Definition Of Comparative Genomics

Comparative genomics is the study of similarities between genomes of different species. Comparative genomics reveals the changes made in genomes during evolution and provides insight into the molecular features and mechanisms responsible for the evolution of all life forms. The resolution possible for a comparative genome map relies on the type of reagents available for the species being studied. The pig human comparative map includes one of the most elegant uses of fluorescent in situ hybridization (FISH) in a livestock species.[2] Research-ers[2] used entire single human chromosomes labeled with a fluorescent dye as probes on pig metaphase chromosome spreads. Once this was accomplished, entire single pig chromosomes could be used as probes and hybridized to human metaphase chromosome spreads. The bidirectional FISH study provided a detailed comparison of the pig and human genomes. The results of this study are continually refined and available on the web site (http:// www.toulouse.inra.fr/lgc/pig/compare/compare.htm). This study was possible only because the necessary whole chromosome libraries were available for both species. Unfortunately, this methodology is unable to determine conservation of gene order within conserved syntenic chromosomal segments.

The highest-resolution comparative map compares the sequences of the entire genomes of different species. To date, in mammals, this is possible only for a comparison between the human, mouse and rat genomes. However, there are plans to sequence the genomes of several other animal species including chicken and dog. Two additional livestock species (pig and cow) have been placed in the high priority category for genome sequencing by the National Genome Research Institute (http://www.genome. gov/). Until complete genome sequences for livestock species are available, comparative genomics can be conducted by computerized (virtual) mapping using conserved synteny of large segments of the target animal's sequence against the human, mouse, or rat genome sequence. This is essentially one of the projects the National Institute of Health's Intramural Sequencing Center is currently studying (http://www.nisc.nih.gov/).

This large-scale comparison of conserved genomic sequence is a powerful method to identify DNA sequences with specific functions, preserved throughout the evolutionary process. Thus, modern day comparative genomic research is critical to the state of the art of functional genomics studies.

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