4-4.3. Amplified library

•  The primary library created is usually of a low titer and unstable. The stability and titer can be increased by amplification. For this, the phages or bacterial colonies are plated out several times and the resulting progenies are collected to form an amplified library.

•  The amplified library can then be stored almost indefinitely due to long shelf-life of phages.

•  It usually has a much larger volume than the primary library, and consequently may be screened several times.

•  It is possible that the amplification process will result in the composition of the amplified library not truly reflecting the primary one.

•  Certain DNA sequences may be relatively toxic to E. coli cells. As a consequence bacteria harboring such clones will grow more slowly than other bacteria harboring non-toxic DNA sequences. Such problematic DNA sequences present in the primary library may be lost or under-represented after the growth phase required to produce the amplified library.

4-4.4. Subgenomic library

Subgenomic library is a library which represents only a fraction of the genome. Enhancing the fold of purification of target DNA is crucial for subgenomic DNA libraries which can be achieved by multiple, sequential digestion when information of the restriction map of the sequences of interest is known. After initial purification of a given fragment, the purification can further be increased by redigestion with another enzyme generating a smaller (clonable) fragment relative to original DNA.

4-4.5. Advantages of genomic libraries

•  Identification of a clone encoding a particular gene of interest.

•  It is useful for prokaryotic organisms having relatively small genomes.

•  Genomic libraries from eukaryotic organisms are very important to study the genome sequence of a particular gene, including its regulatory sequences and its pattern of introns and exons.