Final answer:
Based on Chargaff's rules, if double-stranded DNA contains 27.8% cytosine, then it also contains 27.8% guanine, and the remaining percentage (44.4%) will be split evenly between adenine and thymine, resulting in 22.2% adenine and 22.2% thymine.
Step-by-step explanation:
If a researcher finds that double-stranded DNA contains 27.8% cytosine, we can draw conclusions based on Chargaff's rules. According to these rules, the amount of cytosine (C) will be equal to the amount of guanine (G) in the DNA because they pair with each other. Therefore, the percentage of guanine is also 27.8%. Knowing that the total percentage of the four nitrogenous bases must add up to 100%, and that adenine (A) pairs with thymine (T), we can calculate the percentages of adenine and thymine.
To find the percentage of adenine and thymine, we first calculate the combined percentage of cytosine and guanine: 27.8% + 27.8% = 55.6%. Subtracting this from 100% gives us the combined percentage for adenine and thymine: 100% - 55.6% = 44.4%. As adenine equals the amount of thymine, we divide this percentage by 2 to get the percentage for each: 44.4% / 2 = 22.2%. Therefore, the DNA contains 22.2% adenine and 22.2% thymine as well.