Your cells contain two types of nucleic acids, ribonucleic acid and deoxyribonucleic acid, or RNA and DNA, respectively. They differ from the other macronutrients in that they are not a source of calories in your diet, and their role is strictly to direct the synthesis of new protein molecules.
The definition of a postgraduate is a student who is studying a subject beyond the college level when he already has a college degree. Someone attending medical school is an example of a postgraduate.
Thus, the graduate study is advanced course work which follows undergraduate course work. An undergraduate degree is awarded for the completion of 2 years (associates) or 4 years (bachelors) of college level study. A graduate degree is awarded after attending graduate school.