The number and sequence of nitrogenous bases in the DNA molecule are key elements in variations that are found in chromosomes. Different organisms have different numbers of the four bases, and the sequence varies in countless ways. In all cases, however, adenine and thymine always stand opposite each other, and cytosine and guanine oppose one another. This means that the percentage of adenine and thymine will always be the same, as will the percentage of cytosine and guanine.
