problem in rotation of sphere with cubes

I write a code which draw 3 cubes and part of sphere.(In the attachment)

Moving mouse while pressed mouse left button will rotate them.
right button will translate them.
rotate mouse scroll will zooming them.

I have 2 problems
1-there is something wrong in rotation as sphere always appear above all cubes.
2-the part of sphere is good for front face while the back is very bad.

Note : I used OpenTK under C# express 2010

(only from picture)
do you request a depth buffer at GL window creation, do you enable depth test, do you enable backface culling ?

yes, I enabled depth test with DepthFunction.Less and CullFaceMode.FrontAndBack

nothing solve the problem

Fine but what about “request a depth buffer at GL window creation” ?

can you help me how to request.

can any one help me to solve this problem shown in the photo?

can I get help to solve this issue.