It is mostly likely the cell membrane.
The cell membrane is a structure of a cell composed of double layer of lipids and proteins. It surrounds a cell and separates the cytoplasm from its surrounding environment. The cell membrane is selectively permeable (semi-permeable), which means that it only lets certain molecules enter and exit. It can also control the amount of some substances that go into or out of the cell.
The base pairing rules in any DNA molecule are these:
• Adenine always pairs with Thymine(A-T)
• Cytosine always pairs with Guanine (C-G)
