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In contrast, Physical mapping also determines the physical distance between landmarks on the chromosomes. With its measurement of relative distances between markers, the physical map measures the actual physical distance between chromosomal sites and describes the chemical characteristics of the DNA molecule itself. The earliest of these, the cytogenetic map, relied on microscopic analysis of the banding patterns of stained chromosomes to determine the location of genes relative to visible markers. The most accurate mapping is now done with robotics, lasers, and computers to measure the distance between genetic markers. The DNA fragments can be duplicated numerous times in the laboratory so that the resulting identical copies called clones can be tested for the presence or absence of specific genetic landmarks. This has brought about higher resolution analysis. The high-resolution physical maps now assembled use a variety of approaches for purifying pieces of DNA. One method relies on restriction enzymes isolated from bacteria whose natural function is to attack viral and other foreign DNA to recognize short sequences of DNA and cut the molecules at those sites. In one approach, “Top-down” mapping, the small fragments then can be amplified by cloning or by polymerase chain reaction techniques. Polymerase chain reaction (PCR) is technique for rapid production of millions of copies of a particular stretch of DNA. Next, the pieces are reassembled in various ways and their order and distances are established. “Bottom-Up” mapping also involves cutting the chromosome into fragments, then cloning and ordering the fragments into overlapping sequences of DNA known as contigs (see illustration four)(Gibbons, 1998). Contigs are linked into a library of chromosomal segments and reassembled by sophisticated automated sequencing machines. Contig mapping of the Human genome favored the following strategy: to map the genome one chromosome at a time, dividing and subdividing each chromosome into smaller and smaller segments before beginning restriction enzyme mapping and ordering of clones. After subdivision, restriction maps of these smaller segments would be determined and the information linked together to form continuous maps of whole chromosomes. In principle, this strategy could be broken down into five consecutive steps (Gibbons, 1998):

1. isolation of each human chromosome,

2. division of each chromosome into a collection of overlapping DNA fragments 0.5 to 5 million base pairs in length,

3. subdivision and isolation of each of these chromosomal fragments into overlapping DNA fragments about 40,000 base pairs in length,

4. determination of the order of the 40,000-base-pair DNA fragments as they appear in the chromosomes and determination of the positions of cutting sites for a restriction enzyme within each of these fragments, and

5. use of the mapping information gained in step 4 to link together each of the overlapping 0.5-to 5-million-base-pair fragments isolated in step 2.

The goal of the progressively more sophisticated mapping is the ability to definitely connect a gene and a disease.

In addition to physical maps, genetic maps were useful to the project in two ways. First, the linkage studies tracking disease – causing genes by positional cloning. Secondly, by providing new markers for physical mapping (Gibbons, 1998).

Duplicating DNA accurately and quickly is important to both mapping and sequencing. Scientists first replicated fragments of human DNA by cloning them in single – celled organisms that divide rapidly, such as bacteria or yeast. Now the common use of cloning DNA is known as the polymerase chain reaction, which is easily automated and can copy a single molecule of DNA many millions of times in a few hours. However, the entire DNA has not been sequenced from libraries of different tissues at different stages of life. In fact, only short unique regions, called expressed – sequence tags, have been developed to act as chromosome markers (Gibbons, 1998).

Some benefits, in general, are the genetic information we have and when the Genome Project is completed, it will have generated a catalog describing 50,000 to 100,000 human genes at some level of detail; high-resolution maps of the chromosomes, including hundred of thousand of landmarks (see illustration five). Three billion base pairs of DNA sequence information. The project has laboratory information management systems, robotics, database management systems, and graphical user interfaces that are for the computing tools required for helping researchers make sense of this flood of data (Purves, et al. 1998).

A new field of research, known as bioinfomatics, has developed to address the computing challenges raised by the project. Moreover, the researchers in bioinfomatics have developed public databases connected to the Internet to make Genome Project data available to scientists worldwide. Regardless of who completes the sequencing, by what method, and when, the mapping of the 8 billion base pairs that together make up human structure, functioning and propensity to disease is only the beginning. The real challenge will lie in interpreting the information and relating it to human health and disease (Schook, et al. 1991).

In conclusion, the HGP has enormous possible benefits for the identification of the genes that are altered in diseases and may even lead to cures. It can tell us about ourselves and is a strong advocate of reproduction and procreative freedom. This research

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