λ phage packaging constraints: What total DNA size can be packaged efficiently into a lambda phage head during in vitro packaging?
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A50 kb (fixed and exact only)
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BApproximately 35–53 kb (defined lower and upper packaging limits)
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CApproximately 40–50 kb only
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DAny size can be packaged without constraint
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ELess than 25 kb only
Answer
Correct Answer: Approximately 35–53 kb (defined lower and upper packaging limits)
Explanation
Introduction / Context:Lambda phage (λ) vectors are used to clone relatively large DNA fragments. However, the phage head imposes strict size constraints: DNA that is too small or too large will not be packaged efficiently. Recognizing these limits is essential when constructing lambda replacement or insertion libraries.
Given Data / Assumptions:
- λ headful capacity sets upper and lower boundaries for DNA length packaged.
- Replacement vectors remove a stuffer fragment to accommodate larger inserts.
- Insertion vectors accept smaller inserts and maintain most λ genome sequences.
Concept / Approach:The lambda head requires DNA within a specific size window (roughly mid-30s to low-50s kb). Too little DNA is not recognized properly by packaging machinery; too much DNA cannot be physically accommodated. Therefore, inserts must be sized such that the total recombinant genome fits the packaging window.
Step-by-Step Solution:
Identify usable size range → approximately 35–53 kb total DNA.Evaluate vector type → insertion vectors handle smaller inserts; replacement vectors accept larger inserts by removing stuffer DNA.Select the answer that explicitly captures the window (not a single fixed size).Verification / Alternative check:Protocols for λ library construction specify calculating total recombinant size to fall within the packaging limits for efficient in vitro packaging and titer recovery.
Why Other Options Are Wrong:
50 kb exact only: Overly restrictive; the system tolerates a range.40–50 kb only: Too narrow; underestimates the window.Any size: Incorrect; packaging has strict limits.< 25 kb only: Too small for efficient packaging.Common Pitfalls:Ignoring total genome size after ligation; miscalculating stuffer removal; attempting to package constructs outside the viable window leading to no plaques.
Final Answer:Approximately 35–53 kb (defined lower and upper packaging limits).