Unwound Dna

Since A•T base pairs contain two hydrogen bonds and C•G base pairs contain three, A+T-rich tracts are less thermally stable that C+G-rich tracts in DNA. Under denaturing conditions (heat or alkali), the DNA begins to "melt" (separate), and unwound regions of DNA will form, and it is the A+T-rich sequences that melt first. In addition, in the presence of superhelical energy (a high-energy state of DNA resulting from its supercoiling, which is the natural form of DNA in the chromosomes of most organisms), A+T-rich regions can unwind and remain unwound under conditions normally found in the cell. Such sites often provide places for DNA replication proteins to enter DNA to begin the process of chromosome duplication.