If you want to study promoter regions across the genome, which type of library would you use?

• A. Genomic library
• B. cDNA library
• C. Protein library
• D. RNA interference library

Answer: (A)

To study promoter regions across the genome, you need access to the DNA sequences that make up the genome, including regulatory elements upstream of genes. A genomic DNA library is built from fragments of the entire genome and cloned into vectors, so it represents all genomic regions, including promoters, enhancers, introns, and intergenic sequences. This makes it the best resource for examining promoter architecture on a genome-wide scale. In contrast, a cDNA library comes from expressed mRNA and reflects only the transcribed portions of the genome, lacking promoters and other noncoding regulatory regions. A protein library contains expressed proteins, not DNA sequences, and an RNA interference library is designed to perturb gene expression rather than survey genomic sequences. Therefore, the genomic library best fits the goal.