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• A DNA library (3) is a collection of clones of DNA designed so that there is a high probability of finding any particular piece of the source DNA in the collection. When each clone has been placed in a precise physical location relative to others (such as in wells of microtiter plates), the library is said to be ordered.
• DNA libraries can be made (2) using highly efficient cloning vectors such as lambda phages, cosmids, P1 phages and bacterial or yeast artificial chromosomes. When the DNA is simply ligated to the vector and packaged in the phage particles, the library is said to be unamplified. An amplified library is one in which the original DNA's have subsequently been increased by replication in bacteria.
• The DNA cloned (1) in libraries depends on the purposes of the investigators.
  • Genomic libraries are made from total nuclear DNA of an organism. In making these libraries, it is important to cut the DNA into clonable size pieces as randomly as possible. Shearing or partial digestion with a frequently cutting restriction endonuclease are often used.
  • Chromosome-specific libraries are made from the DNA of purified isolated chromosomes.
  • cDNA libraries are made from the mRNA populations of particular tissues. There are thus liver, heart, leaf, root, etc. cDNA libraries.
  • cDNA libraries may be normalized. Normalization eliminates many copies of highly abundant cDNAs. It can be achieved by incubating denatured double stranded cDNAs to intermediate Cot values in hybridization reactions (see technical details).
  • cDNA libraries are often expression libraries, in which clone construction is such that part or all of the encoded protein is expressed in bacteria harboring the cloned DNA. Such expression is needed in screening libraries using antibodies or enzyme activities.
• How good a genomic library is can be calculated from the formula:

      ln(1-P) 
N = -----------
     ln(1 -1/n)

where N is the number of clones needed to have a probability, P, of finding any particular sequence in the library, when the ratio of the genome size to the average cloned fragment size is n. The same formula applies to cDNA libraries, but 1/n is defined as the fractional proportion that a specific mRNA is of the total mRNA.