They consist of at least one copy of every DNA sequence in the genome. It is possible to isolate and study a particular gene using a genomic library. The first step of constructing a genomic library involves digesting the DNA sample with restriction enzymes. Next step follows cloning the DNA fragment obtained from restriction digestion into a vector. The vector consists of a specific restriction site. First, the restriction site in the vector undergoes cleavage due to the action of a specific restriction endonuclease. The vector now has a space for new DNA insert. Upon treating the vector with new DNA fragment and specific ligating enzymes, the DNA gets inserted into the vector. Hence, the genomic library gets prepared. However, a limitation exists to the above method. Suppose, if a gene consists of one or more restriction sites for a specific restriction enzyme, it gets cleaved into two or more fragments smaller in sizes. In this way, it produces a large number of fragments. Cloning the longer DNA fragments into a vector helps in solving the problem. Mechanical shearing gives longer DNA fragments. Another approach involves partial digestion. The restriction enzymes recognize four to six base pair nucleotide sequences. Partial digestion results in the formation of overlapping fragments.