Molecular Biology • Digital Learning Chapter
Concentration and Purity of Extracted DNA Using Spectrophotometry
Advanced M4S Digital Textbook based on the supplied MBLOGSTU educational content dated 23 February 2024.
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To determine the concentration and purity of extracted DNA using spectrophotometry, a UV-Vis spectrophotometer is typically used. The supplied source describes measurement of absorbance at 260 nm and comparison with absorbance at 280 nm.
Core concept: A260 is used for nucleic-acid measurement, while the A260/A280 ratio is used in the supplied source to assess DNA purity.
- Extracted DNA sample
- UV-Vis spectrophotometer
- Cuvettes or microvolume plates compatible with the spectrophotometer
- Distilled water for blank measurements
- Spectrophotometry software for data analysis
- Turn on the spectrophotometer.
- Allow it to warm up according to the manufacturer's instructions.
- Set the wavelength to 260 nm, corresponding to the absorbance maximum of nucleic acids.
Practical point: The source specifically directs the instrument to 260 nm for DNA measurement.
- Prepare a blank containing only distilled water in a suitable cuvette or microvolume plate.
- Place the blank in the spectrophotometer.
- Use the Blank or Zero function.
Purpose: The blank establishes the measurement baseline before the DNA sample is measured.
- Dilute the extracted DNA if necessary so that it falls within the linear range of the spectrophotometer.
- Pipette an aliquot of the diluted DNA into a clean cuvette or microvolume plate.
- Place the sample in the spectrophotometer.
- Record the absorbance at 260 nm.
The supplied source states that DNA concentration can be calculated from the absorbance using the Beer-Lambert law.
DNA concentration (ng/μL) = Absorbance at 260 nm × DNA extinction coefficient × Dilution factor
The source states that the DNA extinction coefficient typically ranges from 50 to 80 ng/μL for double-stranded DNA.
Record: A260 value, dilution factor, extinction coefficient used, and calculated concentration.
Determine DNA purity by calculating the ratio of absorbance at 260 nm to absorbance at 280 nm.
A260/A280 = Absorbance at 260 nm ÷ Absorbance at 280 nm
| A260/A280 ratio | Interpretation in supplied source |
|---|---|
| 1.8–2.0 | Indicative of pure DNA |
| <1.8 | May indicate contamination with proteins or other organic compounds |
Important: The values above reproduce the interpretation provided in the supplied source.
Data Analysis
- Record the calculated DNA concentration.
- Record the DNA purity ratio.
- Use spectrophotometry software or spreadsheet programs to generate reports or perform further analysis.
Considerations
- Ensure that the spectrophotometer is properly calibrated and maintained according to the manufacturer's instructions.
- Handle DNA samples carefully to avoid contamination.
- Pipette appropriate volumes accurately.
- Perform measurements in triplicate or replicate to support accuracy and reproducibility.
Workflow summary: Prepare instrument → blank → measure DNA at 260 nm → calculate concentration → calculate A260/A280 → record and analyze results.
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