A molecular theory of the helical twisting in chiral liquid crystals is developed, which provides an explanation for the experimentally observed helical sense inversion induced by a change of concentration in binary mixtures of chiral and nonchiral nematic liquid crystals. The theory also describes the sense inversion induced by a change of temperature observed in some single component nematics. The theory present is based on a simple model of a chiral rigid molecule, composed of several equivalent nonchiral sites, which are arranged in the molecule to form a chiral configuration. The macroscopic helical pitch in the chiral nematic phase, twist elastic constant, and nematic order parameters are calculated using the same molecular model. It is shown that the helical sense inversion can be determined by a large biaxiality of chiral molecules. It is also demonstrated that the biaxiality is important in determining the variation of the helical pitch with temperature and concentration.