human genome

human genome, all of the approximately three billion base pairs of deoxyribonucleic acid (DNA) that make up the entire set of chromosomes of the human organism. The human genome includes the coding regions of DNA, which encode all the genes (between 20,000 and 25,000) of the human organism, as well as the noncoding regions of DNA, which do not encode any genes. By 2003 the DNA sequence of the entire human genome was known.

The human genome, like the genomes of all other living animals, is a collection of long polymers of DNA. These polymers are maintained in duplicate copy in the form of chromosomes in every human cell and encode in their sequence of constituent bases (guanine [G], adenine [A], thymine [T], and cytosine [C]) the details of the molecular and physical characteristics that form the corresponding organism. The sequence of these polymers, their organization and structure, and the chemical modifications they contain not only provide the machinery needed to express the information held within the genome but also provide the genome with the capability to replicate, repair, package, and otherwise maintain itself. In addition, the genome is essential for the survival of the human organism; without it no cell or tissue could live beyond a short period of time. For example, red blood cells (erythrocytes), which live for only about 120 days, and skin cells, which on average live for only about 17 days, must be renewed to maintain the viability of the human body, and it is within the genome that the fundamental information for the renewal of these cells, and many other types of cells, is found.

The human genome is not uniform. Excepting identical (monozygous) twins, no two humans on Earth share exactly the same genomic sequence. Further, the human genome is not static. Subtle and sometimes not so subtle changes arise with startling frequency. Some of these changes are neutral or even advantageous; these are passed from parent to child and eventually become commonplace in the population. Other changes may be detrimental, resulting in reduced survival or decreased fertility of those individuals who harbour them; these changes tend to be rare in the population. The genome of modern humans, therefore, is a record of the trials and successes of the generations that have come before. Reflected in the variation of the modern genome is the range of diversity that underlies what are typical traits of the human species. There is also evidence in the human genome of the continuing burden of detrimental variations that sometimes lead to disease.

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Knowledge of the human genome provides an understanding of the origin of the human species, the relationships between subpopulations of humans, and the health tendencies or disease risks of individual humans. Indeed, in the past 20 years knowledge of the sequence and structure of the human genome has revolutionized many fields of study, including medicine, anthropology, and forensics. With technological advances that enable inexpensive and expanded access to genomic information, the amount of and the potential applications for the information that is extracted from the human genome is extraordinary.

Role of the human genome in research

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Since the 1980s there has been an explosion in genetic and genomic research. The combination of the discovery of the polymerase chain reaction, improvements in DNA sequencing technologies, advances in bioinformatics (mathematical biological analysis), and increased availability of faster, cheaper computing power has given scientists the ability to discern and interpret vast amounts of genetic information from tiny samples of biological material. Further, methodologies such as fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) have enabled the detection of the organization and copy number of specific sequences in a given genome.

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Branches of Genetics

The Human Genome Project (HGP), which operated from 1990 to 2003, provided researchers with basic information about the sequences of the three billion chemical base pairs (i.e., adenine [A], thymine [T], guanine [G], and cytosine [C]) that make up human genomic DNA. An outgrowth of the HGP was the International HapMap Project (2002-3), an international collaboration that made use of the genome sequence data published by the HGP for the purpose of identifying genetic variations contributing to human disease. Coincident with the completion of these two projects and with the development of computer databases capable of storing the full human genome sequence and known variations came genome-wide association studies, aimed at identifying associations between the variants and particular diseases.

Understanding the origin of the human genome is also of particular interest to many researchers since the genome is indicative of the evolution of humans. The public availability of full or almost full genomic sequence databases for humans and a multitude of other species has allowed researchers to compare and contrast genomic information between individuals, populations, and species. From the similarities and differences observed, it is possible to track the origins of the human genome and to see evidence of how the human species has expanded and migrated to occupy the planet.

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