Question

Bacteria and other microbes can be used to "clean up" an oil spill by breaking down oil into carbon dioxide and water. Two samples isolated from the Deepwater Horizon leak in the Gulf of Mexico were labeled A and B. The DNA of each was isolated and the percent thymine measured in each sample. Sample A contains 20.7 % thymine and sample B contains 30.9 % thymine. Assume the organisms contain normal double‑stranded DNA and predict the composition of the other bases.

Answer 1

Sample A: Thymine= 20.7% ; Adenine= 20.7%; Guanine = 29.3% ; Cytosine= 29.3%

Sample B: Thymine= 30.9% ; Adenine= 30.9% ; Guanine= 19.1% ; Cytosine base= 19.1%

Explanation:

In a double-stranded DNA molecule, the percentage of adenine base is equal to that of the thymine base while the percentage of guanine base is equal to that of the cytosine base. It is called Chargaff's rule and is based on the complementary base pairing of purine and pyrimidine bases.

Sample A: Percentage of thymine= 20.7%

This means that the sample has= 20.7% adenine base

Total thymine and adenine bases in sample A= 20.7 + 20.7 = 41.4%

Therefore, proportion of guanine + cytosine bases in the sample= 100-41.4= 58.6%

Percentage of guanine base= 58.6/2 = 29.3%

Percentage of cytosine base= 29.3%

Sample B: Percentage of thymine= 30.9%

This means that the sample has= 30.9% adenine base

Total thymine and adenine bases in sample A= 30.9 + 30.9 = 61.8%

Therefore, proportion of guanine + cytosine bases in the sample= 100-61.8= 38.2%

Percentage of guanine base= 38.2/2 = 19.1%

Percentage of cytosine base= 19.1%