The Importance of Vectors

Several types of DNA libraries have been developed for specific purposes, but all share some common features. The DNA fragments that make up the library are attached to other DNA sequences that are used as "handles" to maintain the fragments. These "handles," called vectors, allow the DNA to be replicated and stored, typically within model organisms such as yeast or bacteria.

vectors carriers Different types of vectors can be used to store DNA fragments of dif-

plasmid a small ring of ferent lengths. For example, plasmidvectors can store small fragments (from DNA found in many bac- a few hundred bases up to ten or twenty thousand bases of sequence), while viral vectors, or viral-plasmid hybrids such as cosmids, can store up to fifty thousand bases, and yeast artificial chromosome (YAC) vectors can store hundreds of thousands of bases. In general, plasmid-based vectors are the easiest to manipulate but store the smallest fragments. They are commonly used for applications that involve complex manipulations, such as cloning or gene expression, but that require only small DNA fragments (e.g., cDNA libraries, as described below).

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