The DNA library is a collection of DNA fragments that have been cloned into vectors to recognize and isolate desired DNA fragments. There are two types of libraries – cDNA and genomic libraries.
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The genomic DNA libraries comprise large DNA fragments. On the other hand, cDNA libraries are constituted by cloned, reverse-transcribed mRNA. As a result, they are devoid of the DNA sequences relative to the genomic areas which are not expressed such as the introns.
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cDNA Library (Complementary DNA library)
- cDNA – It is a DNA copy of an mRNA molecule generated by reverse transcriptase, a DNA polymerase that can use either RNA or DNA as their template
- These are prepared using mRNA as templates, their starting material
- They are representative of only those genes of the genome that are expressed given specific conditions
- cDNA does not have introns, and hence can be expressed in prokaryotic cells
Genomic DNA library
- Genomic DNA is the chromosomal DNA of an entity representative of the collection of its genomic content. They are different from that of complementary DNA, mitochondrial DNA or bacterial plasmid DNA. These are directly prepared from the genomic DNA, and represent the complete genome of an entity
- For their construction, ligases and restriction endonucleases are vital
- As it carries introns, they are incapable of expression in the prokaryotes. Moreover, prokaryotes lack the machinery for processing introns
Key Difference between cDNA and Genomic DNA library
The table below depicts the differences between cDNA and Genomic DNA library.
| Collection of clones having complementary DNA to the mRNA of an entity | Collection of clones having the complete genomic DNA of an entity |
| Represents genes expressed in a particular cell at a given period of time | Represents all genes |
| Smaller compared to genomic DNA library | Vast in comparison to cDNA library |
| Clone contains sequences seen on mRNA only, not the complete gene. | Contains sequences for introns and exons. Genomic clones can have sequences of the complete gene |
| Reverse transcriptase enzyme | Ligases and restriction endonucleases |
| The number is way lesser in comparison to the genomic DNA library | Way greater number of recombinants compared to cDNA library |
| mRNA | DNA |
| Can be directly expressed as it has only coding sequences | Gene expression extracted from the genomic library is challenging in the prokaryotic systems, as they are devoid of a splicing mechanism |
| Occurs in the synthesis of the first cDNA strand | Not involved |
| Phagemids, Plasmids, Lambda phage for harbouring smaller fragments due to the absence of introns | Cosmids, plasmids, Lambda phage, YAC, BAC to harbour larger fragments |
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