What is a genomic library and how does it differ from a cDNA library?

• A. A genomic library contains random fragments of the entire genome; a cDNA library contains expressed gene sequences reverse-transcribed from mRNA
• B. A genomic library contains only mRNA; a cDNA library contains genomic fragments
• C. A genomic library is only from bacteria; a cDNA library is from viruses
• D. A genomic library is the entire protein complement; a cDNA library is the entire genome

Answer: (A)

A genomic library stores the organism’s entire genome as many random DNA fragments cloned into vectors. This means it includes both coding regions and noncoding regions (introns, promoters, regulatory elements, intergenic DNA) and represents the genome as a whole, regardless of whether the genes are being expressed.

A cDNA library, on the other hand, is built from mRNA that was present in a cell or tissue. The mRNA is reverse-transcribed into complementary DNA (cDNA) and then cloned. Because mRNA comes from expressed genes and is processed, the resulting cDNA library contains only the expressed gene sequences and typically lacks introns and promoter regions. It reflects the genes that were active in the specific tissue at the time the RNA was collected.

So the correct description states that a genomic library contains random fragments of the entire genome, while a cDNA library contains expressed gene sequences reverse-transcribed from mRNA. Other descriptions that say the genomic library contains only mRNA or that the cDNA library contains genomic fragments aren’t accurate.