Report of the Committee on Mapping and Sequencing the Human Genome (1988) / Chapter Skim
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MAPPING
MAPPING
Pages 34-51
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From page 34... ...
Genetic linkage maps and physical maps (even when incomplete) , as well as partial sequences, have been of value in research on Escherichia coli and Drosophila.
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From page 35... ...
Methods of cloning DNA molecules from any organism into microbial cells, of cleaving molecules at specific sites, and of separating DNA fragments that differ only slightly in size have all contributed to present mapping capabilities. Also of importance are DNA-probe techniques that allow a particular DNA sequence, usually obtained from a DNA clone, to be used to detect other DNA molecules with similar or identical sequences in uncloned DNA that is extracted from human or other cells.
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From page 36... ...
FUNDAMENTALS OF GENOME MAPPING Physical Maps Describe Chromosomal DNA Molecules. Whereas Genetic Linkage Maps Describe Patterns of Inheritance Physical maps specify the distances between landmarks along a chromosome.
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From page 37... ...
Indeed, the great variety of chromosomes that can be produced by exchange and independent assortment is responsible for much of the genetic individuality of different humans. The order of genes on a chromosome measured by linkage maps is the same as the order in physical maps, but there is no constant scale factor that relates physical and genetic distances.
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From page 38... ...
Having ordered clone collections is also a prerequisite to most methods of sequencing the genome since the clones would provide the actual DNA fragments that would be purified and prepared for DNA sequencing. Both Physical and Genetic Linkage Maps Can Be Constructed with Various Degrees of Resolution and Connectivity All types of mapping presuppose an inherent trade-off between the level of detail, or resolution, in a map and the extent to which the map provides a convenient overview of the mapping objective (its connectivity)
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From page 39... ...
GENETIC LINKAGE MAPPING Restriction Fragment Length Polymorphisms Are Convenient Landmarks for Genetic Linkage Mapping Human beings differ from one another at many points in their genomes: Some of these differences account for differences in traits such as eye color, blood type, height at maturity, or susceptibility to a particular disease. Most differences, however, have few or no consequences in terms of the appearance or function of the individual.
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From page 40... ...
The probability of such an exchange increases with the physical distance between the genes, thereby accounting for the observation that genes are ordered in the same way by genetic linkage and by physical mapping. To measure the degree of exchange between two genes, the frequency of co-inheritance of parental allele combinations must be measured on a statistically significant sample.
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From page 41... ...
This will greatly facilitate finding the actual DNA sequences that correspond to a gene once such a gene is localized on the genetic linkage map. In addition, making maps continuous across entire chromosomes will be easier by genetic linkage mapping, whereas maps of higher resolution (finer than a million nucleotides)
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From page 42... ...
. Equally important, the RFLP approach, because of its ability to interrelate genetic linkage and physical mapping, has laid the groundwork for locating and analyzing the actual DNA sequences responsible for the diseases by coupled strategies of physical mapping and cloning, starting with the DNA clones used to probe for the linked RFLPs.
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From page 43... ...
Restriction enzymes have been discovered that cleave DNA into fragments with average sizes ranging from 100,000 to 1 million nucleotides. In addition, a method known as pulsed-field gel electrophoresis, which allows the separation of DNA fragments as large as 10 million nucleotides, has been introduced (Schwartz and Cantor, 1984)
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From page 44... ...
Particularly for high-resolution mapping, the preferred source of DNA samples for physical mapping will be ordered collections of DNA clones -- a set of cloned DNA fragments that have been sufficiently analyzed that they can be arranged to reflect the order of their corresponding DNA fragments on the original chromosomes. Since the clones are usually generated in a way that produces cloned DNA fragments that start and stop at random sites along the chromosome, each member of the collection will normally overlap 44
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From page 45... ...
. Fingerprinting Methods Can Be Used to Order DNA Clones Preparing an ordered-clone collection involves cloning DNA fragments as molecules that can replicate in a microbial host, determining the order of these fragments in the genome, and propagating the fragments in pure form to make them widely available for subsequent analysis.
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From page 46... ...
For example, it should be possible to prepare DNA clone collections by using a single restriction enzyme that cuts DNA infrequently; this procedure would generate a single family of large DNA fragments that are then cloned. This family would be much less complex than the collection of randomly cut clones required for the fingerprinting method.
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From page 47... ...
Initially, the map assignments for the expressed genes could be based on the existing cytogenetic map and could be carried out by somatic cell genetic techniques, as well as by in situ hybridization of cDNAs to chromosomes. As the physical mapping and sequencing of the genome proceeded, it would require relatively little effort to refine these assignments.
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From page 48... ...
Consequently, attaining high connectivity in the physical map should be a major priority of the overall human genome project. Because the technology needed for genetic linkage mapping with RFLPs is more advanced than that for physical mapping, an immediate emphasis should be placed on completing the genetic linkage map.
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From page 49... ...
Despite recent advances, many limitations on physical mapping methods still exist. For example, DNA fragments as large as 10 million nucleotides can be handled, but only with considerable difficulty, and such large fragments cannot yet be cloned.
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From page 50... ...
1980. Construction of a genetic linkage map in man using restriction fragment length polymorphisms.
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From page 51... ...
1985. Construction of linkage maps with DNA markers for human chromosomes.
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