datagridviewcomboboxcolumn

Question

Hi,
My requirement is to set combobox column for deptno column of the table dept.
When insert button is clicked,i write the following code
DataGridViewComboBoxColumn cell = new DataGridViewComboBoxColumn();
cell.DataPropertyName = "DEPTNO";
cell.HeaderText = "dno";
cell.Items.Add(10);
cell.Items.Add(20);
cell.DataSource = ds.Tables["dept"];
cell.ValueMember = "DEPTNO";
cell.DisplayMember = "deptno";
dataGridView1.Columns.Add(cell);
dt=new DataTable();
dt = (DataTable)dataGridView1.DataSource;
DataRow dr = dt.NewRow();
dt.Rows.Add();

dataGridView1.DataSource = dt;

My problem is that combobox column is displayed as a separate column.But it value member set to deptno column of dept table.And when i select 30 from that combo box control goes to deptno=30 row.
I want combox cell is set for deptno column and when we select a value from that combo box column the velue is displayed for that row.
How to do this.
Thanks in advance.