Introduction

The extraordinary versatility of proteins in catalyzing chemical reactions and building cellular structures was featured in the previous chapter. In this chapter we consider the nucleic acids, the molecules that (1) contain the information prescribing amino acid sequence in proteins and (2) serve in the several cellular structures that choose, and then link into the correct order, the amino acids of a protein chain. Deoxyribonucleic acid (DNA) is the storehouse, or cellular library, that contains all the information required to build the cells and tissues of an organism. The exact duplication of this information in any species from generation to generation assures the genetic continuity of that species. The information is arranged in units identified by classical geneticists from Gregor Mendel through Thomas Hunt Morgan, and known now as genes, hereditary units controlling identifiable traits of an organism. In the process of transcription, the information stored in DNA is copied into ribonucleic acid (RNA), which has three distinct roles in protein synthesis. Messenger RNA (mRNA) carries the instructions from DNA that specify the correct order of amino acids during protein synthesis. The remarkably accurate, stepwise assembly of amino acids into proteins occurs by translation of mRNA. In this process, the information in mRNA is interpreted by a second type of RNA called transfer RNA (tRNA) with the aid of a third type of RNA, ribosomal RNA (rRNA), and its associated proteins. As the correct amino acids are brought into sequence by tRNAs, they are linked by peptide bonds to make proteins.

Discovery of the structure of DNA in 1953 and the subsequent elucidation of the steps in the synthesis of DNA, RNA, and protein are the monumental achievements of the early days of molecular biology. To understand how DNA directs synthesis of RNA, which then directs assembly of proteins - the so-called central dogma of molecular biology  - we first discuss the building blocks composing DNA and RNA, the cell’s two primary nucleic acids, and their general structures. We then introduce the basic mechanisms of DNA and RNA chain synthesis, including a brief discussion of gene structure, which reveals why molecular processing is required to make functional RNA molecules. Next we outline the roles of mRNA, tRNA, and rRNA in protein synthesis. The chapter closes with a detailed description of the components and biochemical steps in the formation of proteins. Since the events of macromolecular synthesis are so central to all biological functions  - growth control, differentiation, and the specialized chemical and physical properties of cells - they will arise again and again in later chapters. A firm grasp of the fundamentals of DNA, RNA, and protein synthesis is necessary to follow the subsequent discussions without difficulty.

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