Junk DNA: Why humans, chimps are different?

GEORGIA TECH (US) — While the DNA sequence of genes between humans and chimpanzees is nearly identical, a new study finds the insertion and deletion of large pieces of DNA near  genes are highly variable.

This may account for major differences between the two species, according to work by researchers at the Georgia Institute of Technology.

For years, scientists believed the vast phenotypic differences between humans and chimpanzees would be easily explained—the two species must have significantly different genetic makeups. However, when their genomes were later sequenced, researchers were surprised to learn that the DNA sequences of human and chimpanzee genes are nearly identical.


Chimp in the wild. (Credit: Georgia Tech)

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What then is responsible for the many morphological and behavioral differences between the two species?

The research team lead by biology professor John McDonald has verified there are large genomic “gaps” in areas adjacent to genes that can affect the extent to which genes are “turned on” and “turned off.”

The research shows that these genomic “gaps” between the two species are predominantly due to the insertion or deletion (INDEL) of viral-like sequences called retrotransposons that are known to comprise about half of the genomes of both species. The findings are reported in the most recent issue of the online, open-access journal Mobile DNA.

“These genetic gaps have primarily been caused by the activity of retroviral-like transposable element sequences,” says McDonald. “Transposable elements were once considered ‘junk DNA’ with little or no function. Now it appears that they may be one of the major reasons why we are so different from chimpanzees.”

McDonald’s research team, comprised of graduate students Nalini Polavarapu, Gaurav Arora and Vinay Mittal, examined the genomic gaps in both species and determined that they are significantly correlated with differences in gene expression reported previously by researchers at the Max Plank Institute for Evolutionary Anthropology in Germany.

“Our findings are generally consistent with the notion that the morphological and behavioral differences between humans and chimpanzees are predominately due to differences in the regulation of genes rather than to differences in the sequence of the genes themselves,” says McDonald.

The current analysis of the genetic differences between humans and chimpanzees was motivated by the group’s previously published findings (2009) that the higher propensity for cancer in humans vs. chimpanzees may have been a by-product of selection for increased brain size in humans.

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