If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.
30%
Explanation
This question is based on Chargaff's base pairing rules mentioned in the context. The textbook states that 'Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds.' This means in double-stranded DNA, the amount of Adenine equals Thymine (A=T) and the amount of Guanine equals Cytosine (G=C). Since Cytosine is 20%, Guanine is also 20%. The remaining 60% must be equally divided between Adenine and Thymine, giving 30% each.
Solution Steps
Step 1: According to base pairing rules in double-stranded DNA, Adenine pairs with Thymine (A = T) and Guanine pairs with Cytosine (G = C).
Step 2: Given: Cytosine = 20%. Since G = C, therefore Guanine = 20%.
Step 3: Total percentage of G + C = 20% + 20% = 40%.
Step 4: Remaining percentage = 100% - 40% = 60%. This 60% is shared equally between Adenine and Thymine.
Step 5: Therefore, Adenine = 60% ÷ 2 = 30%.