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.
The normal term of engagement is 22 years; however, the minimum service required before resignation is 4 years, plus, in the case of the Army, any service person below the age of 18. At present, the yearly intake into the armed forces is 11,880 (per the 12 months to 31 March 2014).