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Cloning Developmental Genes
Clones of developmental genes are required for many studies. As in any research, the techniques devised for the studies and the results obtained greatly aided later and similar efforts. The only information available to assist cloning a developmental gene the first time was the known chromosomal location of the gene. Such genes could be mapped both genetically and cytologically. The cytological mapping utilized the char-acteristic banding pattern seen in the polytene chromosomes from Drosophila salivary glands and the existence of deletions and inversionsaffecting a developmental complex like bithorax.
Cloning of bithorax by Bender began with a collection of random Drosophila clones. These could be located approximately on the Droso-phila chromosomes by in situ hybridization. Then a chromosomal walkwas done to reach the gene complex. One of the major problems with walking along a chromosome is knowing when you get to your gene. Sometimes deletions will delineate a gene. Inversions that inactivate the desired gene are better, however. The end points are easy to locate in a walk and they must lie within or very near the gene.
One approach to complete the identification of a clone is to seek RNAs to which the cloned DNA hybridizes. Hybridizations of radioac-tive DNA from candidate clones to mRNA can be done in situ to slices of embryos or even adult flies to identify any RNAs with expression patterns suggestive of developmental genes. Once a candidate RNA can be detected, RNA extracted at the appropriate time from embryos of the the right tissue can be used to make a cDNA library that is then screened with the cloned DNA. In the case of Antennapedia, the probe to the gene hybridized not only to the gene, but also to another region of the same chromosome, a region containing the bithorax developmental gene complex.
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