Protein synthesis, also known as translation, represents the culmination of the transfer of genetic information, stored as nucleotide bases in deoxyribonucleic acid (DNA), to protein molecules that are the major structural and functional components of living cells. It is during translation that this information, expressed as a specific nucleotide sequence in a ribonucleic acid (RNA) molecule, is used to direct the synthesis of a protein. The protein then folds into a three-dimensional structure that is defined, in large part, by its amino acid sequence. The interaction between the RNA to be translated and the protein synthetic machinery involves three main components:
- ribosomes,
- messenger RNA (mRNA),
- transfer RNA (tRNA).
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The ribosome is the machine on which all proteins are synthesized. mRNA contains the information required to direct the synthesis of the primary sequence of the protein, although only a portion of that information is used to encode the protein. tRNAs carry the amino acids that are to be incorporated into the protein. The ribosome brings together the tRNA molecule and mRNA so that the correct amino acid is incorporated into the protein. In general, the translation of mRNA begins near the 5' end and moves towards the 3' end, and proteins are synthesized starting with their amino-terminal ends and progressing toward the carboxy-terminal end; thus the 5' end of the RNA corresponds to the amino-terminal end of the protein; the 3' end of the RNA corresponds to the carboxy-terminal end of the protein. In this chapter, we begin our discussion of translation by looking at the general characteristics of the genetic code. Next, we will discuss the structure and function of the ribosome, the macromolecular ribonucleoprotein particle that synchronizes the activity of mRNA and tRNAs to produce a protein. The process of translation (initiation, elongation, and termination of protein synthesis) will be explained, and the mechanism by which proteins are targeted to specific locations in the cell will be discussed. Finally, the many modifications that proteins undergo after synthesis so that they can attain their full activity will be described briefly. During the course of this discussion, we will acknowledge that protein molecules have a defined half-life and address the role of a second macromolecular complex, the proteasome, in protein turnover.
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