asp.net dropdownlist güncelleme işlemi

Merhaba arkadaşlar aşağıda bulunan kodlarda güncelleme işlemi yaparken dropdownliste kayıtlı veriyi getiremiyorum.Bu konuda yardımcı olur musunuz.

default.aspx

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" BackColor="White" BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px" CellPadding="3" DataSourceID="SqlDataSource1">
<Columns>
<asp:TemplateField HeaderText="DETAY">
<ItemTemplate>
<asp:ImageButton ID="ImageButton3" ImageUrl="~/image/view.png" runat="server" Width="35" Height="35" OnClick="Imagebutton3_Click" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="DÜZENLE">
<ItemTemplate>
<asp:ImageButton ID="ImageButton4" ImageUrl="~/image/edit.png" runat="server" Width="35" Height="35" OnClick="Imagebutton4_Click" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="sicil" HeaderText="SİCİL" SortExpression="sicil" />
<asp:BoundField DataField="adsoyad" HeaderText="AD SOYAD" SortExpression="adsoyad" />
<asp:BoundField DataField="unvan" HeaderText="ÜNVAN" SortExpression="unvan" />
</Columns>
<FooterStyle BackColor="White" ForeColor="#000066" />
<HeaderStyle BackColor="#006699" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="White" ForeColor="#000066" HorizontalAlign="Left" />
<RowStyle ForeColor="#000066" />
<SelectedRowStyle BackColor="#669999" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F1F1F1" />
<SortedAscendingHeaderStyle BackColor="#007DBB" />
<SortedDescendingCellStyle BackColor="#CAC9C9" />
<SortedDescendingHeaderStyle BackColor="#00547E" />
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="SELECT * FROM [personel] order by[sicil] asc"></asp:SqlDataSource><asp:Button ID="btn2ShowPopup" runat="server" style="display:none" />
<ajx:ModalPopupExtender ID="mp2" runat="server" PopupControlID="Panel3" TargetControlID="btn2ShowPopup"
BackgroundCssClass="modalBackground" >
</ajx:ModalPopupExtender>
<asp:Panel ID="Panel3" runat="server" CssClass="modalPopup" align="left" style = "display:none">
<div class="form">
<div class="formtitle">Personel Düzenle</div>
<table class="style1">
<tr>
<td>

<table class="style1">
<tr>
<td class="style2">
Sicil</td>
<td>

<asp:TextBox ID="TextBox1" runat="server" Width="306px"></asp:TextBox>

</td>
</tr>
</table>
<table class="style1">
<tr>
<td class="style2">
Ad Soyad</td>
<td>

<asp:TextBox ID="TextBox2" runat="server" Width="306px"></asp:TextBox>

</td>
</tr>
</table>
<table class="style1">
<tr>
<td class="style2">
Ünvan</td>
<td>

<asp:DropDownList ID="DropDownList1" runat="server" Width="310px">
</asp:DropDownList>

</td>
</tr>

default.aspx.cs

protected void Imagebutton4_Click(object sender, ImageClickEventArgs e)
{

ImageButton btndetails = sender as ImageButton;
GridViewRow gvrow = (GridViewRow)btndetails.NamingContainer;
TextBox1.Text = HttpUtility.HtmlDecode(gvrow.Cells[2].Text);
TextBox2.Text = HttpUtility.HtmlDecode(gvrow.Cells[3].Text);
DropDownList1.SelectedValue = gvrow.Cells[4].ToString();

this.mp2.Show();

}