Q62.Purity of DNA samples can be checked by measuring the ratio of absorbances at 260 and 280 nm. For a pure DNA sample the ratio should be
(1) 1.8
(2) < 1.8
(3) < 1.0
(4) > 2
DNA Purity Check: A260/A280 Ratio Explained
The standard method to assess DNA sample purity involves the absorbance ratio at 260 nm and 280 nm, where a pure DNA sample typically shows a ratio of 1.8.
Correct Answer
Option (1) 1.8 is the correct choice for a pure DNA sample.
This ratio indicates minimal protein or other contaminants, as nucleic acids absorb maximally at 260 nm while proteins peak at 280 nm.
Option Breakdown
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(1) 1.8: Represents pure DNA; widely accepted as ~1.8 (range 1.7-2.0), confirming high-quality extraction suitable for downstream applications like PCR or sequencing.
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(2) < 1.8: Signals contamination, often by proteins, phenol, or other 280 nm-absorbing impurities, reducing sample usability.
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(3) < 1.0: Indicates severe contamination; extremely low ratios suggest heavy protein presence or failed purification, rendering DNA ineffective.
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(4) > 2: Points to RNA contamination or single-stranded DNA; pure double-stranded DNA stays below 2.0.
Why Measure A260/A280?
Nucleic acids exhibit peak absorbance at 260 nm due to their aromatic ring structures, while proteins (e.g., via tryptophan) absorb at 280 nm.
The ratio formula is A260/A280 (corrected for background at 320 nm), providing a quick, non-destructive purity check via spectrophotometer.
Pure ratios ensure reliable quantification: DNA concentration ≈ 50 μg/mL × A260 × dilution factor.
Practical Tips
Always scan 220-320 nm for a smooth peak at 260 nm; pair with A260/A230 (ideal 2.0-2.2) for organic contaminant detection.
