Lambda replacement vectors: maximum foreign DNA size For a bacteriophage λ replacement vector, what is the typical maximum size of foreign DNA that can be inserted while still allowing phage packaging?

Introduction / Context:
Lambda (λ) vectors are classic cloning tools. Two main classes exist: insertion vectors and replacement vectors. Replacement vectors remove a dispensable chunk (“stuffer”) and replace it with foreign DNA. Correct insert sizes are critical for successful packaging into phage heads.

Given Data / Assumptions:

• λ headful packaging tolerates a limited genome length window.
• Replacement vectors can accept larger inserts than insertion vectors.
• Typical teaching values place the maximum practical insert for replacement vectors around the low-to-mid 20 kb range.

Concept / Approach:
λ packaging requires DNA between approximately 78% and 105% of the wild-type length. In replacement vectors, removing the stuffer frees capacity for a larger insert than insertion vectors, commonly enabling 20–25 kb inserts without compromising packaging efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Standard molecular cloning texts and vector manuals list ~20–23 kb as a common maximum for λ replacement systems, corroborating this choice.

Why Other Options Are Wrong:

• 18–20 kb is closer to some insertion vector capacities.
• 25–30 kb and 40–50 kb exceed reliable λ packaging ranges.
• 8–10 kb is too small for replacement vectors’ potential.

Common Pitfalls:
Forgetting both lower and upper packaging limits; too small or too large inserts prevent phage assembly.

Final Answer:
20–25 kb