Human cDNA libraries in genetic engineering What best describes a human cDNA library used in recombinant DNA technology and functional genomics?

Introduction / Context:
A complementary DNA (cDNA) library is a foundational resource in molecular biology. It is generated by reverse transcribing messenger RNAs (mRNAs) into DNA, capturing the protein coding repertoire that a particular human cell type expresses under the conditions sampled. This question checks whether you can distinguish a cDNA library from a genomic library and recognize its appropriate uses in cloning and expression.

Given Data / Assumptions:

• cDNA is synthesized from mRNA via reverse transcriptase.
• Only expressed genes present as mRNA at the time of RNA isolation are represented.
• cDNA lacks introns and most regulatory/intergenic sequences.
• Libraries are cloned into vectors (e.g., plasmids) for propagation and screening.

Concept / Approach:

A cDNA library reflects the transcriptome of the source tissue, not the entire human genome. Because introns are spliced out of mRNA, cDNAs correspond to exons joined together, which is advantageous for expressing eukaryotic proteins in prokaryotes. Radioactivity is not intrinsic to cDNA libraries; radiolabeled probes may be used in screening, but the library itself is simply DNA cloned in vectors.

Step-by-Step Solution:

Verification / Alternative check:

Comparing to a genomic library clarifies differences: genomic libraries include introns, promoters, and intergenic DNA; cDNA libraries do not. Expression screening of cDNA libraries is routine for isolating coding sequences.

Why Other Options Are Wrong:

Option B overclaims universality; no single cDNA library covers all proteins. Option C confuses screening radioactivity with the library itself. Option D is false because a correct description exists. Option E describes a genomic library, not cDNA.

Common Pitfalls:

Assuming cDNA equals genome; forgetting tissue and condition dependence; thinking introns remain in cDNA.

Final Answer:

A collection of DNA copies corresponding to mRNAs for specific human proteins expressed in the sampled cells or tissues