In molecular cloning, M13 (often written as M13 or M13mp series) is best described as which type of vector and virion morphology? Choose the most accurate characterization for routine lab use.

Introduction / Context:
M13 is a classic cloning workhorse in molecular biology. Understanding what M13 is—both as a virus and as a vector—helps students connect phage biology with practical applications such as Sanger sequencing, site-directed mutagenesis, and display technologies. This question tests recognition of M13’s morphology and genome type in the context of vector design.

Given Data / Assumptions:

• M13 infects Escherichia coli carrying the F pilus.
• M13 particles are long and filamentous rather than icosahedral.
• M13-based vectors (e.g., M13mp, f1-based plasmids) provide single-stranded DNA useful for sequencing and mutagenesis.

Concept / Approach:
Natural M13 is a filamentous bacteriophage with a circular single-stranded DNA genome. Laboratory derivatives are engineered to facilitate cloning, blue–white screening, and generation of ssDNA. Because M13 produces ssDNA packaged into filamentous particles and secretes virions without lysing the host, it is especially suited to applications needing high-quality ssDNA templates.

Step-by-Step Solution:

Verification / Alternative check:
Standard cloning manuals describe M13 as a filamentous phage producing circular ssDNA and as the basis for M13mp vectors and f1-origin phagemids used to recover ssDNA from cultures after helper phage rescue.

Why Other Options Are Wrong:

• D: A plasmid is a non-viral circular dsDNA replicon; M13 is a phage.
• E: Cosmids are plasmids with lambda cos sites for packaging large dsDNA fragments, not filamentous ssDNA phage.
• A or B alone are incomplete; the best answer must cover both properties.

Common Pitfalls:
Confusing M13 with lambda (a dsDNA head–tail phage) or assuming all phage vectors yield dsDNA only.

Final Answer:
Both a filamentous phage and an ssDNA vector