cDNA vs. Genomic DNA library

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.

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.

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
  • It can also represent the DNA of both eukaryotic and prokaryotic entities and carries introns
  • 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.

cDNA library

Genomic DNA library

What it is

Collection of clones having complementary DNA to the mRNA of an entity

Collection of clones having the complete genomic DNA of an entity

What does it contain?

Represents genes expressed in a particular cell at a given period of time

Represents all genes

Size

Smaller compared to genomic DNA library

Vast in comparison to cDNA library

Coding and non-coding sequences

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

Important substance required for their construction

Reverse transcriptase enzyme

Ligases and restriction endonucleases

How many recombinants to be screened?

The number is way lesser in comparison to the genomic DNA library

Way greater number of recombinants compared to cDNA library

Material required to start

mRNA

DNA

Expression in prokaryotic system

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

Role in reverse transcriptase

Occurs in the synthesis of the first cDNA strand

Not involved

Vectors used

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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