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What vector can be used to clone a very large DNA fragment?


The standard plasmid and phage lambda vectors can accept donor DNA of sizes as large as 25 to 30 kb.However,many experiments require inserts well in excess of this upper limit.To meet these needs,the following special vectors have been engineered.In each case,after the DNAs have been delivered into the bacterium ,they replicate as large plasmids.

  1. Cosmids are vectors that can carry 35 to 45 kb inserts.They are engineered hybrids of lambda DNA and bacterial plasmid DNA.Cosmids are inserted into lambda phage particles ,which act as "syringes"that introduce these big pieces of recombinant DNA into recipient E.coli cells.The plasmid component of the cosmid provides sequences necessary for the cosmid's replication.Once in the cell,these hybrids form circular molecules that replicate extrachromosomally in the same manner as plasmids do.
  2. PAC:-(P1 artificial chromosome)vectors deliver DNA by a similar system but can accept inserts ranging from 80 to 100kb.In this case,the vector is a derivative of bacteriophage P1,a type that has a larger genome than that of lambda.
  3. BAC :-(Bacterial Artificial Chromosome) vectors, derived from F-plasmid ,can carry inserts ranging from 150-300 kb.The DNA to be cloned is inserted into the plasmid,and this large circular recombinant DNA is introduced into the bacterium by a special type of transformation.
  4. YAC:-(Yeast Artificial Chromosome): Inserts larger than 300 kb require an eukaryotic vector system called YAC.
  5. Expression vector: For cloning the gene for human insulin, a plasmid host was selected to carry the relatively short cDNA inserts  of approximately 450 bp.This host was a special type of plasmid called a plasmid expression vector.Expression vectors contain bacterial promoters that will initiate transcription at high levels when the appropriate allosteric regulation is added in the growth medium.  

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