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A genomic library is a collection of plasmid
clones or phage lysates contatining recombinant DNA molecules so
that the sum total of DNA inserts in this collection represents
the entire genome of the concerned organism. Genomic libraries
are prepared by purifying total DNA & then makin a partial
restriction digest resulting in fragments that can be cloned into
a suitable vector, usually a lambda replacement vector, a cosmid
or possibly a yeats artificial chromosome ( YAC), Bacterial
artificial chromosome ( BAC) or p1 vector.
For Bacteria, Yeast & fungi, the number of clones needed for a
complete genomic library is not as large as to be unmanageable.
But for animals and plants, though a complete library contains so
many different clones that identification of the desired one is a
difficult task.
IMPORTANCE
OF GENOMIC LIBRARIES
Genomic libraries & clones are necessary in the addition
to cDNA clones because cDNA clones are generated from mRNA, which
lack introns & other sequences surrounding the gene.
And if the gene are to be modified & returned to plants, it is
likely that genomic sequences will be more useful. Moreover, the
genome of plants & animals are remarkably complex, & a particular
fragment of interest comprise only a small fraction the whole
genome. Therefore construction of a useful & representative
recombinant genomic library is dependant upon the generation of
large population of clones. However inspite of all the care taken
in the production of genomic libraries, certain DNA fragment
should be expected to be under or overrepresented or even missing.
The possible reason for this may be that certain fragments code
for a toxic product, or might replicate slowly or might have been
altered by recombinants events during cloning. |
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