Chromosome physical maps

Schematic drawing showing the human Y chromosome physical map. In all, 196 individual YAC clones were assembled into 10 contiguous segements separated by nine short gaps. The location of several known genes and the putative centromere region are shown.

Radiation hybrid mapping Now the most rapid method of localizing a gene or DNA fragment to a subregion of a human chromosome and constructing a physical map. 

The directions of transcription of certain operons are indicated by the arrows. The 1983 version of this map76 contained 1027 loci and we now know the location of all of the more than 4300 genes. (B) Physical map of the E. coli chromosome obtained by study of large 20-100 kb fragments obtained by digestion with restriction endonuclease Notl and separated by pulsed field gel... 

Let us now consider how knowledge of bacterial sex factors and of phage K permitted the mapping of bacterial chromosomes. Together with the use of restriction endonucleases these techniques gave us the first precise physical maps of bacterial chromosomes and pointed the way toward the determination of complete genome sequences. 

DNA has been substituted in one strand.98 Since distances can be measured accurately on the electron micrographs, rather precise ( 50-100 bp) physical maps can be obtained. The chromosome map of phage K was mapped in this way initially now its complete nucleotide sequence is known. Another electron microscopic method is useful for location of AT-rich regions that denature readily. In a suitable concentration of formamide these regions melt to form visible single-stranded denaturation loops similar to the bubbles in Figure 26-6. 

The National Academy of Sciences il. S.i endorsed a massive national effort to map and sequence the human genome The project target—to produce genetic and physical maps of increasing resolution, with a fully detailed map of the chromosomes—the project to he completed within a decade and ai a cost estimated to be S3 billion,... 

Hirai, H. and LoVerde, P.T. (1995) FISH techniques for constructing physical maps on schistosome chromosomes. Parasitology Today 11, 310-314. 

Restriction maps of entire genomes can be obtained by two approaches. In the top-down approach, infrequently-cutting restriction enzymes and pulsed-field gel electrophoresis are used to map a chromosome much as one would a plasmid. In the bottom-up approach, analysis of many clones in parallel leads to the accumulation of a set of clones which represent the whole genome. Restriction maps of the clones can then be merged to make the genome map. The clones themselves are also a valuable resource. The earliest complete and nearly complete physical mapping projects, both top-down E. co//[10], Schizosaccharomycespombe 19 ) and bottom-up ( . coli[ 1], Saccharomyces cerevisiae[ 2, Caenorhabditis [80,81 ]), were on genomes which were already... 

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