When scientists use viral enzymes to make cDNA from RNA isolated from the cells and tissues that they are studying, it does not contain introns due to being spliced out in mRNA. cDNA also does not contain any other gDNA that does not directly code for a protein (referred to as non coding DNA). Lastly, not all genes in the gDNA are being transcribed into mRNA at any given time. As a result, cDNA will only contain genes that are actively being used by a specific cell or tissue at a point in time. There is much less total information in cDNA than gDNA, but what information remains can be a lot more relevant to what a researcher is looking at since it doesn’t contain sequences that are unnecessary to the functioning and replication of the DNA.
Once isolated, gDNA can be used to make genomic libraries for DNA sequencing, fingerprinting, differentiation and other applications with both clinical and research fields.
cDNA can also be used also be used to make cDNA libraries, permanent collections of cDNA that can be copied and/or stored long term, and it is commonly used to clone eukaryotic genes in a prokaryote. This way a protein expressed in a eukaryotic organism can be introduced into a prokaryote. For this process cDNA is used over gDNA, since prokaryotes cannot spice out introns contained in gDNA.
In order to isolate cDNA, first the RNA of an organism must be isolated. Then, using areverse transcriptaseenzyme, cDNA can be made. This is the process retroviruses use to incorporate into their host’s cells. Retroviruses, such as Simian Immunodeficiency Virus (SIV) and Avian Myeloblastosis Virus (AMV), use their cDNA to produce mRNA in the host, leading to the production of viral proteins. This is possible because retroviruses use RNA as their genomic material instead of DNA, and it is reverse transcribed into the cDNA, which then undergoes normal transcription and leads to the viral protein in the host. The life cycle of a retrovirus is shown in Figure 4.