Question

Which vector is not suitable for Genomic DNA library construction?

• A. \(\lambda\) replacement vector
• B. Yeast Artificial Chromosomes
• C. Bacterial Artificial Chromosomes
• D. Plasmids

Hint:

• Choice of vector for genomic library depends on insert size capacity:
  • Plasmids Small inserts (up to ~15 kb).
  • Lambda (\(\lambda\)) phage vectors Moderate inserts (~9-23 kb for replacement vectors).
  • Cosmids Larger inserts (~30-45 kb).
  • Bacterial Artificial Chromosomes (BACs) Large inserts (~100-300 kb). Stable.

Answer 1

The Correct Option is D

A genomic DNA library is a collection of cloned DNA fragments that ideally represents the entire genome of an organism. The choice of vector for constructing a genomic library depends on the size of the genome and the desired size of the cloned fragments. For large genomes (e.g., mammalian, plant), vectors that can accommodate large DNA inserts are preferred to ensure the library is representative and manageable in terms of the number of clones. Let's analyze the suitability of the vectors listed:

• (a) \(\lambda\) replacement vector (Lambda phage replacement vectors): These are derived from bacteriophage lambda. They can accommodate DNA inserts typically in the range of 9-23 kb (kilobase pairs) by replacing a non-essential "stuffer" fragment of the phage genome. They are suitable for genomic libraries, especially for organisms with moderately sized genomes or for specific sub-genomic libraries.
• (b) Yeast Artificial Chromosomes (YACs): These vectors can clone very large DNA fragments, typically from 100 kb up to 1 Mb (megabase) or more. They are excellent for constructing genomic libraries of organisms with large, complex genomes (e.g., human, mouse) and for physical mapping.
• (c) Bacterial Artificial Chromosomes (BACs): These are based on the F-plasmid of E. coli and can stably maintain large DNA inserts, typically 100-300 kb. BACs are widely used for genomic libraries of complex genomes due to their stability and ease of manipulation compared to YACs.
• (d) Plasmids: Standard plasmid vectors (e.g., pBR322, pUC series) typically accommodate relatively small DNA inserts, usually up to about 10-15 kb. While they can be used for genomic libraries of organisms with very small genomes (e.g., some viruses or bacteria), they are generally not suitable for constructing representative genomic libraries of organisms with large genomes. This is because a very large number of plasmid clones would be required to cover the entire genome, making the library unwieldy and difficult to screen.

The question asks which vector is "not suitable" for genomic DNA library construction. While technically plasmids *can* be used for very small genomes, compared to the other options which are all designed for larger inserts (typical for "genomic library construction" in a general sense, implying larger genomes), plasmids are the least suitable or often unsuitable for representative libraries of complex genomes. Thus, plasmids are generally considered less suitable for constructing comprehensive genomic libraries of most organisms compared to lambda vectors, BACs, or YACs. \[ \boxed{\text{Plasmids}} \]