Final answer:
A cDNA library includes DNA from expressed genes, making it suitable for expressing cloned genes and producing functional proteins in a host organism. Genomic libraries contain all genomic sequences, not specifically those for expressed genes.
Step-by-step explanation:
A cDNA library is the correct choice for cloning a gene to produce functioning proteins in a cloning host. This is because a cDNA library represents only those genes that are actually being expressed and translated into proteins in a given tissue or organism at a specific time. This makes it suitable for protein production in cloning hosts like bacteria. In contrast, a genomic library contains all sequences present in the organism, including non-coding regions and regulatory sequences, which are not necessarily relevant for protein expression.
Since mRNA only comes from expressed genes, this cDNA can be used in cloning vectors to express the corresponding proteins in bacteria or other host organisms. This technology has practical applications, such as the production of human insulin from expressed human cDNAs, rather than using insulin from other sources like pigs.