A combined experimental and theoretical study is conducted on guanine molecules adsorbed on graphite substrate. The main effort is to understand the scanning tunneling microscopic images and the effect of scanning tip force on guanine molecules and their images. The force on guanine molecule is determined from the tip-induced deformation in the substrate and is found to be a few nano-Newtons per guanine molecule. A jellium substrate is used to model the graphite substrate in ab initio calculations of electronic density of states. Both the equilibrium height of guanine and tip force on guanine are determined using pseudopotential density functional calculations of system’s total energy. The calculated height of guanine molecule above the substrate is in good agreement with measurements. Electronic states are calculated and it is shown that features in surface density of states are mostly due to admixture with the guanine LUMO and the admixture increases with increasing scanning tip force.