Question

Cosmids contain
(A). Replication origin
(B). Unique restriction sites
(C). A selectable marker from a plasmid
(D). Cos site from phage \(\lambda\) genome
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (C) and (D) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

Think of a cosmid as a large plasmid with a special "handle"—the cos site. The plasmid parts (ori, marker, restriction sites) allow it to function inside the cell, while the cos site "handle" allows it to be efficiently packaged and delivered into the cell like a virus.

Answer 1

The Correct Option is C

Step 1: Cosmid Overview:
Cosmids are hybrid cloning vectors, combining plasmid and bacteriophage lambda (\(\lambda\)) characteristics. They are engineered to clone large DNA fragments, typically ranging from 35-45 kb.
Step 2: Cosmid Components:
Key cosmid components are:
(A) Replication origin (ori): Like plasmids, cosmids require an origin of replication for host bacterial cell (e.g., *E. coli*) replication.
(B) Unique restriction sites: A multiple cloning site (MCS) with unique restriction sites is needed for inserting foreign DNA.
(C) Selectable marker: An antibiotic resistance gene (e.g., amp\(^R\)) from a plasmid allows selection of cells with the vector.
(D) Cos site from phage \(\lambda\) genome: The "cos" (cohesive ends) site, from the lambda phage genome, is recognized by the phage packaging machinery, allowing large recombinant cosmid DNA to be packaged into phage heads, enabling efficient bacterial cell infection.
Step 3: Conclusion:
All four listed components are essential for cosmid vector functionality. Therefore, (A), (B), (C), and (D) are all correct.