Despite the wide morphological diversity, the main contents of all cells of living organisms are biological macromolecules, including proteins, nucleic acids, lipids, and carbohydrates. These macromolecules are the basis of all cellular functions and forms. Nucleic acids, proteins, and carbohydrates are produced by linking monomers of similar type by a common chemical bond. Nucleic acids are made from nucleotide building blocks joined together by phospho–diester bonds, proteins are made by joining together amino acids by peptide bonds, and carbohydrates are made from monomeric sugar molecules joined by glycosidic bonds. Lipids make an exception. They consist of glycerol forming ester bonds with fatty acids on each of its three hydroxyl groups. In this chapter, the chemical nature of the nucleic acids DNA and RNA and of proteins are discussed. In addition, the processes of DNA synthesis by replication in cells and by PCR in test tubes, the process of RNA synthesis in cells by transcription and of protein synthesis by translation are explained. Methods of genetic engineering are depicted on this basis. Finally, the chemical nature of carbohydrates and lipids is discussed.