Molecular biology / məˈlɛkjʊlər / is a branch of biology that seeks to understand the molecular structures and chemical processes that are the basis of biological activity within and between cells. It is centered largely on the study of nucleic acids (such as DNA and RNA) and proteins.
If a substance has molecular weight M, then M grams of the substance is termed one mole. The number of molecules in one mole is the same for all substances; this number is known as Avogadro’s number (6.022140857 × 10 23).