We study the evolution of solar (Z = 0.016) and metal-poor stars (Z = 0.001) of initial masses corresponding to 2M⊙, and 4M⊙, from the main sequence until the formation of degenerate carbon-oxygen cores. We explore the nucleosynthesis during the core hydrogen burning phase through the CNO cycle and during the core helium burning phase through the 3α reaction. We determine the combined effects of the nucleosynthesis and of the convective mixing episodes on the surface composition of the considered stars. This work is intended to be a starting point to analyze the effects of more sophisticated treatments of mixing by convection, making special emphasis in the case of metal-poor stars.