The Cell's Design: How Chemistry Reveals the Creator's Artistry by Fazale Rana

Author:Fazale Rana [Rana, Fazale]
Language: eng
Format: epub
Tags: Christian Books & Bibles, Theology, Apologetics, Creationism, Religion & Spirituality, Religious Studies, Science & Religion, Science & Math, Biological Sciences, Biochemistry, Evolution, History & Philosophy, Professional & Technical, Medical eBooks, Basic Science, Religious Studies & Reference, Christian
Amazon: B00B8569S8
Publisher: Baker Books
Published: 2008-06-01T22:00:00+00:00

Figure 10.2. Protein Synthesis at the Ribosome

The mechanism of protein synthesis involves the binding of tRNA–amino acid adducts to the A site and the subsequent transfer of the amino acid to the growing polypeptide chain in the P site.

Once positioned in the P and A sites, a region of rRNA in the large subunit (referred to as peptidyl transferase) forms a chemical bond between the first and second amino acids in the polypeptide chain. When this occurs, the amino acid in the P site dissociates from its tRNA. The tRNA in the P site leaves the ribosomes and becomes available to bind another amino acid.

The tRNA in the A site, which has the growing polypeptide chain attached to it, translocates to the P site. And, the tRNA–amino acid complex for the third position in the polypeptide chain enters the A site. Then bond formation, tRNA dissociation, and transfer from A to P site repeats. This entire process occurs over and over again until all the information in the mRNA is read and the entire polypeptide is synthesized. For each step in this assembly-line process, the ribosome complex advances along the mRNA length—one codon at a time.

Quality Control Procedures

As with any well-designed production process, the cell’s protein synthetic machinery employs quality assurance protocols. Checkpoints occur at several critical junctures during protein manufacture, including (1) tRNA and rRNA production, (2) mRNA production, (3) amino acid attachment to tRNA, (4) the movement of tRNA to the ribosome, and (5) the positioning of tRNA at the ribosome’s A site.

Maintaining the Protein Production Machinery

Biochemists refer to tRNA and rRNA as stable RNAs because these molecules, once produced by the cell’s machinery, persist for a long period of time under normal growth conditions. In contrast, mRNA has a high turnover rate (see chapter 6, p. 119).

The biosynthesis of tRNAs and rRNAs is highly accurate. Still, from time to time errors creep into the production process. If left unchecked, defective tRNAs and rRNAs will create havoc for the cell because these molecules are key cogs in the biochemical machinery that manufactures proteins. In any production process, if the machinery that makes the product doesn’t work properly, the product either can’t be produced or won’t be assembled correctly. The stability of these biomolecules further exacerbates the potential damage effected by defective tRNAs and rRNAs because, even if they’re flawed, these molecules will persist in the cell. (In contrast, when flawed proteins are accidentally made, the cell’s machinery eliminates them.)

In recent years biochemists have discovered the strict quality control governing the production of tRNAs and rRNAs in all cell types.10 When improperly made, the protein poly(A) polymerase adds several adenine nucleotides to the defective RNAs to form what biochemists call a poly (A) tail (see chapter 5, p. 102). The addition of this poly (A) tail (polyadenylation) flags the faulty RNAs for destruction.11 Studies on the bacterium E. coli show that when mistakes occur in the biosynthesis of tRNA and rRNA, cooperative activity between the proteins RNAase R and PNPase destroys the defective molecules.

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