Normal and superconducting state spectral properties of cuprates are theoretically described within the extended <i>t-J</i> model. The method is based on the equations of motion for projected fermionic operators and the mode-coupling approximation for the self-energy matrix. The dynamical spin susceptibility at various doping is considered as an input, extracted from experiments. The analysis shows that the onset of superconductivity is dominated by the spin-fluctuation contribution. The coupling to spin fluctuations directly involves the next-nearest-neighbor hopping <i>t'</i>, hence <i>T<sub>c</sub></i> shows a pronounced dependence on <i>t'</i>. The latter can offer an explanation for the variation of <i>T<sub>c</sub></i> among different families of hole-doped cuprates. A formula for maximum <i>T<sub>c</sub></i> is given and it is shown that optimum doping, where maximum <i>T<sub>c</sub></i> is reached, is with increasing <i>-t'</i> progresively increased.