Examples of DNA regions that are present in Genomic DNA
libraries but may not be present in cDNA libraries are:
- Promoter Sequences: Promoter sequences are not transcribed to
RNA and would be absent from cDNA libraries while being present in
Genomic DNA libraries.
- Genes not expressed in tissue/developmental stage: Genomic DNA
libraries contain all the DNA sequences present in an organism,
regardless of expression. However, all genes are not expressed in
all tissue types. For example, the Insulin gene is only expressed
in Pancreatic beta cells and therefore the sequence of the insulin
gene may not be present in a cDNA library from another tissue/cell
type. Another example is Haemoglobin Alpha gene which is only
expressed in Red Blood Cells and may not be found in cDNA from
another tissue.
- Alternate Splicing products:Several genes exist that are
expressed as one isoform in one tissue and as another isoform in
another tissue. This variation is achieved by alternate splicing,
which causes different Exons to be stitched together to form the
isoform for that particular cell type. A Genomic DNA library will
contain all exon sequences, while a tissue specific cDNA may not
have all sequences.