Key Difference – cDNA vs Genomic Library
There are two major types of DNA libraries constructed by scientists using genetic engineering techniques. Those are cDNA library and Genomic library. The key difference between cDNA and Genomic library is that cDNA library contains the cloned complementary DNA of total mRNA of an organism while the genomic DNA library contains the cloned fragments of the entire genome of an organism. The genomic DNA library is larger than the cDNA library.
CONTENTS
1. Overview and Key Difference
2. What is cDNA Library
3. What is Genomic Library
4. Side by Side Comparison – cDNA vs Genomic Library
5. Summary
What is a Genomic Library?
A genomic DNA library is a collection of clones bearing the fragments of total genomic DNA of an organism. It contains the entire genomic DNA of that organism, including coding and noncoding sequences. The construction of a genomic library is performed by the recombinant DNA technology followed by cloning (genetic engineering). There are different steps involved in the construction as shown in figure 01. The process starts with the genomic DNA isolation. Using a suitable DNA extraction protocol, total genomic DNA of an organism should be isolated. Then the DNA should be converted into manageable sizes or into the specific fragments by restriction endonucleases (DNA cutting enzymes). Fragmented DNA should be inserted into vectors using DNA ligases (DNA joining enzymes). A vector is a self-replicating organism. Plasmids and bacteriophages are commonly used vectors in the recombinant DNA technology. These ligated vectors are known as recombinant DNA molecules since they carry both own and inserted DNA sequences. Recombinant vectors are added to a host bacterium and made to uptake the recombinant vectors inside the bacterial cell. Bacteria with recombinant vectors (plasmids) should be grown in a culture medium. During the bacterial multiplication, bacterial DNA, together with recombinant plasmids, replicate their genomes and produces clones. These clones contain the entire genome of the source organism. Hence, it is called a genomic library. Plasmids can easily be separated from the bacterial chromosomal DNA to construct the genomic library of that organism. If a particular organism contains interested genes, it is easy to detect it in the genomic library by hybridization using molecular probes (markers).
Genomic libraries are important to study the genomic structure and function, specific genes, gene location, gene mapping, mutations, gene sequencing, identification of novel therapeutic genes etc.
Figure_1: Construction of a genomic library
What is a cDNA library?
A cDNA library is a collection of complementary DNA (cDNA) clones synthesised from total mRNA of an organism. The construction procedure involves different steps. Purification of total mRNA from an organism is the first step involved. Isolated mRNA are converted into cDNA strands by a process called reverse transcription. Reverse transcription is facilitated by an enzyme called reverse transcriptase. It uses a small 3’ primer and initiates the synthesis of first cDNA strand complementary to template mRNA strand. Resulting double stranded cDNA are converted into smaller fragments using restriction endonucleases and inserted into suitable vectors. These constructed recombinant molecules are then added into a host organism and grown in a culture medium to produce clones. The collection of clones containing cDNA fragments of an organism is known as a cDNA library. Fully sliced mature mRNA does not contain introns and regulatory regions. Therefore non-coding fragments are not present in cDNA libraries unlike in a genomic library.
cDNA libraries are important for the analysis of coding regions, gene functions, expression of genes etc.
Figure_2: Construction of a cDNA library
What is the difference between cDNA and Genomic Library?
cDNA vs Genomic Library |
|
| cDNA library is a collection of the clones bearing the complementary DNA to the mRNA of an organism | Genomic library is a collection of the clones bearing the total genomic DNA of an organism. |
| Coding vs Noncoding Sequences | |
| cDNA library contains only the coding sequences; it does not contain introns. | Genomic library consists of the entire genomic DNA including noncoding (introns and regulatory) DNA. |
| Size | |
| cDNA library is small. | Genomic library is large. |
| Starting Material | |
| Starting material is mRNA | The starting material is DNA. |
| Involvement of Reverse Transcription. | |
| Reverse transcription happens in the first cDNA strand synthesis. | Reverse transcription does not happen. |
Summary – cDNA and Genomic Library
The genomic library represents a population of clones bearing the fragmented total genomic DNA of an organism. cDNA library represents the population of clones bearing complementary DNA of the total mRNA of an organism. A cDNA clone contains only the sequences found in mRNA while the genomic clone contains the sequences of the entire genome. This is the difference between cDNA and genomic library.
Reference:
1. Haneef, Deena T KochunniJazir. “Difference between Genomic and cDNA Library.” Difference between Genomic and cDNA Library. N.p., n.d. Web. 15 Feb. 2017
2. Stekel, Dov J., Yoav Git, and Francesco Falciani. “The Comparison of Gene Expression from Multiple cDNA Libraries.” Genome Research. Cold Spring Harbor Laboratory Press, Dec. 2000. Web. 16 Feb. 2017
3. “Genomic Library Screens for Genes Involved in n-Butanol Tolerance in Escherichia coli.” Genomic Library Screens for Genes Involved in n-Butanol Tolerance in Escherichia coli. N.p., n.d. Web. 16 Feb. 2017
Image Courtesy:
1. “Formation of a cDNA Library”By PhD Dre at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Genomic Library Construction”By Aluquette – Own work (CC BY-SA 3.0) via Commons Wikimedia