<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;"><img src="http://www.genome.gov/Images/feature_images/DNA_helix_1.jpg" alt="D N A Double Helix" border="0" align="left" /></p>

<h4 style="font-family: Arial, Helvetica, sans-serif; font-weight: normal; font-size: 18.399999618530273px; color: rgb(153, 102, 0); margin: 9.600000381469727px 0px; line-height: 21.600000381469727px;">DNA, Genes and Genomes</h4>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">Located on 23 pairs of chromosomes packed into the nucleus of a human cell, genes direct the production of proteins with the assistance of enzymes and messenger molecules. Specifically, an enzyme copies the information in a gene's DNA into a molecule called messenger ribonucleic acid RNA (mRNA). The mRNA travels out of the nucleus and into the cell's cytoplasm, where the mRNA is read by a tiny molecular machine called a ribosome, and the information is used to link together small molecules called amino acids in the right order to form a specific protein.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">Proteins make up body structures like organs and tissue, as well as control chemical reactions and carry signals between cells. If a cell's DNA is mutated, an abnormal protein may be produced, which can disrupt the body's usual processes and lead to a disease, such as cancer.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">Sequencing simply means determining the exact order of the bases in a strand of DNA. Because bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, researchers do not have to report both bases of the pair.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">In the most common type of sequencing used today, called the chain termination method, a DNA strand is treated with a variety of nucleotides, a set of enzymes, and a specific primer to generate a collection of smaller DNA fragments. Four fluorescent tags, each specific for a given base, is part of the mixture. Each of the fragments differs in length by one base and is marked with a fluorescent tag that identifies the last base of the fragment. The fragments are then separated according to size and passed by a detector that reads the fluorescent tag. Then, a computer reconstructs the entire sequence of the long DNA strand by identifying the base at each position from the size of each fragment and the particular fluorescent signal at its end.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">At present, this technology only can determine the order of up to 800 base pairs of DNA at a time. So, to assemble the sequence of all the bases in a large piece of DNA, such as a gene, researchers need to read the sequence of overlapping segments. This allows the longer sequence to be assembled from shorter pieces, somewhat like putting together a linear jigsaw puzzle. In this process, each base has to be read not just once, but at least several times in the overlapping segments to ensure accuracy.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">Researchers can use DNA sequencing to search for genetic variations and/or mutations that may play a role in the development or progression of a disease. The disease-causing change may be as small as the substitution, deletion, or addition of a single base pair or as large as a deletion of thousands of bases.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">The Human Genome Project, which was led at the National Institutes of Health (NIH) by the National Human Genome Research Institute, produced a very high-quality version of the human genome sequence that is freely available in public databases. That international project was successfully completed in April 2003, under budget and more than two years ahead of schedule.</p>

<p style="font-family: Verdana, Geneva, sans-serif; line-height: 18.399999618530273px;">The sequence is not that of one person, but is a composite derived from several individuals. Therefore, it is a &quot;representative&quot; or generic sequence. To ensure anonymity of the DNA donors, more blood samples (nearly 100) were collected from volunteers than were used, and no names were attached to the samples that were analyzed. Thus, not even the donors knew whether their samples were actually used.</p>