Ubuntu iso

(sorry for bad english ... !)
Problem with make bootable iso .
I downloaded Kubuntu/Ubuntu desktop iso ;
I can make bootable USB with this command :
dd if=Ubuntu-12.04-Desktop.iso of=/dev/sdc

I only extract and repack this iso with this command :

sudo genisoimage -r -V "$IMAGE_NAME" -cache-inodes -J -l \
  -b isolinux/isolinux.bin -c isolinux/boot.cat \
  -no-emul-boot -boot-load-size 4 -boot-info-table -o ../ubuntu-remix.iso .

but can not make usb bootable with this new ISO !
I mean : dd command can not make usb bootable !
How can fix this problem ?
What command make original Ubuntu iso ?