Question

pBluescript II KS is a

• A. Plasmid
• B. Cosmid
• C. Phagemid
• D. Artificial chromosome

Hint:

• Phagemid A plasmid vector that also contains an origin of replication from a filamentous bacteriophage (e.g., M13, f1).
• Allows for replication as a double-stranded plasmid and, upon infection with a helper phage, packaging of single-stranded DNA into phage particles.
• Useful for ssDNA production for sequencing, mutagenesis, etc.
• pBluescript is a common example.

Answer 1

The Correct Option is C

pBluescript II KS (and its counterpart SK) are commercially available cloning vectors that are classified as phagemids. A phagemid is a type of plasmid vector that contains an origin of replication for a filamentous bacteriophage (like M13 or f1) in addition to its own plasmid origin of replication. Key features of phagemids like pBluescript:

• They can replicate as a normal plasmid in E. coli.
• When the E. coli host containing the phagemid is infected with a helper phage (which provides the necessary phage proteins), the phagemid can be packaged into phage particles as single-stranded DNA (ssDNA).
• This ability to produce ssDNA is useful for DNA sequencing (Sanger method), site-directed mutagenesis, and constructing probes.
• pBluescript vectors also typically contain a multiple cloning site (MCS) within the lacZ\(\alpha\) gene for blue-white screening, ampicillin resistance gene for selection, and promoters (e.g., T7 and T3 RNA polymerase promoters) flanking the MCS for in vitro RNA synthesis.

Let's analyze the options: (a) Plasmid: While a phagemid functions as a plasmid for general replication, its defining feature is the phage origin that allows ssDNA production. So, "phagemid" is more specific and accurate. (b) Cosmid: A hybrid vector containing lambda phage cos sites and a plasmid origin, used for cloning large DNA fragments (30-45 kb). Not pBluescript. (c) Phagemid: Correct. pBluescript is a classic example of a phagemid. (d) Artificial chromosome (e.g., YAC, BAC): Vectors designed to clone very large DNA fragments (hundreds of kb to Mb). Not pBluescript. Therefore, pBluescript II KS is a phagemid. \[ \boxed{\text{Phagemid}} \]