Hi. In this video, I'm going to be talking about DNA libraries. So, DNA libraries are collections of DNA fragments, and there are two types that I want to talk about. The first is a genomic library. So, this is going to be a collection of genomic DNA in fragment form. Here's how you do this: you take an organism and isolate its genome. You then extract all the DNA it has and cut it with special enzymes called restriction enzymes. These enzymes cut the DNA into fragments. These fragments then go into some type of vector, one possibility being called a yeast artificial chromosome, which accommodates really large DNA fragments. This is especially important for genomic DNA because the fragments are large, as you're dealing with the entire genomic DNA. These vectors can hold up to 1,000,000 base pairs. When I mention vectors, in case you are unfamiliar with the term, they are typically circular DNA used to insert or remove DNA and to facilitate the transfer of DNA into bacteria or other organisms.
Once you have these fragments in vectors like yeast artificial chromosomes, they can be sequenced, which simplifies the sequencing process. You can express them in bacteria or other organisms. Here's the process: first, you have an organism and you extract its DNA, then digest it with those restriction enzymes, resulting in numerous fragments. These fragments are placed into cloning vectors, circular DNA molecules easily introduced into organisms like bacteria for sequencing or expression.
The second type I want to discuss is the cDNA library. Unlike a genomic library, which begins with genomic material, a cDNA library starts with mRNA, the RNA that is actively being produced in the cell. The process involves isolating the RNA from an organism to capture the set of genes being actively expressed by the cell. This RNA is then converted into DNA through a process called reverse transcription. This step is crucial because it removes introns and splices together exons, yielding the final RNA transcript that would be translated into a protein. The resultant DNA is then put into another vector and introduced into bacteria for sequencing, to determine which genes are active. Furthermore, because the RNA profile changes over time, if cells are exposed to a specific chemical and the RNA is extracted at different intervals, the resulting cDNA libraries will differ since various genes are expressed differently over time and under varying conditions. This makes cDNA libraries a powerful method for examining gene expression under specific conditions.
Summarizing, whether extracting and cutting DNA to create genomic libraries or transcribing mRNA into cDNA for expression analysis, both techniques are invaluable in understanding gene function and regulation. Now, let's move on to the next topic.
