  Given a graph $G$, the parameters $\chi(G)$ and $\omega(G)$ respectively
denote the chromatic number and the clique number of $G$. A function $f :
\mathbb{N} \rightarrow \mathbb{N}$ such that $f(1) = 1$ and $f(x) \geq x$, for
all $x \in \mathbb{N}$ is called a $\chi$-binding function for the given class
of graphs $\cal{G}$ if every $G \in \cal{G}$ satisfies $\chi(G) \leq
f(\omega(G))$, and the \emph{smallest $\chi$-binding function} $f^*$ for
$\cal{G}$ is defined as $f^*(x) := \max\{\chi(G)\mid G\in {\cal G} \mbox{ and }
\omega(G)=x\}$. In general, the problem of obtaining the smallest
$\chi$-binding function for the given class of graphs seems to be extremely
hard, and only a few classes of graphs are studied in this direction. In this
paper, we study the class of ($P_2+ P_3$, gem)-free graphs, and prove that the
function $\phi:\mathbb{N}\rightarrow \mathbb{N}$ defined by $\phi(1)=1$,
$\phi(2)=4$, $\phi(3)=6$ and
$\phi(x)=\left\lceil\frac{1}{4}(5x-1)\right\rceil$, for $x\geq 4$ is the
smallest $\chi$-binding function for the class of ($P_2+ P_3$, gem)-free
graphs.
