GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND INHERITANCE OF TRAITS

19.1. PROTEIN SYNTHESIS AND THE GENETIC CODE

19.1.5. mRNA Translation: Protein Synthesis

Amino Acids are linked into a polypeptide chain in the order specified by the mRNA triplets. This process involves mRNA, tRNA, Ribosomes, a set of Enzymes, and ATP. Initially, amino acids are activated with the participation of ATP to form aminoacyl-AMP:

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From AMP, the aminoacyl group is transferred to the terminal nucleotide of tRNA. The activation and attachment of The amino acid to the corresponding tRNA are mediated by the enzyme aminoacyl-tRNA synthetase. This enzyme recognizes both the Amino Acid and its specific tRNA. There are 20 such synthetases—one for each amino acid. If an amino acid has multiple codons, there are correspondingly multiple tRNAs for it. Different tRNAs designated for the same amino acid are called isoacceptor tRNAs. Consequently, the respective synthetase can attach the amino acid to several isoacceptor tRNAs. The tRNA molecule contains a region complementary to the mRNA codon (the anticodon). Amino acids are joined on the ribosomes (Fig. 19.2). The ribosome moves along the mRNA starting from the 5' end; with each shift by one triplet, the next amino acid is brought into position by the transport RNA and attaches via its amino group to the carboxyl group of the preceding amino acid (forming a peptide bond). Several ribosomes attach to an mRNA molecule, so that multiple polypeptide chains are synthesized simultaneously on the same template. Such a complex of a single mRNA with ribosomes is called a polysome. At the end of the mRNA, There is a codon that signals the release of the completed polypeptide chain from the ribosome (UAA, UAG, UGA).

Genetic Code

Note. The triplets UAA ("ochre"), UAG ("amber"), and UGA designate the termination of synthesis and the release of the polypeptide chain from the ribosome.

Abbreviations:

Ala — Alanine

Arg — Arginine

Asn — asparagine

Asp — aspartic acid

Val — valine

His — Histidine

Gly — Glycine

Gln — glutamine

Glu — glutamic acid

Ile — isoleucine

Leu — leucine

Lys — Lysine

Met — Methionine

Pro — Proline

Ser — Serine

Tyr — Tyrosine

Thr — Threonine

Trp — Tryptophan

Phe — phenylalanine

Cys — Cysteine

Thus, The nucleotide sequence of DNA represents an "encoded instruction" that determines (via mRNA) The Structure of a specific protein. The transfer of information from DNA through RNA to protein is known as the "central dogma" of molecular biology. This flow of information occurs in all organisms in which DNA serves as the genetic material (eukaryotes, prokaryotes, and DNA Viruses).

Among RNA viruses, there are those in which RNA replicates directly on an RNA template. However, in certain RNA oncoviruses, DNA Synthesis occurs first; that is, RNA serves as a template for DNA synthesis. The information contained in viral RNA is transferred to DNA via reverse METABOLISM/31.html">Transcription (mediated by the enzyme Reverse Transcriptase). This enzyme is used in Introduction/32.html">Genetic Engineering (if mRNA, rather than DNA, is isolated as the information carrier, it must be transcribed into DNA, which is then integrated into a plasmid). Reverse transcription has not been detected in prokaryotes.

Fig. 192. Protein Biosynthesis:

the transfer of Genetic information takes place in two stages: first, mRNA is synthesized on the DNA template strand; then, as the ribosome moves along the mRNA (from left to right in the diagrams), various tRNAs deliver Amino Acids and position them According to the mRNA triplets. The amino acids are linked together by peptide bonds