Consider four different kinds of human libraries: a genomic library, a brain cDN

Question

Consider four different kinds of human libraries: a genomic library, a
brain cDNA library, a liver cDNA library, and a unique genomic sequence (no repetitive DNA) library.
a. Assuming inserts of approximately equal size, which would contain the
greatest number of different clones?
b. Would you expect any of these not to overlap the other at all in terms of the
sequences it contains? Explain.
c. How do these four libraries differ in terms of the starting material for
constructing the clones in the library?

Explanation / Answer

a) Genomic library has the greatest number of different clones because a cDNA (complementary DNA) library is a DNA library that has been created from mRNAs that are present in the cell. Since a cDNA is created from mRNA transcripts, that means that in Eukaryotic organisms there will be no introns or transcriptional factors present in the cDNA library, only exons. Only protein coding regions will be present in a cDNA library. This also means that a cDNA library is often times tissue specific. Since the expression of mRNAs will be different in different tissues of the organism it will appear different then a genomic library. Often times to offset this problem a cDNA library will be composed of different tissues (brain, liver, heart) to encompass a greater variety of the proteins that are expressed. A genomic library will contain all the exons, introns, and transcriptional factors that are not found in the cDNA library.

Again, the genomic library will have all DNA of the organism. So anything present in either cDNA library will also be in the genomic library. Comparing the brain and liver libraries, there are a lot of "housekeeping genes" that should be expressed in both tissues--genes for basic metabolism, for cytoskeletal proteins, for organelle proteins. So those should overlap nicely between the two cDNA libraries. Other genes are expressed in one but not the other tissue.

Whereas the a unique genomic sequnces is the part of seq uense which are non repitative so we may exclude some partrs and characterise those parts which code for certain genes.

b)As we know the cDNA library oonly contains the intronic part and the a unique genomic sequnces contains only selected genes which all presnt in the genomic library which comprises all the exons, introns, and transcriptional factors etc.

c)As stated above, the genomic library uses nuclear DNA, the other two are produced from mRNA in each tissue.

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