A cDNA library is made using mRNA instead of DNA as the starting material. The mRNA can be extracted from cells of specific tissues from the organism of interest. The “c” in cDNA stands for copy because a double stranded DNA copy is made from a mRNA . Because of the different starting material, cDNA libraries will have some key distinctions from a genomic library.
The reasons for the differences above are because of the nature of gene expression. When mRNA is made, RNA polymerase will transcribe the DNA sequences from the end of the promoter to the termination sequence. Furthermore, if the mRNA is isolated from the cytoplasm of eukaryotes, intron sequences will have been removed and will not be copied into a cDNA clone. Thus the cDNA clone will be missing parts of the gene sequence.
The clones in a cDNA library will be a small subset of the organism’s total gene complement because the organism will only turn on certain genes in certain cells at certain times. For example, a root cDNA library will be expected to have many clones of genes that would not be found in a leaf cDNA library. A cDNA library made from mRNA isolated from young leaves would also have clones of different genes compared to a cDNA library made from senescing leaf mRNA. Some cDNA gene clones would be found in most cDNA libraries. These would be clones of genes that are expressed in all cells in the plant.
Genes are expressed at different levels in cells. Therefore some genes will have thousands of mRNA copies in a cell while other genes will have a few mRNA copies. When the mRNA sample is used to make a cDNA library, the library will contain proportionally more copies of clones of some genes compared to others.
A summary of key differences between cDNA and genomic libraries is given below.
cDNA library vs. Genomic Library
1) The cDNA clone will only contain the sequences found in the mRNA, not the entire gene while the genomic clone could have the sequences of the entire gene.
2) A cDNA library will not contain a clone of every gene of the organism. A genomic library could have a clone of every gene for that organism.
3) If a gene is expressed at a high level in certain tissues, a cDNA library made from mRNA of that tissue will contain many copies of that gene. Other clones will rarely or never be found in a cDNA library because the gene is not expressed or is rarely expressed. A genomic library will have clones of genes represented as often as they occur in the chromosomes of that organism.