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Thursday, April 27, 2006

Lactose Helps Explain Evolution

All of us with lactose intolerance should know that some of the bacteria in our colons manufacture the lactase enzyme, which helps digest any lactose that reaches them, reducing symptoms.

Few of us realize that learning how the bacteria do this was one of the breakthroughs in understanding how genes function.

The new issue of the New York Review of Books has a review by Edward Ziff and Israel Rosenfield of three new books on evolution. As usual, the review is an actual review of the subject as much as or more than a critical review of the book's involved. The authors explain the lactose collection as follows:

The number of genes in a given species, therefore, is not a measure of its complexity. Why had biologists so overestimated the number of genes in the human genome? Why is it unnecessary for complex animals such as mammals to have ten times as many genes as worms?

The answers to these questions were already hinted at more than four decades ago. At the time it was known that the bacteria E. coli, which normally live off the sugar glucose, are also capable of producing enzymes that digest other sugars, such as lactose. But biologists noticed that the bacteria only produce the enzyme when lactose is present in their immediate environment. Scientists could not explain how the E. coli somehow "knew" when the lactose-digesting enzyme would be needed.

In 1961, Jacques Monod and Fran├žois Jacob discovered that E. coli bacteria actually have a mechanism that controls the production of the enzyme for digesting lactose. As unicellular organisms, E. coli bacteria have only several thousand genes, each of which is made up of a specific sequence of DNA. A single one of these genes, present in all E. coli, carries in its DNA the genetic instructions needed to assemble the enzyme that can digest lactose; the DNA is copied into RNA, which is then "translated" to produce the enzyme itself. When there is no lactose present in the bacteria's immediate environment, the gene is switched off: its DNA is not copied into RNA and the enzyme is not produced. The reason for this, the scientists discovered, is that a protein called a repressor molecule attaches itself to the DNA site where the copying into RNA begins, thus blocking off the DNA and preventing the gene from producing the RNA responsible for the synthesis of the enzyme.

On the other hand, when the E. coli bacteria encounter lactose, the lactose binds itself to this repressor molecule, causing the repressor to be detached from the DNA site. This unblocks the DNA, allowing the gene to be copied into RNA, and produce the enzyme that can digest lactose. In other words, the repressor molecule acts as a switch that controls the gene's production of the enzyme. Since only a fraction of the total number of genes present in an organism are expressed, or turned on, at any given time, Monod and Jacob conjectured that other genes must be similarly turned on and off.

Although they had not yet found systematic evidence to support these ideas, the discovery of the repressor molecule allowed the two scientists to form a powerful new hypothesis about how genes function. As Jacob recently wrote, in a brief description of the new hypothesis:
    It proposed a model to explain one of the oldest problems in biology: in organisms made up of millions, even billions of cells, every cell possesses a complete set of genes; how, then, is it that all the genes do not function in the same way in all tissues? That the nerve cells do not use the same genes as the muscle cells or the liver cells? In short, [we] presented a new view of the genetic landscape.

The deeper significance of the Monod-Jacob model of gene function, and its implications for the nature of evolution, became apparent with the new field of embryo research that arose almost twenty years later.


There's much more to the article. Read it for a better understanding of the evolution of evolution.

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