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: Understanding the Concept:
Cosmids are hybrid cloning vectors that combine features of plasmids and bacteriophage lambda (\(\lambda\)). They are designed to clone large DNA fragments (typically 35-45 kb).
Step 2: Detailed Explanation:
Let's analyze the essential components of a cosmid:
(A) Replication origin (ori): Like a plasmid, a cosmid needs an origin of replication to be able to replicate within the host bacterial cell (e.g., *E. coli*). This is a fundamental requirement for any vector.
(B) Unique restriction sites: To insert a foreign DNA fragment, the vector must be cut with a restriction enzyme. A multiple cloning site (MCS) containing unique restriction sites is a standard feature.
(C) A selectable marker: After transformation, it is necessary to select for the host cells that have successfully taken up the vector. A selectable marker, such as an antibiotic resistance gene (e.g., amp\(^R\)), is derived from a plasmid and serves this purpose.
(D) Cos site from phage \(\lambda\) genome: This is the defining feature of a cosmid. The "cos" (cohesive ends) site is a sequence from the lambda phage genome that is recognized by the phage packaging machinery. This allows the large recombinant cosmid DNA to be packaged into empty phage heads, which can then efficiently infect bacterial cells.
Step 3: Final Answer:
All four listed components are essential features of a functional cosmid vector. Therefore, (A), (B), (C), and (D) are all correct.