Question:
A DNA molecule is 140 base pairs long. It has 20% thymine. How many guanine bases are present in the molecule ?
A DNA molecule is 140 base pairs long. It has 20% thymine. How many guanine bases are present in the molecule ?
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A = T = 20%, so G = C = 30% of 280 nucleotides = 84.
Updated On: Oct 22, 2026
- 64
- 84
- 32
- 56
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
DNA is double stranded. By Chargaff's rule, A pairs with T, and G pairs with C. So A = T and G = C.
Step 2: Count the bases:
140 base pairs means \( 140 \times 2 = 280 \) nucleotides in total.
Step 3: Use the percentages:
Thymine is 20%, so adenine is also 20%. Together A + T = 40%.
The rest is G + C = \( 100 - 40 = 60\% \). So G = C = 30% each.
Step 4: Find guanine:
\[ G = \frac{30}{100} \times 280 = 84 \]
Step 5: Check the options:
56 is the number of thymine bases (20% of 280), not guanine, so (D) is wrong.
64 and 32 do not match any base count here. So only (B) 84 fits.
Final Answer:
Guanine bases = 84, option (B). \[ \boxed{84} \]
DNA is double stranded. By Chargaff's rule, A pairs with T, and G pairs with C. So A = T and G = C.
Step 2: Count the bases:
140 base pairs means \( 140 \times 2 = 280 \) nucleotides in total.
Step 3: Use the percentages:
Thymine is 20%, so adenine is also 20%. Together A + T = 40%.
The rest is G + C = \( 100 - 40 = 60\% \). So G = C = 30% each.
Step 4: Find guanine:
\[ G = \frac{30}{100} \times 280 = 84 \]
Step 5: Check the options:
56 is the number of thymine bases (20% of 280), not guanine, so (D) is wrong.
64 and 32 do not match any base count here. So only (B) 84 fits.
Final Answer:
Guanine bases = 84, option (B). \[ \boxed{84} \]
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