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Introduction

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High-Yield Terms

  • Genetic code: the genetic code defines the set of rules (the nucleotide triplets) by which information encoded within the genes is translated into proteins

  • Wobble hypothesis: explains the non–Watson-Crick base-pairing that can occur between the 3′-nucleotide of a codon and the 5′-nucleotide of an anticodon

  • Ternary complex: translational initiation complex composed of eIF-2 with GTP bound and the initiator methionine tRNA

  • Peptidyltransferase: an RNA-mediated (ribozyme) enzymatic activity of the 60S ribosome that catalyzes transpeptidation during protein elongation.

  • Selenoprotein: any of a family of proteins that contain a modified selenocysteine amino acid

  • Signal sequence: the N-terminal 15-25 hydrophobic amino acids in proteins that are destined for secretion of membrane insertion; the sequences are recognized by the signal recognition particle which assists in binding of the ribosome to the ER membrane

  • Prenylation: posttranslational modification by the isoprenoid molecules farnesyl or geranyl allowing proteins to associate with membranes

  • Ubiquitin: a 76-amino-acid peptide enzymatically attached to proteins which, in most cases, targets the modified protein for degradation in the proteosome

  • Proteosome: a large protein complex whose function is to proteolytically degrade damaged or unneeded proteins

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Translation is the RNA-directed synthesis of proteins. The process requires not only the template RNA, the mRNA, but also the participation of the tRNAs and rRNAs. The tRNAs are necessary to carry activated amino acids into the ribosome which itself is composed of rRNAs and ribosomal proteins. Although the chemistry of peptide bond formation is relatively simple, the processes leading to the ability to form a peptide bond are exceedingly complex. During translation the ribosome is intimately associated with the mRNA ensuring correct access of activated tRNAs and the necessary enzymatic activities to catalyze peptide bond formation.

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Translation proceeds in an ordered process. First, accurate and efficient initiation must take place, then peptide elongation, and finally accurate and efficient termination must occur. All three of these processes require specific proteins, some of which are ribosome associated and some of which are separate from the ribosome, but may be temporarily associated with it.

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Determination of the Genetic Code

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Early experiments, designed to ascertain how the information in the DNA of genes was transmitted via RNAs to proteins, demonstrated several key facts that resulted in the definition of the genetic code. In addition, it was determined that the genetic code is read in a sequential manner starting from a fixed point in the mRNA and that the 5′-end of the mRNA corresponded to the amino terminus of the encoded protein. This means that translation proceeds along the mRNA in the 5′ → 3′ direction which corresponds to the N-terminal to C-terminal direction of the amino acid sequences within proteins.

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The code was shown to be a triplet of nucleotides and all 64 possible combinations of the 4 nucleotides could code for amino acids. This latter fact is defined as the ...

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