Question regarding gear shifting on a motorcycle?

I just recently got my motorcycle license and a new bike? Quick questions in regards to gear shifting. Is it possible to hold the clutch down the entire time while shifting ( lets say your in 4th.. you need to go to 1st to slow down, can you simple hold down the clutch and click down to 1st? or do you have to let go of the clutch after each gear shift down and then grab it again?) also in regards to any turns, what gear should you be in? can you be in 3rd or 4th while in a turn or so you have to be in 2nd?