normal dNTPs over ddNTPs due to the missing 3′-OH on ddNTPs. A low ddNTPs to dNTPs ratio (typically 1:10 or lower, e.g., 0.1 mM ddNTP vs. 1 mM dNTP) minimizes premature stops while sampling all positions across millions of template molecules. This balance produces evenly distributed fragments for gel or capillary electrophoresis analysis.
Common Ratio Pitfalls
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Ratios =1 or >1 overload terminations, skewing toward short reads.
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Ratios near 0 eliminate terminations entirely.
Practical Tip: Modern kits use fluorescent ddNTPs at optimized ratios (e.g., 1:300), with modified polymerases like Sequenase to reduce bias. For biotech students or researchers in molecular biology, always verify ratios per protocol to avoid sequencing artifacts.
