Now that the genomes of many different living things, including humans, have been decoded, a central focus of genetics research involves identifying particular genes, determining which proteins they code for, and understanding how the body uses those proteins.

When scientists study the DNA of living things, they often rely on databases called DNA libraries. Like community libraries, DNA libraries contain lots of stored and accessible information. To build a DNA library, researchers must first isolate DNA from a cell and then break the long, double-stranded molecule into small pieces, or fragments. They then insert each fragment into a vector, which can store the DNA fragment, transport it into a host cell or virus, and allow for it to be replicated. Commonly used vectors are chromosomes or plasmids (circular DNA). Single-celled organisms such as bacteria and yeast are commonly used as hosts for these vectors. When grown in culture, these vectors replicate quickly inside their hosts, generating large quantities of copied, or cloned, DNA fragments that make up the DNA library.

There are two basic types of DNA libraries. A genomic DNA library contains cloned DNA that, taken together, represents the entire genome of an organism. This type of library is used primarily in genome-sequencing projects, such as the Human Genome Project, whose objective was to catalog all human DNA. A cDNA library, on the other hand, contains copies of complementary DNA (cDNA) molecules. Complementary DNA molecules are double-stranded versions of messenger RNA (mRNA) molecules. This means that cDNA libraries represent only the coding regions of a genome—the parts with instructions for making proteins. These regions make up a very small percentage of the overall genome of most organisms. Creating a cDNA library is similar to creating a genomic DNA library, except that the process begins with mRNA, not DNA. An enzyme called reverse transcriptase copies the single-stranded mRNA into the double-stranded DNA form. Then, as with genomic libraries, the DNA is cut up into fragments, inserted into a vector, and cloned.

A cDNA library reflects the mRNA content of an organism's cell at a specific moment in time. Scientists can use this information to understand how certain genes are expressed in the body and how the proteins they produce function and sometimes malfunction, resulting in disease. A comparison between cDNA libraries from healthy cells and diseased cells can reveal which genes are involved in a disease. This helps identify many mutations that directly cause diseases, as well as genes that are either over- or underexpressed in diseases such as cancer. Complementary DNA libraries also help identify factors that can change gene expression and possibly cause a disease, as well as therapies that can restore normal gene activity and cure a disease, or prevent its progression.

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