  The flexibility and the extension along the direction of the force are shown
to be related to the bubble number fluctuation and the average number of
bubbles respectively, when the strands of the DNA are subjected to a force
along the same direction, here we call a stretching force. The
force-temperature phase diagram shows the existence of a tricritical point
(TCP), where the first-order force induced zipping transition becomes
continuous. On the other hand, when the forces are being applied in opposite
directions, here we call an unzipping force, the transition remains
first-order,with the possibility of vanishing of the low-temperature re-entrant
phase for a semiflexible DNA. Moreover, we found that the bulk elasticity
changes only if an external force penetrates the bound phase and affect the
bubble states
