Question

Which among the following is incorrect with reference to Yeast Artificial Chromosomes?

• A. YACs have ORS, Telomere and Centromere
• B. They are used for physical mapping of complex genomes
• C. They can be used to express eukaryotic proteins that require post translational modifications
• D. YAC do not favor cloning of large fragments of DNA

Hint:

• Yeast Artificial Chromosomes (YACs) Cloning vectors that replicate as linear chromosomes in yeast.
• Key features ARS (origin), CEN (centromere), TEL (telomeres), selectable markers.
• Main advantage Can clone very large DNA inserts (hundreds of kilobases to megabases).
• Used for genomic libraries of complex genomes, physical mapping, and expressing large genes or eukaryotic proteins.

Answer 1

The Correct Option is D

Yeast Artificial Chromosomes (YACs) are cloning vectors designed to clone very large fragments of DNA (typically 100 kb to over 1 Mb) in yeast (Saccharomyces cerevisiae) host cells. Key features and uses of YACs:

• (a) YACs have ORS, Telomere and Centromere: TRUE. To function as a chromosome in yeast, a YAC vector must contain:
  • ARS (Autonomously Replicating Sequence): Yeast origin of replication. (ORS might be a typo for ARS, or Origin of Replication Site).
  • TEL (Telomeres): Sequences at the ends of linear chromosomes, required for stability and complete replication.
  • CEN (Centromere): Sequence required for proper segregation of the chromosome during yeast cell division.
  • They also contain selectable markers for yeast and often for E. coli (for initial cloning steps).
• (b) They are used for physical mapping of complex genomes: TRUE. Due to their ability to clone very large DNA fragments, YACs were crucial in early genome mapping projects (e.g., the Human Genome Project) for creating ordered libraries of large genomic segments.
• (c) They can be used to express eukaryotic proteins that require post translational modifications: TRUE. Since yeast is a eukaryotic host, it possesses the cellular machinery for many post-translational modifications (e.g., glycosylation, phosphorylation) that are common in other eukaryotes, including mammals. This makes YACs (and yeast expression systems in general) suitable for expressing some complex eukaryotic proteins that might not fold or modify correctly in prokaryotic hosts like E. coli.
• (d) YAC do not favor cloning of large fragments of DNA: FALSE. This is the opposite of their main advantage. YACs are specifically designed and used because they do favor and can accommodate very large DNA fragments, much larger than plasmids, phages, or cosmids.

The question asks for the incorrect statement. Statement (d) is incorrect. \[ \boxed{\text{YAC do not favor cloning of large fragments of DNA}} \]