Q.25 Cosmid vector is made up of:

1. Bacterial plasmid and lambda phage DNA

2. Lambda phage DNA and Bacterial Fertility factor

3. Bacterial fertility factor and bacterial plasmid

4. Yeast telomeres and Yeast ARS, DNA

Cosmid Vector Composition: Bacterial Plasmid and Lambda Phage DNA

Cosmid vectors combine plasmid replication with lambda phage packaging for cloning large DNA inserts up to 45 kb. They revolutionized genomic library construction in molecular biology.

Correct Answer

1. Bacterial plasmid and lambda phage DNA defines cosmid vectors. These hybrids include a bacterial plasmid backbone (ori, selectable markers) plus lambda phage cos sites for in vitro packaging into phage particles, enabling efficient E. coli transduction.

Option Breakdown

Bacterial Plasmid and Lambda Phage DNA (Option 1)

Cosmids derive from plasmids like pBR322 with added lambda cos sequences (~200 bp). This allows 37-52 kb inserts to package into lambda heads for infection, then circularize and replicate as plasmids inside bacteria.​​

Lambda Phage DNA and Bacterial Fertility Factor (Option 2)

Lambda phage provides only cos sites in cosmids, not full DNA. F-factor (Fertility factor) supports bacterial conjugation but lacks plasmid ori or cos features needed for cosmid function.​

Bacterial Fertility Factor and Bacterial Plasmid (Option 3)

F-factor enables large-insert cosmids (fosmids) but standard cosmids use plasmid ori, not F-factor. This combo misses lambda cos for packaging, limiting insert size to ~20 kb like regular plasmids.​

Yeast Telomeres and Yeast ARS DNA (Option 4)

Yeast Artificial Chromosomes (YACs) incorporate ARS (Autonomously Replicating Sequence) and telomeres for eukaryotic cloning up to 1 Mb. Cosmids target prokaryotic systems without these yeast elements.​