Reducing insert size when introns are present — vector system choice If a eukaryotic gene contains multiple introns, which vector system is most naturally aligned with using cDNA (introns removed) and thereby reduces the effective insert size for cloning or expression?

Introduction / Context:
Many eukaryotic genes are interrupted by introns, which increase genomic DNA size and complicate expression in systems that cannot process introns. A common strategy is to use complementary DNA (cDNA) synthesized from spliced mRNA, yielding intron-free coding sequences.

Given Data / Assumptions:

• Retroviral replication involves reverse transcription of spliced viral RNAs into DNA.
• cDNA lacks introns, making inserts smaller and more suitable for expression in non-splicing hosts or for size-limited vectors.
• The question asks which platform aligns with using cDNA to reduce insert size.

Concept / Approach:

Retroviral vectors are typically built from cDNA, mirroring the biology of retroviruses that package RNA and reverse transcribe it. Using cDNA removes introns, compressing large genomic loci into compact ORFs that fit within vector size constraints and express correctly without host splicing.

Step-by-Step Solution:

Verification / Alternative check:

Standard gene therapy practice uses retroviral or lentiviral vectors with cDNA inserts to stay within packaging limits and ensure correct expression in target cells.

Why Other Options Are Wrong:

Vaccinia (option b) and baculovirus (option c) accommodate large inserts, often including introns; YACs (option e) expand size rather than reduce it; option d is false because cDNA cloning removes introns routinely.

Common Pitfalls:

Assuming intron-containing genomic clones are required for expression; many systems prefer intron-free cDNA for efficiency.

Final Answer:

Retrovirus-based vector (cDNA derived from spliced mRNA)