11.4 Protein synthesis (translation)  (Page 26/12)

Learning objectives

• Describe the genetic code and explain why it is considered almost universal
• Explain the process of translation and the functions of the molecular machinery of translation
• Compare translation in eukaryotes and prokaryotes

The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins account for more mass than any other macromolecule of living organisms. They perform virtually every function of a cell, serving as both functional (e.g., enzymes) and structural elements. The process of translation , or protein synthesis , the second part of gene expression, involves the decoding by a ribosome of an mRNA message into a polypeptide product.

The genetic code

Translation of the mRNA template converts nucleotide-based genetic information into the “language” of amino acids to create a protein product. A protein sequence consists of 20 commonly occurring amino acids . Each amino acid is defined within the mRNA by a triplet of nucleotides called a codon . The relationship between an mRNA codon and its corresponding amino acid is called the genetic code .

The three-nucleotide code means that there is a total of 64 possible combinations (4 ³ , with four different nucleotides possible at each of the three different positions within the codon). This number is greater than the number of amino acids and a given amino acid is encoded by more than one codon ( [link] ). This redundancy in the genetic code is called degeneracy . Typically, whereas the first two positions in a codon are important for determining which amino acid will be incorporated into a growing polypeptide, the third position, called the wobble position , is less critical. In some cases, if the nucleotide in the third position is changed, the same amino acid is still incorporated.

Whereas 61 of the 64 possible triplets code for amino acids, three of the 64 codons do not code for an amino acid; they terminate protein synthesis, releasing the polypeptide from the translation machinery. These are called stop codon s or nonsense codon s . Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also typically serves as the start codon to initiate translation. The reading frame , the way nucleotides in mRNA are grouped into codons, for translation is set by the AUG start codon near the 5’ end of the mRNA. Each set of three nucleotides following this start codon is a codon in the mRNA message.

The genetic code is nearly universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all extant life on earth shares a common origin. However, unusual amino acids such as selenocysteine and pyrrolysine have been observed in archaea and bacteria. In the case of selenocysteine, the codon used is UGA (normally a stop codon). However, UGA can encode for selenocysteine using a stem-loop structure (known as the selenocysteine insertion sequence, or SECIS element), which is found at the 3’ untranslated region of the mRNA. Pyrrolysine uses a different stop codon, UAG. The incorporation of pyrrolysine requires the pylS gene and a unique transfer RNA (tRNA) with a CUA anticodon.