Molecular Genetics

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Translation is the process of converting the sequence of nucleotide residues in mRNA into the sequence of amino acid residues in a polypeptide. Since protein is synthesized as a consequence of translation, the process is also called protein synthesis.

• Protein synthesis can be reconstituted in vitro from purified components. The components necessary for translation are:
  • The messenger RNA (mRNA), the product of transcription and splicing, carries the code for the order of insertion of amino acids. This genetic code is a triplet code without punctuation. The absence of punctuation makes reading the mRNA susceptible to changes in the frame of reading the nucleotide residues. The mRNA must also contain signals for initiation and termination of a polypeptide chain. The initiation signals are near its 5' end, while the termination signals are near its 3' end.
  • The transfer RNA (tRNA) molecules actually do the translation. There is at least one tRNA for each of the 20 common amino acids. At one end of the tRNAs three contiguous nucleotides, the anticodon, specifically base pair with only a specific subset of codon triplets. The opposite ends have a place for covalent attachment of an amino acyl residue.
  • There are at least 20 aminoacyl tRNA synthetases, at least one per type of amino acid. Each synthetase will only activate and covalently attach one kind of amino acid to a tRNA. The synthetase also is specific about which tRNAs it will add the amino acid to. The synthetases, thus, are at the center of translation, recognizing both amino acid and nucleotide residues.
  • Ribosomes are multimolecular aggregates containing RNAs and proteins. They can be separated into two subunits, a large and a small subunit. The ribosomes contain binding sites for mRNA, for tRNAs and for a variety of protein factors. Ribosomes are also the site of a catalytic activity that is probably responsible for the formation of the peptide bond during protein synthesis. The peptide bond is made between the C-terminal alpha-carbonyl of the nascent peptide chain and the alpha-amino group of the incoming aminoacyl residue. Thus the polarity of polypeptide synthesis is from N- to C-terminus.
  • A variety of protein factors, usually loosely bound to ribosomes, are also required. Many are needed for proper initiation of polypeptide chain synthesis, while others are required for binding incoming aminoacyl tRNAs to the ribosome and moving the ribosome relative to the mRNA (translocation) after each addition step. Factors may assist the termination reaction.
  • Biochemical energy for translation is derived from nucleoside triphosphates at three steps. Charging of tRNA requires ATP to form an aminoacyl adenylate intermediate. Adenylate is later released as AMP. GTP hydrolysis occurs during the cycle of binding the amino acyl tRNA to the ribosome-mRNA complex and also during the translocation reaction.
• The completed polypeptide chain must be processed in one or more ways before it assumes its role as a mature protein or peptide. The activities, enzymatic or otherwise, of the proteins can be regulated by regulating their synthesis in translation.