Question

When it comes to naturally occurring double-stranded DNA molecules, which of these is always true?
1: [G] + [C] = 50%
2: [T] + [C] = 50%
3: [G] + [T] = 50%

urgent ill upvote

Answer 1

According to Chargaff's Rule which states that in every DNA of every organisms the base pairs i.e. of pyrimidine and purine bases are in the ratio of 1:1. Adenine is always in equal quantity with Thymine and Guanine is always in equal quantity with Cytosine. When it comes to naturally occurring double-stranded DNA molecules, concentration of guanine[G] will be equal to the concentration of cytosine[C] and the concentration of Adenine[A] equal to that of Thymine [T].The whole of the DNA strand would be constituting of only these 4 base pairs.

Thus [A]+[G]+[T]+[C] = 100% --------------(1)

and

[A]=[T] & [G]=[C] .----------------------(2)

Thus, using (1) and (2)

2[T]+2[G] = 100%

[T]+[G]=100/2 = 50%

& similarly,

2[T]+2[C]=100%

[T]+[C] = 50%

while with just information about G and C we cannot find the percentage.

Thus the always true options are:

2: [T] + [C] = 50%
3: [G] + [T] = 50%