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Q.What will be the amount of Cytosine in a DNA, if the total amount of Thiamine and Adenine in the DNA is 55 % ?

(i) 27.5 %
(ii) 22.5 %
(iii) 45 %
(iv) 55 %
Mizoram MbseMizoram Board of School Education HSSLC 2025MCQ· 1mImportance★★★★★
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By Chargaff's rule, %A = %T and %G = %C; since A + T = 55%, each is 27.5%, leaving G + C = 45%, so Cytosine = 22.5%.

In double-stranded DNA, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is Chargaff's rule, which means:

%A=%Tand%G=%C\%A = \%T \quad \text{and} \quad \%G = \%C

Given: %T+%A=55%\%T + \%A = 55\%.

Since %A=%T\%A = \%T, each individually is:

%A=%T=552=27.5%\%A = \%T = \dfrac{55}{2} = 27.5\% …

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