A molecule of DNA may contain thousands of genes. DNA stores within its chemical structure the information that determines an organism's hereditary properties. The physical structure of DNA is much the same in all organisms. The DNA molecule is shaped like a twisted rope ladder, called a double helix. A pair of bases forms each rung.
Most genes consist of several thousand base pairs.
The order of the bases, or the base sequence, provides the information necessary for a cell to make a specific protein. The form and function of a cell depend on the proteins it produces. Thus, the base sequences of an organism's DNA make the organism different from all other living things. Before a cell divides, enzymes split the DNA ladder lengthwise, separating the base pairs. Then, the base sequence in each half ladder directs the production of a new matching half.