What is a phagemid? Choose the most accurate description of a phagemid's genetic architecture and capability.

Introduction / Context:
Phagemids combine features of plasmids and filamentous phage, allowing researchers to prepare single-stranded DNA for sequencing, mutagenesis, and display technologies while maintaining standard plasmid benefits (selection, high copy number).

Given Data / Assumptions:

• Phagemids retain a bacterial plasmid origin for dsDNA replication.
• They also include an f1 phage origin that, with helper phage, yields ssDNA.
• Selectable markers and MCS sites are typical.

Concept / Approach:
The defining feature is the additional f1 origin. Upon superinfection with a helper phage, the phagemid DNA is packaged as ssDNA and secreted, which can be isolated easily. This capability distinguishes phagemids from ordinary plasmids and from λ-based cosmids.

Step-by-Step Solution:

Verification / Alternative check:
Vectors such as pBluescript and pUC derivatives with an f1 origin are widely used as phagemids, validating the description.

Why Other Options Are Wrong:

• λ att or cos sites are λ features; phagemids are based on filamentous phage f1, not λ.
• Vectors lacking a plasmid origin would not replicate as plasmids.
• “Defective phage genome” does not match phagemid function.

Common Pitfalls:
Confusing cosmids (λ cos-based, large insert) with phagemids (f1 ori-based, ssDNA production).

Final Answer:
A plasmid cloning vector that also carries the filamentous phage (f1) origin, enabling recovery of single-stranded DNA