Question

You analyze a DNA sample and find that its base composition is 30% A, 20% T, 30% G, and 20% C. What can you conclude about the structure of this DNA?
Because A equals G, and T equals C, this must be a double-stranded DNA molecule.
Because A does not equal T, and G does not equal C, this cannot be a double-stranded DNA molecule. It must therefore be a single-stranded DNA.
Because A is half as much again as T, and G is half as much again as C, this must be a mutated double-stranded DNA molecule.
There is not enough information to determine whether it is a single-stranded or a double-stranded DNA molecule.

Answer 1

Answer:

Because A does not equal T, and G does not equal C, this cannot be a double-stranded DNA molecule. It must therefore be a single-stranded DNA.

Explanation:

The base composition in the observed DNA sample does not follow the Chargaff rule. If it was a double-stranded DNA, the percent proportion of adenine base and thymine base should be equal (either 30% or 20% for both). Similarly, the percent proportion of guanine and cytosine bases must also be equal  (either 30% or 20% for both). Since the given sample does not follow it, it is not a double-stranded DNA molecule but is present as a single strand.