Chapter 25: Information Decoding – Translation and Posttranslational Protein Processing

25.1 An Overview of Translation

Translation is the process by which the genetic information encoded in messenger RNA (mRNA) is used to synthesize proteins. This process is central to the flow of genetic information in the cell and involves several key molecular players and steps.

• Central Dogma of Molecular Biology: The flow of genetic information follows the sequence: DNA replication (copying DNA), transcription (synthesizing RNA from DNA), and translation (synthesizing protein from mRNA).

• Key Steps in Each Process: Each process involves three main stages: initiation, elongation, and termination.

• Translation Mechanism: The ribosome reads the mRNA in the 5' to 3' direction, matching each codon to the appropriate aminoacyl-tRNA, and synthesizes the polypeptide from the N-terminus to the C-terminus.

• Directionality: Proteins are always synthesized from the amino (N) terminus to the carboxyl (C) terminus.

Example: During translation, the ribosome moves along the mRNA, and each codon is recognized by a complementary anticodon on a tRNA molecule carrying the correct amino acid.

25.2 Activation of Amino Acids for Incorporation into Proteins by Attachment to a tRNA

Before amino acids can be incorporated into proteins, they must be activated and attached to their corresponding transfer RNA (tRNA) molecules. This process is catalyzed by enzymes known as aminoacyl-tRNA synthetases.

• Amino Acid Activation: Each amino acid is attached to the 3' end of its specific tRNA by an ester bond, forming an aminoacyl-tRNA.

• Reaction: The activation occurs in two steps:

  1. The amino acid reacts with ATP to form an aminoacyl-AMP intermediate and pyrophosphate (PPi).

  2. The aminoacyl group is then transferred to the tRNA, releasing AMP.

• Equation:

• Anticodon: The tRNA contains a trinucleotide sequence called the anticodon, which is complementary to the codon in the mRNA.

Example: The enzyme alanyl-tRNA synthetase attaches alanine to its corresponding tRNAAla.

25.3 The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of proteins. It is nearly universal among organisms and is characterized by several important features.

• Codons: A codon is a sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal during translation.

• Deciphering the Code: Experiments using synthetic polynucleotides helped determine which codons correspond to which amino acids.

• Start and Stop Codons:

  • AUG is the start codon, coding for methionine (in eukaryotes) or formylmethionine (in prokaryotes).

  • Three stop codons (UAA, UAG, UGA) signal the termination of translation and do not code for any amino acid.

• Degeneracy: The code is degenerate, meaning that most amino acids are encoded by more than one codon.

• Wobble Hypothesis: Flexibility in base pairing at the third position of the codon allows some tRNAs to recognize multiple codons.

Example: The codons UUU and UUC both code for phenylalanine.

Additional info: The genetic code's redundancy helps protect against mutations, as changes in the third base of a codon often do not alter the encoded amino acid.