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.
Undergraduate degree is a degree you get from graduating a four-year university for the first time. A graduate degree is a degree you get from graduating from a graduate school, which you can only attend after you have graduated from a four-year university.