Human Genome Project Completion: Frequently Asked Questions

The Human Genome Project Completion: Frequently Asked Questions

On April 14, 2003 the National Human Genome Research Institute (NHGRI), the Department of Energy (DOE) and their partners in the International Human Genome Sequencing Consortium announced the successful completion of the Human Genome Project.

A genome is an organism's complete set of deoxyribonucleic acid (DNA), a chemical compound that contains the genetic instructions needed to develop and direct the activities of every organism. DNA molecules are made of two twisting, paired strands. Each strand is made of four chemical units, called nucleotide bases. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases on opposite strands pair specifically; an A always pairs with a T, and a C always with a G.

The human genome contains approximately 3 billion of these base pairs, which reside in the 23 pairs of chromosomes within the nucleus of all our cells. Each chromosome contains hundreds to thousands of genes, which carry the instructions for making proteins. Each of the estimated 30,000 genes in the human genome makes an average of three proteins.

Sequencing means determining the exact order of the base pairs in a segment of DNA. Human chromosomes range in size from about 50,000,000 to 300,000,000 base pairs. Because the bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, scientists do not have to report both bases of the pair.

The primary method used by the HGP to produce the finished version of the human genetic code is map-based, or BAC-based, sequencing. BAC is the acronym for "bacterial artificial chromosome." Human DNA is fragmented into pieces that are relatively large but still manageable in size (between 150,000 and 200,000 base pairs). The fragments are cloned in bacteria, which store and replicate the human DNA so that it can be prepared in quantities large enough for sequencing. If carefully chosen to minimize overlap, it takes about 20,000 different BAC clones to contain the 3 billion pairs of bases of the human genome. A collection of BAC clones containing the entire human genome is called a "BAC library."

In the BAC-based method, each BAC clone is "mapped" to determine where the DNA in BAC clones comes from in the human genome. Using this approach ensures that scientists know both the precise location of the DNA letters that are sequenced from each clone and their spatial relation to sequenced human DNA in other BAC clones.

For sequencing, each BAC clone is cut into still smaller fragments that are about 2,000 bases in length. These pieces are called "subclones." A "sequencing reaction" is carried out on these subclones. The products of the sequencing reaction are then loaded into the sequencing machine (sequencer). The sequencer generates about 500 to 800 base pairs of A, T, C and G from each sequencing reaction, so that each base is sequenced about 10 times. A computer then assembles these short sequences into contiguous stretches of sequence representing the human DNA in the BAC clone.

This is intentionally not known to protect the volunteers who provided DNA samples for sequencing. The sequence is derived from the DNA of several volunteers. To ensure that the identities of the volunteers cannot be revealed, a careful process was developed to recruit the volunteers and to collect and maintain the blood samples that were the source of the DNA.

The volunteers responded to local public advertisements near the laboratories where the DNA "libraries" were prepared. Candidates were recruited from a diverse population. The volunteers provided blood samples after being extensively counseled and then giving their informed consent. About 5 to 10 times as many volunteers donated blood as were eventually used, so that not even the volunteers would know whether their sample was used. All labels were removed before the actual samples were chosen.

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Human Genome Project Completion: Frequently Asked Questions