What advantages do cDNA libraries provide over genomic DNA libraries? Describe cloning applications where the use of a genomic library is necessary to provide information that a cDNA library cannot.

cDNA libraries are collections of complementary DNA (cDNA) molecules synthesized from mRNA templates in a cell using the enzyme reverse transcriptase, representing all the genes that are actively expressed in a particular cell or tissue at a specific time. In contrast, genomic DNA libraries are collections of DNA fragments representing the entire genome of an organism, including both coding and non-coding regions.

1. Only expressed genes: cDNA libraries only contain expressed genes (transcribed into mRNA), whereas genomic DNA libraries contain all genes, regardless of whether they are expressed or not. This makes cDNA libraries suitable for studying gene expression and comparing gene expression levels in different tissues or stages of development. 2. No introns: cDNA libraries are generated from mRNA, which has undergone splicing to remove introns (non-coding sequences). Thus, cDNA libraries contain only the coding sequences (exons) of genes, while genomic DNA libraries consist of both introns and exons. This makes cDNA libraries easier to analyze when studying protein-coding genes, as there is no need to separate the coding regions from the non-coding regions. 3. Smaller size: cDNA libraries are smaller in size compared to genomic DNA libraries, as they only contain the expressed genes. This makes the manipulation and analysis of cDNA libraries more manageable and efficient.

1. Studying non-coding regions: Genomic DNA libraries provide information about the non-coding regions of an organism's genome, such as promoters, enhancers, and other regulatory elements. These regions cannot be studied using a cDNA library since they are not transcribed into mRNA. 2. Studying genes with low or no expression: Some genes may have very low levels of expression or maybe expressed only under specific conditions. These genes may not be well-represented in a cDNA library, making it difficult to study them using this approach. In contrast, genomic DNA libraries contain all genes, irrespective of their expression levels. 3. Investigating structural variations: Genomic DNA libraries allow for the study of structural variations in the genome, such as insertions, deletions, and duplications, which cannot be detected using a cDNA library since they only contain coding sequences.