The owner of any interest in registered land may mortgage the interest by executing a mortgage thereof. Such a mortgage may be assigned, extended, discharged, released in whole or in part, or otherwise dealt with by the mortgagee by any form of instrument sufficient in law for the purpose. Except as provided in part II, the mortgage, and all instruments assigning, extending, discharging, and otherwise dealing with the mortgage, shall be registered and shall take effect upon the title of the mortgaged property only from the time of registration.