Gene Banks or Libraries

Clones containing genomic DNA of a particular sequence may be identified by plaque hybridization, which is similar in principle to colony hybridization for detection of particular plasmid recombinants. A lawn of E. coli containing several thousand plaques of independent recombinants is blotted on to a nitrocellulose filter. The DNA is released from the blotted recombinant phage, denatured by alkali, and neutralized. A radioactive probe of nucleic acid containing sequences of the gene of interest is then hybridized to the DNA plaques on the filter. Positive plaques, detected by autoradiography as radioactive DNA-RNA or DNA—DNA hybrids, are then picked off and amplified at lower plaque density (approx. 100 per plate). 

Many mRNAs in eukaryotes possess a poly (A) sequence at their 3 ends, so it is possible to selectively clone a bank of cDNAs representing the total poly(A) RNA populations in a eukaryotic cell.  

Partial digestion with any one or more restriction nuclease 

Methylation of EcoRl recognition sequences by EcoRl methylase to protect DNA from EcoRl nuclease 

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