How to make javascript work along with Ajax UpdatePanel

Question

I am trying to add validation on my form. I am using AJAX controls in my form fields. When I remove the Update panel and AJAX control, my validation starts working, but when keeping both the things together, my validation is not working. How could I make them work together?

<script type="text/javascript">

    function Validate() {
        var QuestionTextArea = document.getElementById("ctl00_ctl00_cphBody_midbox_fvInsert_txtQuestion");

        varError = "";
        if (!IsTextBoxEmpty(QuestionTextArea, "\nQuestion Text Area  not be Empty.")) {
            alert(varError);
            document.getElementById("ctl00_ctl00_cphBody_midbox_fvInsert_txtQuestion").focus();
            return false;
        } return true;
    }
</script>



    <asp:UpdatePanel ID="UpdatePanel2" runat="server">
            <ContentTemplate>

             <asp:TextBox ID="txtQuestion" runat="server" MaxLength="1000" Columns="50" Rows="5" Style="width: 380px;
             float: none" Text='<%# Bind("Description") %>' TextMode="MultiLine" />
             <AjaxControl:TextBoxWatermarkExtender runat="server" TargetControlID="txtQuestion"
              WatermarkCssClass="water" WatermarkText="Type your Question Here.">
             </AjaxControl:TextBoxWatermarkExtender>


           </ContentTemplate>
        </asp:UpdatePanel>

When I removes ajax extender and Update Panel. My textbox gets validated and when using UpdatePanel. No javascript function created by me is called.