Just because a banana is brown doesn’t mean it’s gone bad. Even black bananas, as scary as they look, can be used in smoothies or banana ice cream. Leaving bananas in their bunch is the best way to store them - hanging or placed in a bowl with their curved side facing down to avoid bruises. 

Bananas release ethylene gas through their stems. The more gas that is released, the riper the fruit. So by wrapping the ends of the stems in plastic wrap, you prevent or slow down this gas from escaping.