Yeast cloning systems: Which yeast plasmid types are typically maintained at high copy number inside Saccharomyces cerevisiae?

Introduction / Context:
Different yeast vector backbones differ in stability and copy number. Understanding YEp, YRp, and YCp behavior is essential for designing experiments that need either strong expression (high copy) or tight control (low copy) of cloned genes in Saccharomyces cerevisiae.

Given Data / Assumptions:

• YEp vectors derive from the 2μ plasmid system.
• YRp vectors rely on ARS elements for replication.
• YCp vectors carry a centromere (CEN) and behave like mini-chromosomes (low copy).

Concept / Approach:
YEp vectors typically contain the 2μ origin and associated functions, supporting high copy numbers (often 20–50+ per cell). YRp vectors include ARS sequences that also allow autonomous replication, commonly resulting in relatively higher copy numbers than centromere-based plasmids, though sometimes less stable. In contrast, YCp vectors incorporate CEN and ARS, are segregated like chromosomes, and are maintained at a low copy (usually ~1–2 per cell).

Step-by-Step Solution:

Verification / Alternative check:
Expression studies routinely use YEp for overexpression and YCp for near-genomic expression levels, confirming the copy-number distinction.

Why Other Options Are Wrong:

• YEp only or YRp only: incomplete; both can be high copy.
• YCp: intentionally low copy due to centromere segregation.
• YAC: not a plasmid; behaves as a chromosome for very large inserts.

Common Pitfalls:
Equating stability with copy number; YRp may be high copy yet less stable than YEp.

Final Answer:
Both (a) and (b)