If 30% of the nitrogenous bases in a sample of DNA from a particular organism are thymine, what percentage should be cytosine?

Question

BlackTom AI · Accepted Answer

To solve this, recall Chargaff's rules: in double-stranded DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine. This means A = T and G = C, and A + T + G + C = 100%.

First, T is 30%, so by base pairing A must also be 30%. That leaves 40% for the remaining bases, which are G and C together. Since G = C, each of them must be half of 40%, giving G = 20% and C = 20%.

Now let's evaluate the options:
- 15%: This would imply G and C are not equal and would not fit the 40% remainder after accounting for A and T; it misallocates the base composition.
- 20%: This matches the calculated C value under Chargaff's rules (G would also be 20%), so it is consistent with the DNA base-pairing principle.
- 30%: If C were 30%, then G would also be 30%, totaling 60% for G+C, which would exceed the remaining 40% after A and T, impossible with A = T = 30%. 
- 40%: Interpreting C as 40% would force G to be 0% to keep G=C, which contradicts the need for all four bases summing to 100%.
- 70%: This would violate the equal pairing requirement and would leave negative or zero percentages for the other bases, which is not possible.

In summary, the only option that aligns with base-pairing rules and the given T percentage is 20% for cytosine (and, correspondingly, 20% for guanine), completing the 100% total.

MOLGEN 4500E AU2025 (25000) Exam 4- Final Exam- Requires Respondus LockDown Browser

If 30% of the nitrogenous bases in a sample of DNA from a particular organism are thymine, what percentage should be cytosine?

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