Hello Bala,
The normal way would be to use some scripts which modify the attributes (like Mandatory) depending on the value of a separate field.
Unfortunatey the scripts are not available for extending UIs, only in own UIs. 
What you can use is a UI Switch.
Alessandro has written a blog in which he describes how to set a UI Switch dynaimcally.
May this helps you.
Bye,
Horst