So, let's get started.
This is question #7 taken from the June 2010 exam (nyslivingenvironment.net):
The correct answer here is (2) 35%. DNA is made up of 4 bases: adenine (A), guanine (G), thymine (T), and cytosine (C). Adenine and guanine are known as purines, while thymine and cytosine are pyrimidines. In a DNA molecule, one purine is always paired to one pyrimidine, thus they pair in a 1:1 ratio. Adenine (purine) pairs with thymine (pyrimidine), and guanine (purine) pairs with cytosine (pyrimidine). For every one adenine, there will be one thymine. For every one guanine, there will be one cytosine.
A way I was taught to remember the base pairs was with the phrase At Garden City. Garden City is a town near where I grew up, and if you remembered the phrase AT Garden City, you would remember A pairs with T and G pairs with C.
In any case, if we know that a DNA sample is made up of 15% thymine (T), that means it must also be made up of 15% adenine (T). 15% + 15% = 30%. Therefore, the other 70% (100% - 30% = 70%) of the molecule must be made up of guanine (G) and cytosine (C). And if there is an equal amount of both of these bases, we know that 70% must be divided by 2, to get 35% C and 35% G. Make sense?
For more general information on DNA, check out these videos:
http://www.brainpop.com/science/cellularlifeandgenetics/dna/
http://learn.genetics.utah.edu/content/begin/tour/
For more practice, you can build your own DNA molecule here:
http://learn.genetics.utah.edu/content/begin/dna/builddna/
