Gene Regulation and Protein Synthesis

Gene expression in many bacteria is regulated through the existence of operons. An operon is a cluster of genes whose protein products have related functions. For instance, the lac operon includes one gene that transports lactose sugar into the cell and another that breaks it into two parts. These genes are under the control of the same promoter, and so are transcribed and translated into protein at the same time. RNA polymerase can only reach the promoter if a repressor is not blocking it; the lac repressor is dislodged by lactose. In this way, the bacterium uses its resources to make lactose-digesting enzymes only when lactose is available.

Other genes are expressed constantly at low levels; their protein products are required for "housekeeping" functions such as membrane synthesis and DNA repair. One such enzyme is DNA gyrase, which relieves strain in the double helix during replication and repair. DNA gyrase is the target for the antibiotic ciproflaxin (sold under the name Cipro), effective against Bacillus anthracis, the cause of anthrax. Since eukaryotes do not have this type of DNA gyrase, they are not harmed by the action of this antibiotic.

conjugation a type of DNA exchange between bacteria il genome the total genetic material in a cell or organism promoter DNA

sequence to which RNA polymerase binds to begin transcription enzymes proteins that control a reaction in a cell

As in eukaryotes, translation (protein synthesis) occurs on the ribo-some. Without a nucleus to exclude it, the ribosome can attach to the messenger RNA even while the RNA is still attached to the DNA. Multiple ribosomes can attach to the same mRNA, making multiple copies of the same protein.

The ribosomes of eubacteria are similar in structure to those in eukary-otes and archaea, but differ in molecular detail. This has two important consequences. First, sequencing ribosomal RNA molecules is a useful tool for understanding the evolutionary diversification of the Eubacteria. Organisms with more similar sequences are presumed to be more closely related. The same tool has been used to show that Archaea and Eubacteria are not closely related, despite their outward similarities. Indeed, Archaea are more closely related to eukaryotes (including humans) than they are to Eubacteria.

Second, the differences between bacterial and eukaryotic ribosomes can be exploited in designing antibacterial therapies. Various unique parts of the bacterial ribosome are the targets for numerous antibiotics, including streptomycin, tetracycline, and erythromycin. see also Archaea; Cell, Eukaryotic; Chromosome, Prokaryotic; Conjugation; Escherichia coli; Operon; Plasmid; Transduction; Transformation.

Richard Robinson

Bibliography

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Margulis, Lynn, and Karlene Schwartz. Five Kingdoms, 3rd ed. New York: W. H. Freeman, 1998.

Eugenics

While the idea of improving humans through selective breeding is at least as old as the ancient Greeks, it gained widespread prominence after 1869. In 1883, Sir Francis Galton coined the word "eugenics," from the Greek word eugenes, meaning "well-born" or "hereditarily endowed with noble qualities," to describe this new science of directed human evolution. Gal-ton's work, and the subsequent rediscovery of Gregor Mendel's genetic studies, convinced many scientists and social reformers that eugenic control over heredity could improve human life.

Galton's ideas swept America during the Progressive Era of the early twentieth century. At that time, many scientists and laypeople believed that eugenics could facilitate social progress by eradicating problems ranging from alcoholism and prostitution to poverty and disease. What better way to prevent such misfortunes, eugenicists asked, than to prevent the birth of people genetically susceptible to them? Eugenics seemed to offer an efficient and humane solution to society's ills. Unfounded hope in this imperfect science, however, ultimately contributed to repressive social policies, including marriage and immigration restriction, forced sterilization, segregation, and, in the case of Nazi Germany, euthanasia ("mercy killing") and genocide, all in the name of human betterment.

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