Mapping and Sequencing the Human Genome (1988) / Chapter Skim
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4. Mapping
4. Mapping
Pages 34-55
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From page 34... ...
Genetic linkage maps and physical maps (even when incomplete) , as well as partial sequences, have been 34
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From page 35... ...
From a mapping standpoint, this banding is important in that it allows human chromosomes to be individually recognized by light microscopy and allows an average chromosome to be subdivided into 10 to 20 regions. Banding patterns provide the basis for a physical map of the chromosomes, often referred to as a cytogenetic map.
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From page 36... ...
Also of major 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 unclonecl DNA that is extracted from human or other ceils. Whether chromosome mapping is being done at the level of the chromosomal DNA molecule (physical mapping)
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From page 37... ...
The maps can be calibrated in nucleotides by measuring the sizes of the DNA fragments produced when a chromosomal DNA molecule is cleaved with a restriction enzyme. Restriction mapping has not yet been extended to DNA molecules as large as human chromosomes.
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From page 38... ...
Because of the unique ability of genetic linkage mapping to define and localize disease genes, increasing the number of genetic markers available for this type of mapping should receive major emphasis in any overall program to map the human genome. A type of physical map that provides information on the approximate location of expressed genes is a complementary DNA (cDNA)
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From page 39... ...
This map contains information of great biological and medical significance simply because it represents the expressed portion of the genome. The Development of Ordered Collections of DNA Clones Is an Important Adjunct to Physical Mapping In theory, sensitive DNA-probe technologies make it possible to construct physical maps while cloning only a small fraction of the genome that is being mapped.
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From page 40... ...
cow - an- - - r - - ~ - ~ 7 - - ~ - ~-I 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 41... ...
However, two genes that are farther apart but still on the same chromosome are more likely to be separated by exchange during sperm or egg production. 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.
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From page 42... ...
The data are pooled and distributed at regular intervals to all interested investigators. This international collaboration has greatly speeded human genetic linkage mapping and lowered the entry barriers for new investigators who are interested in joining the effort.
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From page 43... ...
RFLPs Are Useful for Interrelating Physical and Genetic Linkage Maps Genetic linkage mapping allows those genes with no known cellular or molecular effects to be located on the human genome. On the other hand, physical maps describe the DNA molecules present in chromosomes.
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From page 44... ...
Now, as a result of the REAP approach, both genes have been mapped with great precision and shown to have a common genetic basis in most or all cases (Gusella et al., 1983; White, 19864. Equally important, the RAP 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 Ramps.
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From page 45... ...
MAKING PHYSICAL MAPS Medium-Resolution Mapping of Restriction' Sites Is Facilitated by New Methods of Preparing and Separating Large DNA Molecules At low resolution, cytogenetic mapping of banded chromosomes is already advanced. At high resolution, methods such as restrictionsite mapping and DNA sequencing of clones are well established.
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From page 46... ...
In another method, only a single restriction enzyme is used to produce the large DNA fragments. In addition, however, a set of small DNA probes, called linking probes, is generated by selectively cloning the short segments of DNA that surround each of the cleavage sites for the restriction enzyme used to make the large fragments.
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From page 47... ...
For these reasons, doing as much physical mapping as possible on cloned DNA has overwhelming advantages. 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.
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From page 48... ...
Not all overlapping segments are being recovered because of biases inherent in the DNA cloning procedures used, and (2) the fingerprint information collected for the overlapping DNA segments lacks sufficient precision to distinguish all DNA fragments from each other unambiguously.
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From page 49... ...
Further development of systems for cloning large DNA molecules will greatly enhance the efficiency of ordering DNA fragments. 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.
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From page 50... ...
The lower resolution physical maps will provide a framework within which to organize the highly fragmentary sequence data that will be generated by these initial sequencing efforts, while the ordered-clone collections will provide the actual fragments that are subcloned for final sequencing (see Chapter 51. Chromosome maps can also be usefully applied to begin a systematic assignment of expressed genes to map positions.
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From page 51... ...
Both physical and genetic linkage mapping have been invigorated by these developments, and important synergism has arisen between these two approaches to genomic mapping. Consequently, the goal of developing complete physical and genetic linkage maps of the human genome in a relatively short time is now realistic.
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From page 52... ...
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 Ramps 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 53... ...
These improvements, which should be major initial goals of the human genome project, will include increased DNA size range, increased resolution, diminished cost, and improved accuracy. Some of the specific target areas include improving or creating methods for: · Physically separating intact human chromosomes.
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From page 54... ...
1980. Construction of a genetic linkage map in man using restriction fragment length polymorphisms.
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From page 55... ...
1985. Construction of linkage maps with DNA markers for human chromosomes.
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