G G A U C C

mRNA

Ribosome

Figure 6.5 (a) Transcription and (b) translation. In transcription, messenger RNA is formed from the DNA template at the chromosome. Subsequently, the mRNA is transported to the cytoplasm, where it encounters ribosomes and participates in translation. In translation, mRNA is used as a template to produce polypeptides. The numbered arrows indicate the approximate sequence of events.

This has significance in the biodegradation of chemicals. Multicellular eukaryotes can also turn genes permanently on or off, to reflect the specialized roles of particular tissues.

Some enzymes are produced continuously regardless of the environmental conditions. Others are produced only when the substrate is present. The latter is called enzyme induction. For example, the cytochrome P450 enzymes responsible for biotransformation of xenobiotic compounds are induced by many of these substances, including components of tobacco smoke and polynuclear aromatic hydrocarbons.

The production of some enzymes is inhibited by the presence of some molecule in the environment. This is called enzyme repression. Both of these types of regulation are transcription-level control, controlling protein synthesis by affecting the production of mRNA.

Studies of E. coli led the French geneticists Francois Jacob and Jacques Monod to propose the operon model of genetic control in 1961 to explain both induction and repression. In the operon model, a biochemical function is associated with a cluster of genes, including structural genes. Structural genes are genes that actually code for enzymes. Other genes control the expression of the structural genes. The control genes include a repressor

_attachment site for an amino acid binds with amino-acid activating enzyme

_attachment site for an amino acid

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