Electron micrograph of DNA (green arrow) being transcribed into RNA (red arrow). [O. L. Miller, Jr., and Barbara R. Beatty, Oak Ridge National Laboratory.]

The extraordinary versatility of proteins as molecular machines and switches, cellular catalysts, and components of cellular structures was described in Chapter 3. In this chapter we consider the nucleic acids. These macro-molecules (1) contain the information for determining the amino acid sequence and hence the structure and function of all the proteins of a cell, (2) are part of the cellular structures that select and align amino acids in the correct order as a polypeptide chain is being synthesized, and (3) catalyze a number of fundamental chemical reactions in cells, including formation of peptide bonds between amino acids during protein synthesis.

Deoxyribonucleic acid (DNA) contains all the information required to build the cells and tissues of an organism. The exact replication of this information in any species assures its genetic continuity from generation to generation and is critical to the normal development of an individual. The information stored in DNA is arranged in hereditary units, now known as genes, that control identifiable traits of an organism. In the process of transcription, the information stored in DNA is copied into ribonu-cleic 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 subsequent elucidation of how DNA directs synthesis of RNA, which then directs assembly of proteins—the so-called central dogma—were monumental achievements marking the early days of molecular biology. However, the simplified representation of the central dogma as DNAnRNAnprotein does not reflect the role of proteins in the synthesis of nucleic acids. Moreover, as discussed in later chapters, proteins are largely responsible for regulating gene expression, the entire process whereby the information encoded in DNA is decoded into the proteins that characterize various cell types.

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