[SagarikaGhosh]

Properties of cDNAs and cDNA libraries:

- Eukaryotic cDNAs are free from intron sequences
- They are smaller in size than the corresponding genes, i.e., the genes that encoded them
- A comparison of the cDNA sequence with the corresponding genome sequence permits the delineation of intron/ exon boundaries
- The contents of cDNA libraries from a single organism will vary widely depending on the developmental stage and the cell type used for preparation of the library. In contrast, the genomic libraries will remain essentially the same irrespective of the developmental stage and the cell type used.
- A cDNA library will be enriched for abundant mRNAs, but may contain only a few or no clones representing rare mRNAs.

Subtracted cDNA Library:

When a cDNA library is generated by first removing those mRNA or cDNA sequences that are common to two sources, e.g., two different cell types, such a library is called subtracted cDNA library. Such a library is enriched for those mRNA sequences that are expressed in target cell type, but not for those that are expressed in the other tissue, which is used as the source mRNA or cDNA that is used for subtraction.

Applications of cDNA Library:

Use of cDNA is absolutely essential when the expression of an eukaryotic gene is required in a prokaryote, e.g., a bacterium. The gene expression may be needed either
1. for detection of the clone or
2. the polypeptide product may be the primary objective of cloning.

This is because eukaryotic genes have introns, which must be removed from their transcripts to yield mature mRNAs. Bacteria do not possess the enzymes necessary for removal of introns; therefore, they do not have the ability to produce mature ready-to-translate mRNA molecules from the transcripts of complete native eukaryotic genes. In contrast, functional mRNA molecules do not have introns; hence their cDNA is also free from introns and can be cloned and expressed in bacteria. For example, cDNA for interferon, blood clotting factor VII C (both human) and several other mRNAs have been expressed in bacteria. In addition, cDNAs are used to generate expressed sequence tags (ESTs), which are very useful in high throughput genome research.