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.
You can also read more about the Doctor of Philosophy programmes in general, or about studying in Germany. The Ph.D. (also PhD, Doctor of Philosophy) is the highest postgraduate degree that can be achieved at universities. People awarded with a PhD are normally allowed to have a teaching position at a university.