Nucleic acids carry an organism's genetic information and determine the physical characteristics and functions of cells. The two major classes are ribonucleic acids (RNA) and deoxyribonucleic acids (DNA). DNA is found in the nucleus and carries the permanent genetic code. It is large and stable, allowing transmission of genetic information between generations. RNA molecules are smaller and less stable than DNA. They commonly serve as working copies of DNA, with messenger RNA leaving the nucleus to direct protein synthesis in ribosomes.