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  4. 22% per cent of the bases in a sample of DNA extracted from a bacterium is adenine. What percentage of cytosine is present in this DNA?

Q. 22% per cent of the bases in a sample of DNA extracted from a bacterium is adenine. What percentage of cytosine is present in this DNA?

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Solution:

According to Chargaff's Equivalency Rule, in a double-stranded DNA, the amount of adenine is equal to thymine and the amount of cytosine is equal to the amount of guanine.
Adenine = 22%, Thymine = 22%
A + T + G + C = 100
22 + 22 + X + X = 100
2X = 100 - 44 = 56
So, X = 28
Guanine = 28%, Cytosine = 28%