11.4 Protein synthesis (translation)  (Page 5/12)

In summary, there are several key features that distinguish prokaryotic gene expression from that seen in eukaryotes. These are illustrated in [link] and listed in [link] .

Protein targeting, folding, and modification

During and after translation, polypeptides may need to be modified before they are biologically active. Post-translational modifications include:

1. removal of translated signal sequences—short tails of amino acids that aid in directing a protein to a specific cellular compartment
2. proper “folding” of the polypeptide and association of multiple polypeptide subunits, often facilitated by chaperone proteins, into a distinct three-dimensional structure
3. proteolytic processing of an inactive polypeptide to release an active protein component, and
4. various chemical modifications (e.g., phosphorylation, methylation, or glycosylation) of individual amino acids.

Key concepts and summary

• In translation , polypeptides are synthesized using mRNA sequences and cellular machinery, including tRNAs that match mRNA codons to specific amino acids and ribosomes composed of RNA and proteins that catalyze the reaction.
• The genetic code is degenerate in that several mRNA codons code for the same amino acids. The genetic code is almost universal among living organisms.
• Prokaryotic (70S) and cytoplasmic eukaryotic (80S) ribosomes are each composed of a large subunit and a small subunit of differing sizes between the two groups. Each subunit is composed of rRNA and protein. Organelle ribosomes in eukaryotic cells resemble prokaryotic ribosomes.
• Some 60 to 90 species of tRNA exist in bacteria. Each tRNA has a three-nucleotide anticodon as well as a binding site for a cognate amino acid . All tRNAs with a specific anticodon will carry the same amino acid.
• Initiation of translation occurs when the small ribosomal subunit binds with initiation factors and an initiator tRNA at the start codon of an mRNA, followed by the binding to the initiation complex of the large ribosomal subunit.
• In prokaryotic cells, the start codon codes for N-formyl-methionine carried by a special initiator tRNA. In eukaryotic cells, the start codon codes for methionine carried by a special initiator tRNA. In addition, whereas ribosomal binding of the mRNA in prokaryotes is facilitated by the Shine-Dalgarno sequence within the mRNA, eukaryotic ribosomes bind to the 5’ cap of the mRNA.
• During the elongation stage of translation, a charged tRNA binds to mRNA in the A site of the ribosome; a peptide bond is catalyzed between the two adjacent amino acids, breaking the bond between the first amino acid and its tRNA; the ribosome moves one codon along the mRNA; and the first tRNA is moved from the P site of the ribosome to the E site and leaves the ribosomal complex.
• Termination of translation occurs when the ribosome encounters a stop codon , which does not code for a tRNA. Release factors cause the polypeptide to be released, and the ribosomal complex dissociates.
• In prokaryotes, transcription and translation may be coupled, with translation of an mRNA molecule beginning as soon as transcription allows enough mRNA exposure for the binding of a ribosome, prior to transcription termination. Transcription and translation are not coupled in eukaryotes because transcription occurs in the nucleus, whereas translation occurs in the cytoplasm or in association with the rough endoplasmic reticulum.
• Polypeptides often require one or more post-translational modifications to become biologically active.

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