DNA has two complementary strands with each base being bonded to a specific partner, adenine to thymine and guanine to cytosine. When replicating, each strand acts as a template onto which a new complemetray strand is constructed. The nucleotides then bond to form new backbones and two new identical DNA molecules result, both containing 50% of the original bases. After another round of DNA replication this will reduce by 50% again, in other words to 25% overall.