Plasmid copy-number control: Plasmids maintained at a limited number of copies per cell are referred to as which class?

Introduction / Context:
Plasmids differ in copy-number control, which affects plasmid yield, metabolic burden on the host, and expression from cloned genes. Understanding the distinction between stringent and relaxed control helps in selecting vectors for expression versus stability.

Given Data / Assumptions:

• Stringent control maintains only a few copies per cell.
• Relaxed control allows many copies per cell.
• Copy number influences plasmid yield and host physiology.

Concept / Approach:
Stringent plasmids (for example, those with pSC101-type origins) replicate in a tightly regulated manner that yields a low copy number. Relaxed plasmids (for example, pMB1/pUC-type) often reach high copy number, especially with mutations that disrupt negative regulation of replication initiation.

Step-by-Step Solution:

Verification / Alternative check:
Plasmid handbooks categorize origins by copy-number behavior; pSC101-derived systems are classic low-copy, while pUC derivatives are classic high-copy.

Why Other Options Are Wrong:

Common Pitfalls:
Equating antibiotic selection with copy number; copy control is ori-dependent, not marker-dependent.

Final Answer:
Stringent plasmids (low-copy-number, tightly regulated).