Yeast Artificial Chromosome (YAC) vectors: Why can they form artificial chromosomes in eukaryotic hosts? Identify the essential DNA elements they carry that allow stable replication and segregation.

Introduction / Context:
Yeast Artificial Chromosomes (YACs) are engineered cloning vectors that allow very large DNA inserts to be propagated like mini-chromosomes in yeast. This question checks whether you know the three essential chromosomal elements that make a YAC behave like a natural chromosome inside a eukaryotic cell.

Given Data / Assumptions:

• The vector is designed for replication and stable inheritance in yeast.
• Chromosome-like behavior requires specific sequence elements.
• We must decide which features are present in YAC vectors.

Concept / Approach:
For any linear eukaryotic chromosome to replicate and segregate correctly, it needs: (1) an origin of replication to initiate DNA synthesis, (2) a centromere to attach to the spindle apparatus for faithful segregation, and (3) telomeres to protect the ends from degradation and end-to-end fusions. In yeast, the origin is an ARS (autonomously replicating sequence). Therefore, a complete YAC contains telomere sequences at both ends, a functional centromere (CEN) element, and an ARS for replication.

Step-by-Step Solution:

Verification / Alternative check:
When YAC elements are incomplete (e.g., missing telomeres), constructs circularize or become unstable. Inclusion of CEN and TEL elements yields mitotic stability and proper partitioning, confirming the necessity of all three components.

Why Other Options Are Wrong:

• Centromere only: allows segregation but without ARS and telomeres, replication/ends fail.
• Telomere only: protects ends but no replication origin or segregation element.
• ARS only: replication occurs, but DNA is unstable and mis-segregates without CEN and TEL.
• None of these: contradicts the observed design and function of YACs.

Common Pitfalls:
Confusing ARS (yeast origin) with bacterial ori; forgetting that telomeres are essential because YACs are linear, not circular.

Final Answer:
all of these