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Hot Topic Origins Origin & Evolution of Life Ape vs. Man: Volatile DNA?
 
Ape vs. Man: Volatile DNA?
Source: UC Riverside
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Origin & Evolution of Life
Posted:   09/04/04

Summary: An often cited two-percent genetic difference between humans and primates may not tell the whole story, since the quality of human "junk" DNA may make the difference.


Primate evolution tree
Evolutionary branches of the primate species depicted on the tree. Full image.
Credit: Mark A. Klingler, Carnegie Museum of Natural History
A team of biochemists from UC Riverside published a paper in the June 11 issue of the Journal of Molecular Biology that gives one explanation for why humans and primates are so closely related genetically, but so clearly different biologically and intellectually.

It is an established fact that 98 percent of the DNA, or the code of life, is exactly the same between humans and chimpanzees. So the key to what it means to be human resides in that other 2 percent.

According to Achilles Dugaiczyk, professor of Biochemistry at UCR, one important factor resides in something called Alu DNA repeats, sometimes called "junk DNA." These little understood sections of DNA are volatile, and prone to sudden mutations, or genomic rearrangements. At times the results are beneficial in that they give rise to new proteins or an altered gene regulation. Sometimes the mutations result in the growth of a cancer tumor, or some other genetic defect.

The team, which also included Rosaleen Gibbons, Lars J. Dugaiczyk, Thomas Girke, Brian Duistermars and Rita Zielinski, identified over 2,200 new human specific Alu DNA repeats that are absent from the chimpanzee and most likely other primates.

living_universe
The biomolecule, DNA, that twists throughout the cell nucleus
"The explosive expansion of the DNA repeats and the resulting restructuring of our genetic code may be the clue to what makes us human," Dugaiczyk said. "During the same amount of time, humans accumulated more genetic novelties than chimpanzees, making the human/chimpanzee genetic distance larger than that between the chimpanzee and gorilla."

Metaphorically speaking, Dugaiczyk said, "Humans and primates march to the rhythm of a drum that looks identical; the same size, shape and sound. But, the human drum beats faster."

This chemical analysis of DNA structures also showed something else. The spread of the Alu DNA repeats was written into the chemistry of human chromosomes. The process was not random, Dugaiczyk said, and it was not subject to an environmental "natural selection," separating winners and losers as theorized by Darwin.

"We are not contending that natural selection does not exist, but that in this instance it is a chemical process within human chromosomes that explains why humans have an explosive expansion of DNA repeats, and primates do not," Dugaiczyk said.

Determining the genetic differences between humans and primates is important for several reasons, Dugaiczyk said, including advancing knowledge about how life developed and evolved on earth. Other benefits include making it easier to identifying human predisposition to genetic disease, by comparing humans with other primate species. A third possible benefit is to underline the importance of protecting endangered primate species.


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