Biology · Chromosomes and DNA
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The biochemical analysis of a sample of DNA shows that 32% of the nitrogenous bases are cytosine. What is the total % of adenine and uracil in RNA transcribed from this DNA?
- A
16%
- B
18%
- C
32%
- D
36%
According to Chargaff's rule, in DNA, the amount of cytosine (C) equals the amount of guanine (G), and the amount of adenine (A) equals the amount of thymine (T). Given 32% cytosine, there is also 32% guanine, totaling 64% for C and G. This leaves 36% for adenine and thymine combined in DNA. During transcription to RNA, thymine is replaced by uracil, maintaining the 36% total, which is now distributed between adenine and uracil. Therefore, the total percentage of adenine and uracil in RNA is 36%. The incorrect options misunderstand these base pair conversions and proportions.
This option is incorrect because it underestimates the total percentage of adenine and uracil. Remember that adenine and thymine percentages are equal in DNA, and adenine converts to uracil in RNA.
This option represents the percentage of adenine or thymine in DNA alone, but it does not account for the conversion to RNA where adenine pairs with uracil, making the total 36%.
This option is the percentage of cytosine in DNA, and should not be confused with the percentage of adenine or uracil in RNA.
This is the correct answer. Since cytosine is 32%, guanine is also 32% by Chargaff’s rule, leaving 36% for adenine and thymine. In RNA, adenine pairs with uracil, so the total percentage of adenine and uracil is 36%.
Tagged under Biology · Chromosomes and DNA · 2015