The transition from inorganic to organic chemistry in the late 19th century added proteins and nucleic acids, alongside sugars, starches, woods and fats, to the portfolio of components of living organisms. The disciplines of biochemistry and molecular biology emerged in the 20th century with the elucidation of metabolic and biosynthetic pathways and the demonstration that proteins are enzymes. Microbial genetics focused on metabolic enzymes led to the conclusion that genes are synonymous with proteins. Proteins were thought to be the central molecules of life and the active components of the chromosomes that transmitted genetic information; the other main component, DNA, seemed too simple and was thought to act simply as a scaffold. Genetic mapping developed along with the concept of ‘genes’ as discrete entities. Theoretical mathematical biology flourished, influenced by microbial genetics and the ‘lethality’ of mutations. Darwinian evolution and Mendelian inheritance were integrated in the ‘Modern Synthesis’, which asserted that mutations are random and that Lamarckian inheritance of experience does not occur. DNA and RNA were initially confused, but the former was found to be mainly located in the nucleus and the latter in the cytoplasm. DNA was eventually shown to contain the genetic information by transforming the phenotype of bacteria, but this finding was slow to be accepted until the elucidation of the structure of DNA with its evident ability to be replicated according to the rules of base pairing and to encode information in the sequence of its nucleotides.