The determination of the base sequence of a nucleic acid is much more difficult than the determination of the sequence of amino acids of a protein, because there are only 4 types of monomers instead of 20. This explains why one began with the determination of the sequences of transfer ribonucleic acids (more than 200 sequences are known); these do [...]
Pluricellular organisms are characterized by the existence of tissues, i.e. sets of cells which have acquired specialized functions. This specialization is advantageous in the sense that it permits a distribution of work but it necessitates means of controlling and integrating the activities of various tissues. Hormones constitute precisely a means of communication between territories distant from one another. It was [...]
It is not surprising that the metabolism of these two aromatic amino acids is closely linked because they differ only by the presence of a phenolic OH in the tyrosine molecule. We will not study here their biosynthesis which — in organisms where it takes place, especially bacteria — starts from 2 compounds of the carbohydrate metabolism (erythrose-4-℗ and phosphocnolpyruvate) [...]