Discuss the M13 based vectors and its application.
M13 is a filamentous bacteriophage of E. coli having a genome consisting of 6.4 kb long circular DNA packaged in a tubular capsid. Almost the full genome encodes genes (I-X) required for coat protein, viral replication and viral assembly, except for a small non-essential region flanking ori called the intergenic region. M13 infects only E. coli cells harbouring F plasmid.
For M13 vector construction, the double stranded RF (Replicative Form) DNA of the M13 genome is used. The intergenic region is exploited for inserting the DNA so that it does not interfere with M13 replication. The circular single stranded genomic DNA (+) is assembled with capsid proteins during their passage through the host cell without causing cell lysis.
A significant advantage of M13 vectors is that large sized DNA (more than 42 kb) can be cloned by M13 bacteriophage. The double stranded RF DNA is readily isolated from infected cells for use as a vector, and the single stranded genome is abundantly available in the form of phage in the growth medium.
An example of M13 based vector for E. coli is M13mp18 which facilitates blue/white selection of the recombinants. This allows easy identification and selection of recombinant clones during the cloning process.
M13 vectors are more efficient than plasmids for cloning large inserts, and screening bacteriophage plaques is easier than screening bacterial colonies for a specific desired insert.
Explanation
The answer covers all key aspects of M13 vectors from the textbook: structure (filamentous bacteriophage, 6.4 kb circular DNA, tubular capsid), genome organization (genes I-X, intergenic region), host specificity (E. coli with F plasmid), vector construction (using double stranded RF DNA), insertion site (intergenic region), advantages (cloning large DNA >42 kb, no cell lysis, single stranded DNA availability), and example (M13mp18 with blue/white selection). The answer follows the 5-mark format with 5 paragraphs and uses bold for key terms.