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Microbiology: Laboratory Theory & Application, Brief 3rd Edition

Microbiology: Laboratory Theory & Application, Brief (3rd Edition)

Book Edition3rd Edition
Author(s)Leboffe, Pierce
PublisherMorton Publishing
Section 8-1: Extraction of DNA from Escherichia coli Cells
Observations and Interpretations
Section 8-2: Bacteria Transformation: the pGLO System
Observations and Interpretations
Section 8-3: Ultraviolet Radiation Damage and Repair
Observations and Interpretations
Section 8-5: Blood Typing
Observations and Interpretations
Section 8-6: ELISA for Detecting Antibodies in a Patient's Sample (The Antibody Capture Method)
Observations and Interpretations

Chapter 8, Section 8-1, Observations and Interpretations, Exercise 1

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Here is a tip:

The nitrogenous bases present in the Deoxyribonucleic acid and the Ribonucleic acid show a similar absorbance range.


The absorbance of DNA [Deoxyribonucleic acid] is used to estimate its concentration. The absorbance value of a DNA molecule displays its capacity to absorb the light at a specific wavelength [measured in nanometers, nm] of light. The absorbance value of a sample is dependent on its thickness or the turbidity and the ability to absorb the wavelengths of light. This may change with respect to the type of sample used for studying its absorbance.

  • The DNA of bacteria has a 0.50 absorbance value at 220 nm because different types of bonds absorb different wavelengths of light.
  • At 240 nm, the organic compounds and the salts have high absorbance values. The DNA does not have salts, and consequently, the absorbance value dips to 0.36.
  • DNA molecules have the highest absorbance value because the nitrogenous bases [purines and pyrimidine] absorb the light of 260 nm.
  • At 280 nm, the absorbance value decreases to 0.042 because at this wavelength the protein molecules have the highest absorbance. The absorbance value will not decrease if the extracted DNA contains high protein content.
  • The strong absorbance of DNA is between 240 nm-275 nm. Since 300 nm is beyond the absorbance range of the DNA molecules, the value decreases to 0.07.
  • At 320 nm, the DNA molecules did not show any absorbance

Sample Response

  • The absorbance of DNA at 220 nm: 0.50
  • The absorbance of DNA at 240nm: 0.36
  • The absorbance of DNA at 260nm: 0.68
  • The absorbance of DNA at 280nm: 0.42
  • The absorbance of DNA at 300nm: 0.07
  • The absorbance of DNA at 320nm: 0.00
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