If a structural gene in an organism's genome is comprised of 33% adenine nucleotides, what...
If a structural gene in an organism's genome is comprised of 33% adenine nucleotides, what percentage of the gene is comprised of cytosine nucleotides?
- A. 22%
- B. 33%
- C. It cannot be determined from the given information.
- D. 17%
Correct Answer: D. 17%
Explanation
According to Chargaff's rule, DNA nucleotides pair in a 1:1 ratio. Therefore, if we know how much of the particular gene is made up of one nucleotide, we can extrapolate that known variable to find the other three unknown variables. To do so, you must remember that adenine pairs with thymine, and cytosine pairs with guanine (A-T, C-G), and that since the ratio between each pair is 1:1 then a gene with 33% adenine must also have 33% thymine. Combine these numbers and subtract from 100: the number leftover is the % of total cytosine and guanine in the gene. 100% - 66% = 34% Finally, since we know that 34% of the DNA is both C and G, and that the ratio between C-G is 1:1, C and G must both be 17%.