Genome libraries probes

By screening cDNA or genomic libraries with homologous probes, the gene encoding the H2 receptor was first identified in dogs [14] and, subsequently, in other species including human [15, 16]. 

Complementary DNAs (cDNA) are DNA copies of mRNAs. Reverse transcriptase is the RNA-directed DNA polymerase that synthesizes DNA strand, using purified mRNA as the template. DNA polymerase is then used to copy the DNA strand forming a double-stranded cDNA, which is cloned into a suitable vector. Once a cDNA derived from a particular gene has been identified, the cDNA becomes an effective probe for screening genomic libraries (Cowell and Austin, 1997). Annotated human cDNA sequences can be accessed from HUNT at http // www.hri.co.jp/HUNT (Yudate, 2001). 

Characterize retrotransposons. Once a positive clone has been identified, it can be used as a probe to isolate full-length copies of the element from a genomic library. These clones can also be characterized by phylogenetic screening. 

If either of the methods gives evidence for a terminal repeat it is then relatively easy to discover the flanking sequences at the point of insertion. In addition, there is usually a short direct repeat of the genomic DNA sequence at the insertion site. To confirm the terminal repeats, and whether the total element has been determined, a second element has to be isolated from the genomic library using the repetitive sequence as a probe. DNA sequence analysis of the second element will confirm the completeness of the element first isolated. 

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