Asp Grid View Check Box To Presrver On Neigation(paging)?

In a grid view i have used paging for that i have used the view state to store datatale to bind it on GridView1_PageIndexChanging event every thing works fine but the problem happens with the first column which is having the checkbox placed in each row .On navigation all checked check box becomes unchecked how to maintain the state of check box as well.this is the aaspx code

<Columns>
<asp:TemplateField HeaderText="Select Student">
<ItemTemplate>
<asp:CheckBox id="Chek" runat="server" Text="select" ></asp:CheckBox>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField HeaderText="Enrollment No." DataField="enrollment_no" />
<asp:BoundField HeaderText="Course Name" DataField="course_name"/>
<asp:BoundField HeaderText="Branch Name" DataField="branch_name"/>
<asp:BoundField HeaderText="Email Id" DataField="email" />
<asp:BoundField HeaderText="Mobile" DataField="mobile"/>
<asp:BoundField HeaderText="Name" DataField="first_name"/>
<asp:BoundField HeaderText="Surname" DataField="last_name" />
</Columns>
</asp:GridView>