If a DNA molecule of 50 base pairs in length contains a total of 15 cytosine bases, how many thymine bases will it have?

Question

BlackTom AI · Accepted Answer

To solve this, recall base-pairing rules: in double-stranded DNA, C pairs with G and A pairs with T, so the counts of complementary bases are equal.
Option 1: 35. This aligns with the logic that if there are 15 cytosines, there must also be 15 guanines, giving 30 bases accounted for (C+G). The remaining bases must be adenine and thymine in equal numbers, totaling 70 bases left, so A = T = 35. Therefore, the number of thymine bases is 35.
Option 2: 30. If C = 15, then G = 15, summing to 30 for C and G. But the total DNA has 100 bases (50 base pairs × 2 strands). The remaining 70 bases are split evenly between A and T, not all 30. Thus, 30 is too small for T.
Option 3: 15. This would imply no extra bases beyond the C count, but in double-stranded DNA there must also be an equal amount of thymine and adenine to pair with each other, which would add more bases beyond the 15 C and 15 G. Therefore 15 cannot be the thymine count.
Option 4: 10. Similar to option 3, this undercounts the A–T pairings. After accounting for C and G (15 each), there are 70 bases left to be distributed as A and T in equal numbers, so 10 thymine bases is inconsistent with the requirement that A = T and with the total base count.

BIOL_2004_81774_202509 Homework 2

If a DNA molecule of 50 base pairs in length contains a total of 15 cytosine bases, how many thymine bases will it have?

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