\begin{tikzpicture} \tikzstyle{point}=[circle,thick,draw=black,fill=black,inner sep=0pt,minimum width=4pt,minimum height=4pt] \draw [red, thick,domain=90:-90, samples=100] plot ({cos(\x)}, {sin(\x)}); \draw [green,thick,domain=-90:-270,samples=100] plot ({cos(\x)}, {sin(\x)}); \node (a)[point,label=90:$a$] at (0,-1cm) {}; \node (b)[point,label=-90:$b$] at (0, 1cm) {}; \node at (1,0) [anchor=180, red] {$\gamma_1$}; \node at (-1,0) [anchor=0, green] {$\gamma_2$}; \end{tikzpicture}