Properties of Yeast Episomal plasmids (YEp): Which statement correctly describes a typical YEp vector?
Correct Answer: Has a 2μ origin of replication and associated rep functions
Introduction / Context:Multiple classes of yeast vectors exist, each optimized for different performance characteristics. YEp vectors are designed for high-copy episomal maintenance by leveraging the endogenous 2μ plasmid system of Saccharomyces cerevisiae.
Given Data / Assumptions:
- YEp stands for Yeast Episomal plasmid.
- We compare YEp with YRp (ARS-based), YCp (CEN-based), and chromosome-like constructs (YAC).
Concept / Approach:YEp plasmids contain the 2μ origin of replication and rep genes, enabling stable, often high-copy episomal replication in yeast. YRp vectors rely primarily on ARS elements and can be higher copy but less stable. YCp vectors carry a centromere (CEN) and are maintained at low copy with chromosome-like segregation. Telomeres are not typical for plasmids; those appear in YACs. Homologous recombination (crossing over) is a mechanism for integration, not for episomal maintenance, and is not required for YEp replication.
Step-by-Step Solution:
Identify YEp as 2μ-based.Exclude ARS-only description (fits YRp more closely).Exclude CEN/TEL description (fits YCp/YAC).Select the option describing the 2μ ori and rep functions.Verification / Alternative check:Standard yeast molecular cloning guides describe YEps as 2μ-derived, high copy, and selectable via auxotrophic markers or drug resistance.
Why Other Options Are Wrong:
- Crossing over: pertains to integrative vectors, not episomal YEps.
- ARS-only: aligns with YRp vectors.
- CEN/TEL: belongs to YCp (CEN) or YAC (TEL).
- “Cannot replicate without integration”: false for episomal YEps.
Common Pitfalls:Confusing ARS-only vectors with 2μ-based episomal systems; both can be high copy but have distinct backbones.
Final Answer:Has a 2μ origin of replication and associated rep functions