How does ithe final chapter serve to wrap up the theme of the entire story?

Lots of things happen in the last chapter of The
Grapes of Wrath which are significant to the overall themes of the
novel. 

1. A flood displaces the Joads and others
   from their homes, just as another natural disaster prompted their initial move from
   Oklahoma.


2. The children are exposed to aspects of life
   they shouldn't have to see at such a young age (and I'm not making the case that
   childbirth is ugly, of course, just that they would not normally have been observers of
   the process).


3. People are willing to help but must, in the
   end, do what's best for their families.


4. Working hard (as
   in building the levee to hold back the flood water) may end in futility.  Sometimes it's
   just not enough.


5. The family is breaking up even further
   as Al and Aggie leave the Joad camp.


6. There is death and
   sorrow in the form of Rose of Sharon's stillborn child.  More grieving for this family. 
   


7. They leave and have nowhere to
   go.


8. Leaving is hard, and they have to leave things behind
   when they go.


9. BUT, there is hope.  Rose of Sharon is able
   to share a life-giving sustenance to one who is in need, and this scene(though a little
   off-putting to many young readers, for sure) offers us hope that this family will once
   again survive whatever struggles and obstacles are ahead of
   them.

You will undoubtedly find more as you
begin looking for them, and I hope you will.