Concerning the attached file, answer each of the…
Concerning the attached file, answer each of the four questions in paragraph form: 1. Describe in your own words the thesis of the author(s). What do they believe are the important features of this application of genetic biotechnology? 2. How does this biotechnology application work? What features of DNA and cells are being used? 3. Describe two specific examples of experimental evidence that the authors refer to are examples or support for further use of this technology? 4. What is one example of a specific limitation or technical ‘hurdle’ that must be worked around or overcome in furthering this use of technology. What Makes Us Human?.pdf Download Attachment This is an unformatted preview. Please download the attached document for the original format. E VOLUTION WHAT MAKES US HUMAN? Comparisons of the genomes of humans and chimpanzees are revealing those rare stretches of DNA that are ours alone By Katherine S. Pollard S KEY CONCEPTS ? ? ? Chimpanzees are the closest living relatives of humans and share nearly 9 9 percent of our DNA. Efforts to identify those regions of the human g enome that have changed the most since chimps and humans di verged from a common ancestor have helped pin point the DNA sequences that make us human. The ndings have also provided vital insights into how chimps and humans can differ so profoundly, despite having nearly identical DNA blueprints. —The 44 Editors SCIENTIFIC AMERICAN ix years ago I jumped at an opportunity to join the international team that was identifying the sequence of DNA bases, or “letters,” in the genome of the common chimpanzee (Pan troglodytes). A s a biostatistician with a long-standing interest in human origins, I was eager to line up the human DNA sequence next to that of our closest living relative and t ake stock. A humbling truth emerged: our DNA blueprints are nearly 99 percent identical to theirs. That is, of the three billion letters that make up the human genome, only 15 million of them— less than 1 percent— have changed in the six million years or so since the human and chimp lineages diverged. Evolutionary theory holds that the vast majority of these changes had little or no effect on our biology. But somewhere among those roughly 15 million bases lay the differences that made us human. I was determined to nd them. Since then, I and others have made tantalizing progress in identifying a number of DNA sequences that set us apart from chimps. An Early Surprise Despite accounting for just a small percentage of the human genome, millions of bases are still a vast territory to search. To facilitate the hunt, I wrote a computer program that would scan the human genome for the pieces of DNA that have changed the most since humans and chimps split from a common ancestor. Because most random genetic mutations neither bene t nor harm an organism, they accumulate at a steady rate that re ects the amount of time that has passed since two living species had a common forebear (this rate of change is often spoken of as the “ticking of the molecular clock”). Acceleration in that rate of change in some part of the genome, in contrast, is a hallmark of positive s election, in which mutations that help an organism survive and reproduce are more likely to be passed on to future generations. In other words, those parts of the code that have undergone the most modi c ation since the chimphuman split are the sequences that most likely shaped humankind. In November 2004, after months of debugging and optimizing my program to run on a massive computer cluster at the University of California, Santa Cruz, I nally ended up with a le that contained a ranked list of these rapidly evolving sequences. With my mentor David Haussler leaning over my shoulder, I looked at the top hit, a stretch of 118 bases that together became known as human accelerated region 1 (HAR1). Using the U.C. Santa Cruz genome browser, a visualization tool that annotates the © 20 09 SCIENTIFIC AMERIC AN, INC. May 2009 THE 1 P ERCEN T D IFFERENCE: JAMES BALOG G etty Images Humans are distinct from chimpanzees in a number of important respects, despite sharing nearly 99 percent of their DNA. New analyses are revealing which parts of the genome set our species apart. human genome with information from public databases, I zoomed in on HAR1. The browser s howed the HAR1 sequences of a human, chimp, mouse, rat and chicken— all of the vertebrate species whose genomes had been decoded by then. It also revealed that previous largescale screening experiments had detected HAR1 activity in two samples of human brain cells, although no scientist had named or studied the sequence yet. We yelled, “Awesome!” in unison when we saw that HAR1 might be part of a gene new to science that is active in the brain. We had hit the jackpot. The human brain is well known to differ considerably from the chimpanzee brain in terms of size, organization and complexity, among other traits. Yet the developmental and evolutionary mechanisms underlying the characteristics that set the human brain apart are poorly understood. HAR1 had the potential to illuminate this most mysterious aspect of human biology. We spent the next year nding out all we could about the evolutionary history of HAR1 by comparing this region of the genome in variw w w. S c i A m . c o m ous species, including 12 more vertebrates that were sequenced during that time. It turns out that until humans came along, HAR1 evolved extremely slowly. In chickens and chimps — whose lineages diverged some 300 million years ago — only two of the 118 bases differ, compared with 18 differences between humans and chimps, whose lineages diverged far more re cently. The fact that HAR1 was essentially fro zen in time through hundreds of millions of years indicates that it does something very important; that it then underwent abrupt revision in humans suggests that this function was signi cantly modi ed in our lineage. A critical clue to the function of HAR1 in the brain emerged in 2005, after my collaborator Pierre Vanderhaeghen of the Free University of Brussels obtained a vial of HAR1 copies from our laboratory during a visit to Santa Cruz. He used these DNA sequences to design a uorescent molecular tag that would light up when H AR1 was activated in living cells — that is, copied from DNA into RNA. When typical genes are switched on in a cell, the cell rst © 20 09 SCIENTIFIC AMERIC AN, INC. SCIENTIFIC AMERICAN 45 46 SCIENTIFIC AMERICAN [E X PERIM ENT] SC ANNING THE GENOME To nd the parts of our genome that make us human, the author wrote a computer pro gram that searched for the DNA sequences that have changed the most since humans and chimpanzees diverged from their last common ancestor. Topping the list was a 118-letter snippet of code known as human Human accelerated region 1 (HAR1). This Common ancestor of region of the genome changed very humans and chimps little for most of vertebrate evoluChimp 6 million tion, with chimp and chicken se years ago quences differing by just two letters. Human and chimp HAR1s, however, 300 million years ago differ by 18 letters, suggesting that HAR1 acquired an important new Common ancestor of function in humans. Chicken humans and chickens .................................................................................................................................................................................................................................................................................................................................................................................