Final answer:
A cDNA library lacks introns and regulatory sequences because it is constructed from mRNA that has already had these non-coding regions removed during processing. It includes only expressed genes, which are sequences translated into proteins.
Step-by-step explanation:
The type of DNA library that lacks introns and regulatory sequences is a cDNA library. A cDNA library is generated by converting the messenger RNA (mRNA) of a cell or tissue back into DNA, using an enzyme called reverse transcriptase. This approach is typically used to focus on the expressed genes within an organism or a specific tissue.
Since introns and regulatory sequences such as promoters or enhancers are removed during the RNA splicing process, they are not present in the mRNA. Consequently, when a cDNA library is made from mRNA, it only contains the sequences that are actually translated into proteins, known as exons. This means that the introns, control sequences, and various noncoding regions sometimes referred to as 'junk DNA' are absent in cDNA libraries.
Understanding the function of a cDNA library is essential for researchers who want to study gene expression and protein coding without the complexities of noncoding DNA sequences. Furthermore, researchers interested in genomic structure and intron function must turn to genomic DNA libraries instead, as those libraries include all the DNA sequences found within the genome.