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.
A PhD or Doctor of Philosophy is a postgraduate academic degree that is awarded for several subject areas which vary from country to country, time period or the institution. However, the term philosophy does not merely refer to the field of philosophy, but it is used in a broader sense.