Question

If the total amount of adenine and thymine in a double-stranded DNA is $45\%$, the amount of guanine in this DNA will be

• A. $22.5\%$
• B. $27.5\%$
• C. $45\%$
• D. $55\%$

Answer 1

The Correct Option is B

To solve this question, we need to understand the basic principle of DNA base pairing. In a double-stranded DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). This is known as Chargaff's rule.

According to the question, the combined percentage of adenine and thymine in the DNA is \(45\%\). This means:

• \(\%A + \%T = 45\%\)

Since adenine pairs with thymine, \(\%A = \%T\). Hence, the percentage of adenine or thymine alone is:

• \(\%A = \%T = \frac{45}{2} = 22.5\%\)

Now, since the total percentage of all bases in the DNA must add up to \(100\%\), we can write:

• \(\%A + \%T + \%G + \%C = 100\%\)
• Using the known value, \(45\% + \%G + \%C = 100\%\)

Simplifying, the combined percentage of guanine and cytosine is:

• \(\%G + \%C = 100\% - 45\% = 55\%\)

Knowing that \(\%G = \%C\), we find the percentage of guanine:

• \(\%G = \%C = \frac{55}{2} = 27.5\%\)

Therefore, the amount of guanine in this DNA is \(27.5\%\). Thus, the correct answer is \(27.5\%\).