If a DNA double helix is 100 nucleotide pairs long, it contains 100 pairs of bases. According to the base pairing rules:
- Adenine (A) pairs with Thymine (T)
- Guanine (G) pairs with Cytosine (C)
This means the number of adenine bases equals the number of thymine bases, and the number of guanine bases equals the number of cytosine bases
. If the DNA contains a certain number of adenine bases, say xxx, then there are also xxx thymine bases. Since the total number of base pairs is 100, the remaining base pairs must be guanine-cytosine pairs:
Total base pairs=100=A-T pairs+G-C pairs=x+G-C pairs\text{Total base pairs}=100=\text{A-T pairs}+\text{G-C pairs}=x+\text{G-C pairs}Total base pairs=100=A-T pairs+G-C pairs=x+G-C pairs
Therefore, the number of guanine bases (which equals the number of cytosine bases) is:
Guanine bases=100−x\text{Guanine bases}=100-xGuanine bases=100−x
So, if you know the number of adenine bases, subtract that from 100 to find the number of guanine bases in the double helix. For example, if there are 25 adenine bases, then there are 100−25=75100-25=75100−25=75 guanine bases
