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What have we learned from the Human Genome Project?
Space may be the final frontier, but human biology is the original unknown, challenging us to discover who we are and where we came from. DNA, the building block of life, contains the genetic code that informs so much of who we are. This code is written with four letters, each representing a different base. The four bases are adenine (A), which pairs with thymine (T), and cytosine (C), which pairs with guanine (G).
Scientists have long known that these four letters provide the recipes for proteins, which carry out numerous bodily functions. But there are still questions to be answered, including how the 3.2 billion base pairs contained in the human genome are ordered. (The human genome is a person's entire bundle of DNA divided unevenly among 23 pairs of chromosomes.) To that end, the Human Genome Project (HGP) was launched in 1990. Some of the project's ambitious goals included:
- Sequencing the entire human genome
- Identifying human genes
- Charting variations across human genomes
- Sequencing genomes of the mouse and four other "model organisms"
Run by the National Institutes of Health and the U.S. Department of Energy, the project was completed ahead of schedule in 2003. A "final" batch of results was published in 2006, but data produced by the HGP are continually examined, analyzed and occasionally revised. Theoretically, with the main goals achieved, the project is finished. Let's look at some of what we learned.
Only a few years before the completion of the HGP, popular predictions stated that humans had up to 100,000 genes. But recent HGP estimates lowered that number to a more modest range of 20,000 to 25,000 [source: Human Genome Project Information]. In addition, the HGP has helped to narrow the range of possible genes and to isolate certain candidates as contributing to specific diseases. Scientists have also reassessed previous assumptions, such as the idea that genes are self-contained, discrete pieces of DNA with defined roles. That's not always the case. We now know that some multitasking genes make more than one protein in fact, the average gene may make three proteins [source: Genome.gov]. Also, genes appear to grab genetic code from other DNA segments.
Before we look closely at heredity and genes, let's stop to consider what scientists have learned about animal and other genomes. Some of these projects, such as mapping the mouse genome, were included in the original Human Genome Project and can tell us about our evolution and DNA.
Animal, Cancer and Other Genomes
Scientists have mapped many animal genomes, among them that of the chimpanzee, mouse, rat, fruit fly, roundworm and puffer fish. They've also charted some plant and disease genomes. These genomic maps are useful in part because animal genomes can be compared to human genomes. Think of a police procedural TV show where a translucent printout showing the DNA of a suspect is lined up against that of the DNA evidence. When everything lines up, there's a match, and the police have their killer. Similarly, scientists can look for matches between human and animal DNA. They don't expect perfect matches, but by examining where our genomes do line up, researchers can see what we have in common with animals, what we don't, and make determinations about common ancestors and how we've evolved. In cases of diseases afflicting animals, such as the cancer ravaging the Tasmanian devil population, a better understanding of animal DNA can potentially lead to important medical treatments.
We mentioned that one goal of the HGP was to sequence the genomes of five "model organisms." This sequencing is an important part of a field known as comparative genomics. In comparative genomics, the study of an animal with a less complex genome, such as a mouse, may yield important information about genes that mice and humans share since we are, in fact, genetically very similar [source: HGP Information]. Just like other forms of animal testing, examining the genome of another species can tell us more about our own.
One of the most intriguing cases of an animal whose genome has been mapped is that of the platypus. This creature has always been considered an oddity because it's one of the few mammals that lays eggs and nurses its young through its abdominal skin, rather than with nipples. The platypus genome, with its 18,500 genes, is important because it harkens back to an ancient time when mammals were egg layers [source: Hood]. Evolution probably took humans away from the ancestors we share with the platypus around 170 million years ago. Today, this evolutionary marvel has characteristics of mammals, birds and reptiles [source: Hood]. They also have 10 sex chromosomes, as compared to our paltry two.
Outside of the human genome (and of course that of the platypus), perhaps no genetic mapping project is as compelling as that focused on unraveling the genetic code of various cancers. Sequencing of cancer genomes allows scientists and doctors to discover gene mutations that contribute to cancer, potentially leading to better detection methods and treatments.
The first complete cancer genome sequenced was that of acute myeloid leukemia, a severe form of cancer that begins in the bone marrow. The Cancer Genome Atlas, an organization that hopes to sequence many types of cancer, led the mapping using massively parallel sequencing, which compares normal and cancer DNA and looks for mutations [source: Kushnerov].
If cancer-genome sequencing proves the hypothesis that each occurrence of cancer produces unique mutations in a particular person, future doctors may be able to customize treatments for each patient. With many treatments available for some conditions, it's often a process of trial and error to see what works best for one person over another [source: Aetna]. In some cases, this practice can do more harm than good or deprive doctors and patients of valuable time needed to stem a disease's advance.
Human Genome Project Results
Now that the Human Genome Project is over, it's time for scientists to examine the information produced and pursue related research. Much of the post-HGP focus has fallen on genes, spurring new discussions of how heredity works and causing scientists to look at DNA differently, setting aside the traditional focus on genes as the dominant actors within DNA. Some researchers are now looking at the 99 or so percent of DNA that aren't genes, wondering if these previously neglected chunks of the genome have significant roles to play.
The HGP and subsequent research efforts have changed the consensus view of genes and noncoding DNA, casting them as part of an increasingly complex image of genes, DNA and other components of the genome. For example, epigenetic marks, the proteins and other molecules attached to DNA, are receiving more attention, especially for their apparent role in heredity. It seems that these marks can also pass on traits, just like genes, and misplaced or damaged epigenetic marks may increase someone's risk of developing cancer and other disorders [source: Zimmer]. A $190 million National Institutes of Health study hopes to map all epigenetic marks on DNA.
Along with changing how we think about genes, the Human Genome Project spawned lots of other projects. For example, in 2002, the International HapMap Project started charting SNPs among various ethnic groups. From person to person, the genetic code differs at around 10 million points (out of 3.2 billion DNA base pairs) [source: Aetna]. These differences are called SNPs -- single nucleotide polymorphisms. But despite these SNPs, human beings only differ from one another by about 0.1 percent, enough to ensure that no two human beings are genetically identical, even, sometimes, identical twins. Understanding SNPs can help us better understand genetic variation among individuals and ethnic groups produce better genetic testing for predisposition toward disease and contribute to the development of more personalized medical treatments.
Future projects and areas of research related to the HGP are seemingly endless. Many millions of dollars are being poured into projects like Encode, a massively ambitious effort to determine the role of every single piece of DNA in the human genome. (Encode stands for Encyclopedia of DNA Elements.) But while information yielded from the HGP and related projects will likely lead to important medical advances and disease treatments, the relationship between research and practical therapies isn't really one of simple cause and effect. Just one new drug can take 10 years of development time.
In the future, look out for these burgeoning fields of research, much of which owe a great debt to the work of the HGP:
- Improved genetic testing to gauge predisposition for disease
- Tracing genes to diseases and birth defects
- Creating customized therapies based on genetic profiles
- Manipulating or repairing DNA to stave off disease
- The role of RNA, particularly the large amount of noncoding RNA
Despite all of these exciting discoveries and those that await us, we may never fully understand the inner workings of DNA. The rapidly shifting definition of the gene may be testament to that. One researcher told the New York Times that human biology could be "irreducibly complex" [source: Angier]. We humans can do and understand remarkable things -- launch spaceships, build incredibly fast computers, create gorgeous works of art -- but our 3.2 billion pieces of DNA may be too much for our minds to fully comprehend in the end. In the course of human progress, it has been far easier to understand the things we make, rather than what makes us.
For more information about the Human Genome Project and other related topics like epigenetics, please visit the links on the next page.