Question

Yeast artificial chromosomes (YACs) are useful because they

• A. Carry very large DNA fragments
• B. Replicate only in bacteria
• C. Are single-stranded DNA vectors
• D. Lack centromeres

Hint:

Remember the cloning capacities: \[ \boxed{ \text{Plasmid} < \text{Cosmid} < \text{BAC} < \textbf{YAC} } \] YACs are preferred when cloning extremely large DNA fragments.

Answer 1

The Correct Option is A

Concept: Yeast Artificial Chromosomes (YACs) are cloning vectors designed from the natural chromosomes of yeast (Saccharomyces cerevisiae). They possess essential chromosomal elements such as a centromere (CEN), telomeres (TEL), and an origin of replication (ARS). Because of these features, YACs can carry extremely large DNA fragments, making them valuable tools in genome mapping and large-scale sequencing projects.

Step 1: Understand the structure of YACs.
A typical YAC contains:
• Telomeres (TEL)
• Centromere (CEN)
• Autonomous Replicating Sequence (ARS)
• Selectable marker genes These components allow the artificial chromosome to behave like a normal yeast chromosome.

Step 2: Importance of YACs.
YACs are capable of carrying DNA fragments ranging from approximately \[ 200~kb \text{ to more than } 1000~kb \] This large cloning capacity makes them useful for:
• Genome mapping
• Human Genome Project
• Cloning large genes
• Physical mapping of chromosomes

Step 3: Eliminate incorrect options.

• YACs replicate in yeast, not only in bacteria.
• They are double-stranded DNA vectors.
• They possess centromeres, which are essential for chromosome segregation.
• Therefore, their major advantage is carrying very large DNA fragments. Final Answer: \[ \boxed{Option (A) is correct \]