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.
All branches of the U.S. military require you to enlist for eight years, though not all of this must be spent on active duty. When you enlist, you agree to spend a certain portion of your enlistment on active duty and the remainder as a reserve. The minimum length of your active duty time depends on the job you seek.