The whole conformation of a comb-like polymethacrylate (PMA-CH3) has been determined by small-angle neutron scattering (SANS) in the isotropic and nematic phases. Its molecular weight dependence is obtained from six well-defined M values. In the isotropic phase, the persistence length is found to be l(p) = 10 +/- 1.5 Angstrom and the relation between the radius of gyration, R-g, and M or N, the number of monomers, is R-g = 0.143M(1/2) = 2.85N(1/2). Therefore, there is practically no difference between the conformation of the PMA-CH3 backbone and that of the polystyrene. This result is striking since it appears that the backbone conformation of a polymer with a hanging group of 23 Angstrom length every 2.5 Angstrom along the chain is quite the same as that of the standard of linear polymers. In the unoriented nematic phase, the same values of l(p) and R-g are found. This means that the l(p) value remains unchanged in the nematic phase. In the oriented nematic phase, the values of the inertial radii of the chain, R-parallel to and R-perpendicular to, in the direction parallel and perpendicular to the nematic director have been measured as a function of M : R-perpendicular to = 0.12M(0.48+/-0.03) and R-parallel to = 0.015M(0.63+/-0.05). These results are confirmed by the dependence of the form factor on the scattering vector in the intermediate range. They are discussed in term of smectic fluctuations.