In most multicellular organisms, every cell carries the same DNA, but this genetic information is used in varying ways by different types of cells. In other words, what a cell does within an organism dictates which of its genes are expressed.
Most enlisted positions require four years of active duty. If you join the Navy through an officer program, you must serve at least four of your eight years on active duty. However, pilots and flight officers must serve a minimum of eight to 10 years of active duty. Dentists, Medical Service Corps officers and nurses must serve a minimum of three years of active duty.
Currently, the president can only serve two terms of four years. When Franklin D. Roosevelt was president, there was no limit on how many terms the president could serve. But in 1951, an amendment was added to the US Constitution limiting the president to only serving two terms. This source has the whole text of the US Constitution.