The Genetic Code Dictates the Amino Acid Sequence of Proteins

There are 20 amino acids in proteins but only four different bases in the messenger RNA. So nature cannot simply use one base of a nucleic acid to code for a single amino acid when making a protein. During translation, the bases of mRNA are read off in groups of three, which are known as codons. Each codon represents a particular amino acid. Since there are four different bases, there are 64 possible groups of three bases; that is, 64 different codons in the genetic code. However, there are only 20 different amino acids making up proteins, so some amino acids are encoded by more than one codon. In addition, three of the codons are used for punctuation to stop the growing chain of amino acids (Fig.3.20).In addition,the codon,AUG, encoding methionine, acts as a start codon. Thus newly made polypeptide chains start with the amino acid methionine. [Much less often, GUG encoding valine, may also act as the start codon. However, even if the start codon is GUG the first amino acid of the newly made protein is methionine (not valine).] To read the codons a set of adapter molecules is needed. These molecules, known as transfer RNA (tRNA), recognize the codon on the mRNA at one end and carry the corresponding amino acid attached to their other end (Fig. 3.21). These adapters represent a third class of RNA and were named transfer RNA since they transport amino acids to the ribosome in addition to recognizing the codons of mRNA. Since there are numerous codons, there many different tRNAs. [Actually, there are fewer different tRNA molecules than codons as some tRNA molecules can read multiple codons— see Ch. 8 for details.] At one end, the tRNA has an anticodon consisting of three bases that are complementary to the three bases of the codon on the messenger RNA. The anticodon Group of three complementary bases on tRNA that recognize and bind to a codon on the mRNA codon Group of three RNA or DNA bases that encodes a single amino acid genetic code The code for converting the base sequence in nucleic acids, read in groups of three, into the sequence of a polypeptide chain ribosomal RNA (rRNA) Class of RNA molecule that makes up part of the structure of a ribosome transfer RNA (tRNA) RNA molecules that carry amino acids to a ribosome iicocfc

FIGURE 3.21 Transfer RNA Contains the Anticodon

Each transfer RNA molecule has an anticodon that is complementary to the codon carried on the messenger RNA. The codon and anticodon bind together by base pairing. At the far end of the tRNA is the acceptor stem ending in the bases CCA (cytosine, cytosine, adenine). Here is attached the amino acid that corresponds to the codon on the mRNA.

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FIGURE 3.21 Transfer RNA Contains the Anticodon

Each transfer RNA molecule has an anticodon that is complementary to the codon carried on the messenger RNA. The codon and anticodon bind together by base pairing. At the far end of the tRNA is the acceptor stem ending in the bases CCA (cytosine, cytosine, adenine). Here is attached the amino acid that corresponds to the codon on the mRNA.

Acceptor stem

Amino acid attachment site

CCA end

DHU loop

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