Make Javascript Work Along With Ajax UpdatePanel?

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, "
Question 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.