Figure 7.9 The Process of RNA Synthesis
RNA. The termination of transcription should not be confused with the termination of translation, which occurs by a totally different mechanism and will be discussed shortly.
Translation is the process of decoding the information carried on the mRNA to synthesize the specified protein. Proteins are synthesized by adding amino acid subunits sequentially to the carboxyl group at the end of an elongating polypeptide chain. Each amino acid added is specified by one codon of the mRNA, as directed by the genetic code. The process of translation requires three major components—mRNA, ribosomes, and tRNAs—in addition to various accessory proteins (table 7.3). ■ carboxyl group, p. 26
The mRNA is a temporary copy of genetic information; it carries encoded instructions for synthesis of a specific polypeptide, or in the case of a polycistronic message, a specific group of polypeptides. That information is deciphered using the genetic code (figure 7.10). This is a universal code, used by all living things, and correlates each series of three nucleotides, a codon, with one amino acid. Because a codon is a sequence of any combination of the four nucleotides, there are 64 different codons (43). Three of these are stop codons; these will be discussed later. The remaining 61 translate to the 20 different amino acids. This means that more than one codon can encode a specific amino acid. For example, both ACA and ACG encode the amino acid threonine. Because of this redundancy, the genetic code is said to be degenerate. Note, however, that two different amino acids are never coded for by the same codon.
An equally important aspect of mRNA is that it carries the information that indicates where the coding region actually begins. This is critical because the genetic code is read as groups of three nucleotides. Thus, any given sequence has three possible reading frames, or ways in which triplets can be grouped (figure 7.11). If translation occurs in the wrong reading frame, a very different, and generally non-functional, polypeptide would be synthesized.
Ribosomes serve as the sites of translation, and their structure facilitates the joining of one amino acid to another. Ribosomes bring each amino acid into a favorable position so that an
Chapter 7 The Blueprint of Life, from DNA to Protein
PERSPECTIVE 7.1 Making Sense of Antisense RNA
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