[chykmandar]

When Amino acid allow to reacts with urea in water at refluxing temperature, 8hs, cooling & acidifying with HCl gives product with N-carbamoyl amino acid

HOOC-CH(R)-NH2 + H2N-CO-NH2 --- > HOOC-CH(R)-NH-CO-NH2 + NH3
 
explain the mechanism.
any role of being zwitter ion?

[Ligander]

IMHO(!!!) you can try to derive answer  from here.

http://en.wikipedia.org/wiki/File:Amine_plus_Carboxylic_Acid.PNG