It has been shown that the shape of Scatchard isotherms upon competitive binding of two ligands to the same binding site in the three-component ligand 1-ligand 2-DNA system depends crucially on the binding constant values. The binding isotherm of ligand 2 in the presence of the competitive ligand 1 turns back (has a bow-like form) when the binding constant of the first ligand is larger than the binding constant of the second one.