Q75.Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: A Cosmid is a specialised vector composed of cos sites of phage λ.
Reason R: cos sites enable the DNA to get packed in lambda (λ) particles.
In the light of the above statements, choose the most appropriate answer from the options given below:
(1) Both A and R are correct and R is the correct explanation of A
(2) Both A and R are correct but R is NOT the correct explanation of A
(3) A is correct but R is not correct
(4) A is not correct but R is correct
Cosmids and cos sites of phage λ play key roles in DNA cloning vectors. The correct answer is option (1): Both Assertion A and R are correct, and R correctly explains A.
Assertion and Reason Breakdown
Assertion A states that a cosmid is a specialized vector made with cos sites from phage λ. This is accurate, as cosmids combine plasmid replication origins and selectable markers with λ phage cos sequences for efficient large-insert cloning (37–52 kb).
Reason R explains that cos sites allow DNA packaging into λ particles. Cos sites (∼200 bp, with cosQ, cosN, cosB subsites) direct terminase enzyme to cut concatemeric DNA at cosN and package it into phage heads.
Option Analysis
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Option (1): Both correct, R explains A. Cosmid design relies on cos sites precisely for λ packaging in vitro, enabling high-efficiency transformation. This matches exactly.
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Option (2): Both correct, but R not explanation. Incorrect—R directly justifies why cos sites are included in cosmids.
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Option (3): A correct, R incorrect. Wrong—R is factually true per λ biology.
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Option (4): A incorrect, R correct. False—A precisely defines cosmid structure.
Cosmid vectors revolutionized cloning by integrating cos sites of phage λ with plasmid features. Developed by Collins and Hohn (1978), they allow packaging of large DNA inserts (up to 45 kb) into λ particles for superior library construction.
What Are Cosmid Vectors?
Cosmids (“cos” + “plasmid”) contain:
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Plasmid ori and antibiotic resistance for bacterial replication.
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Cos sites of phage λ (cosQ-cosN-cosB) for terminase recognition.
This hybrid enables in vitro packaging, yielding 10^6–10^8 transformants/μg DNA—far better than plasmids alone.
Role of Cos Sites in Phage λ
In λ lifecycle, cos sites on concatemers signal terminase:
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Binds cosB, nicks cosN (12-bp staggered cuts for cohesive ends).
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Packages DNA unidirectionally until cosQ termination.
Reason R holds: cos sites enable DNA to get packed in lambda particles.
Why R Explains A
Cosmids exploit this packaging mechanism: ligated inserts flanked by cos sites form “phage-like” concatemers, packaged into transducing particles for E. coli infection. Without cos, no packaging—R directly explains A’s design. Ideal for genomic libraries.
Exam Relevance
For NEET/UGC NET, this tests vector engineering basics. Option (1) fits as both statements align with molecular cloning principles.
