Prior to the Magic 2010 release, mana that was unspent at the end of a phase caused ‘mana burn,’ meaning that you would lose one life for each unspent mana in your mana pool. While it no longer causes mana burn, mana is still emptied from your pool. In fact, it now empties at the end of every step and phase, and not just at the end of each phase. This means that you can no longer generate mana in the upkeep step and then spend it in your draw step. This also applies to the combat phase and the end phase, which are made up of multiple steps.