Explain human brain

The size of the brain of a chimpanzee (right) is considerably smaller than that of a human brain. Probably multiple stretches of DNA help determine that, geneticists say. Science Photo Library/Corbis hide caption

toggle caption Science Photo Library/Corbis

The size of the brain of a chimpanzee (right) is considerably smaller than that of a human brain. Probably multiple stretches of DNA help determine that, geneticists say.

Science Photo Library/Corbis

Scientists studying the difference between human and chimpanzee DNA have found one stretch of human DNA that can make the brains of mice grow significantly bigger.

The human version of a DNA sequence called HARE5 (inserted into this mouse embryo) turned on a gene that's important for brain development. (Gene activity is stained blue.) By the end of gestation, the embryo's brain was 12 percent larger than the brain of an embryo injected with the chimpanzee version of HARE5. Silver Lab/Duke University hide caption

toggle caption Silver Lab/Duke University

The human version of a DNA sequence called HARE5 (inserted into this mouse embryo) turned on a gene that's important for brain development. (Gene activity is stained blue.) By the end of gestation, the embryo's brain was 12 percent larger than the brain of an embryo injected with the chimpanzee version of HARE5.

Silver Lab/Duke University

It could also help explain why human brains are so much bigger than chimp brains, says Silver, who notes that "there are estimates of anywhere from two to four times as big."

In addition to having bigger brains, Silver says, humans also "have more neurons, and we have more connections between these neurons."

Scientists would like to understand what the genetic basis is for humans' apparently special capacity for logic, abstract thought, complex emotions and language. Humans and chimpanzees have DNA that's remarkably similar — researchers say our genetic code is about 95 percent identical. But Silver and some colleagues recently started looking at pieces of DNA that differ markedly between chimps and humans.

"We went through those and picked out ones that seemed to be likely to be regulating gene activity in a developing brain, " explains Silver.

One stretch of DNA looked promising because it was near a gene that's known to be involved in brain development. The researchers took the chimp version of this DNA and put it into mouse embryos. They took other mouse embryos and put in the human version.

"What we discovered is that the human DNA turned on gene activity in neural stem cells, and these are cells which produce the neurons of our cerebral cortex, " says Silver.

Just before birth, mice with the human DNA had brains that were noticeably larger — about 12 percent bigger than the brains of mice with the chimp DNA, according to a report in the journal Current Biology.

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