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ds or ssDNA has more absorbance? Double stranded and Single stranded DNA UV absorbance and Hyperchromic effect

ds or ss DNA has more absorbance?
  • The absorbance of isolated nucleotides is > ss RNA or ss DNA>ds DNA.
  • The increase in absorbance of ss DNA over ds DNA is termed as hyperchromic effect. In other words, dsDNA is hypochromic with respect to ss DNA. Hyperchromic effect is due to the presence of exposed nitrogenous bases in ss DNA.  See the graph below to get a clear picture.
Hyperchromic and hypochromic effect dsDNA vs ssDNA absorbance graph

Definition: Hyperchromic Shift or Effect 

  • The phenomenon of increased UV absorbance by denatured DNA is known as the hyperchromic shift. 
  • The purine and pyrimidine bases in DNA strongly absorb ultraviolet light. Double-stranded DNA absorbs less strongly than denatured DNA due to the stacking interactions between the bases. On denaturation, the bases are exposed thereby showing increased absorbance.
  • This is the order of UV absorbance from high to low free nucleotides->ssDNA->dsDNA
  • The phenomenon of decreased UV absorbance by ds native DNA compared to ss denatured DNA is known as hypochromic effect
                 *ss=single stranded, ds=double stranded
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