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News 2001

Human Genome Project Primer

Scientists around the world celebrate today the publication of two historical scientific papers on the Human Genome Project (HGP). The papers summarize the monumental work of the International Human Genome Sequencing Consortium and that of Celera Genomics Corporation. The two groups announced the first sequencing phase of the project last June. The current articles describe the first attempts to analyze the human genome sequence. The work represents the latest stage of the analysis of the HGP that will continue in the future. The Human Genome Each of the 100 trillion cells in your body contains 46 chromosomes. Chromosomes are made of DNA. The total collection of DNA in the cell is called the human genome. The building blocks of DNA are four chemical letters called adenine (A), thymine (T), cytosine (C), and guanine (G). There are approximately three billion A, T, C, and Gs repeated over and over again in the human genome. Specific combinations of chemical bases found along the chromosome can encode genes that control all cell functions. Genes are difficult to identify because they are hidden amongst only five per cent of the genome. If the genome was a ‘genetic dictionary’, the entire DNA sequence written in regular size font would consist of one million pages each having 3,000 characters per page. An average gene (or word) would require five pages. The Human Genome Project The HGP is a worldwide research initiative with the goal of analyzing the complete sequence of A, T, C, and Gs in the human DNA to identify all of the genes. The information will represent a source book for biomedical science in the 21st century. It will help scientists understand, diagnose, and eventually treat many of the 5,000 genetic diseases that afflict humankind. The international HGP was officially launched in the early 1990s. Its progress and general strategy can be described in three phases: (i) making chromosome maps; (ii) performing DNA sequencing; and (iii) annotating the DNA sequence which includes isolating all the genes, and then finding those that cause disease. The announcements today document major progress towards completing the DNA sequencing phase and ushering in the annotation phase. DNA Sequencing (Public effort and Celera Genomics) A, T, C andGs cannot be read directly from the chromosomes because they are too small to see even under the most powerful microscopes. However, DNA sequencing technologies have been developed to detect their identity. The difficult part is to determine the original order within the genome of the three billion nucleotides of DNA sequence. The publicly funded HGP accomplished this by breaking the chromosomes into pieces (mapping) and then further subdividing these into smaller components comprised of A, T, C and Gs. Celera Corporation bypassed the mapping step and determined the sequence of the smaller subunit of DNA. Celera then reconstructed their DNA sequence map using their own data combined with the DNA sequence information generated by the public effort. Therefore, Celera essentially has twice as much information in their sequence map, so it will be more complete. If we return to the analogy of the HGP being similar to reassembling the content of a genetic dictionary, it is now at the point where there is a rough order of the pages with sections of words on each page. There are still pages missing, words left out, and spelling mistakes. The exact number and definition of the words (genes) are still largely unknown, but there is good progress. Finding the remainder of the genes and identifying those involved in disease will be a major focus of the HGP over the next five years. Surprise discoveries from genome research Gene number: There appear to be 30,000-40,000 genes in the human genome, which is less than the 80,000-150,000 predicted initially. This is only about two times the number of genes worms or flies have. Our DNA is 99.9% identical: Most DNA sequence variation between humans seems to predate the time when our ancestors migrated out of Africa. Therefore, the concept of homogeneous groups (or races) having major biological differences is not consistent with genetic evidence. Unity and similarity of everything alive: The DNA sequence of our genes is 90% identical to mice and over 98% identical to primates. Many human genes crucial for existence are nearly identical to those in bacteria and yeast. Summary Due to significant advances in the international Human Genome Project over the last decade, we are experiencing a knowledge explosion in human biology. We have observed unprecedented developments in disease research, accompanied with genetic diagnosis, and predictive testing. As a result, novel drugs, disease treatment, and new therapies are being developed. The HGP has also shed light on historical puzzles such as the roots and migrations of ancient peoples, historical demography of cultures, and human genetic diversity. Scientists will continue to work on the HGP with an emphasis on annotation of the DNA sequence, determining the function of the gene products (functional genomics), and the study of common diseases. More difficult than the genetic science will be the social implication of this knowledge. There are issues of genetic privacy and ownership of our genome. It will be the responsibility of everyone in society to ensure that there is proper education, adequate discussion of these topics, and sufficient consensus for the lawmakers to safeguard the acceptable standards. Therefore, programs studying ethical, legal, and societal implications of genetic information will be just as important to the continued success of the HGP as the science. Prepared by: Stephen W. Scherer, PhD Senior Scientist, Genetics & Genomic Biology Associate Director, The Centre for Applied Genomics The Hospital for Sick Children Assistant Professor, Department of Molecular and Medical Genetics University of Toronto

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