List of different vectors

• Plasmids are circular DNA molecules that exist independently of chromosomal DNA and can replicate autonomously. Plasmids carry one or more genes which mostly code for useful characteristic of host. All plasmids have sequence that can act as origin of replication. Plasmids of different sizes and possessing different copy number are present.
  
  
• Phage vectors are consist of mainly DNA molecule (sometimes RNA); that carrys large number of genes and are surrounded by a protein coat called as capsid. They can be used as vehicles for carrying DNA insert after modification to remove pathogenic genes and minimizing the size.
  
  
• Cosmids are hybrid between a phage DNA and bacterial plasmid. They have cos sites which are essentially required for packaging lambda (λ) into phage protein coat. They can carry large DNA insert.
  
  
• Fosmids are cosmid like plasmid but they are based on F-plasmid.
  
  
• Phagemids are plasmids having a part of M13 genome.
  
  
• Artificial chromosomes are artificially constructed DNA construct used for transferring DNA.

1-2.2.3 Preparation of vector DNA:

The vector DNA is cleaved by restriction endonucleases at the site where foreign DNA is desired to be inserted. The restriction enzyme is selected to generate a configuration at the cleavage site compatible with the ends of the foreign DNA. This can be achieved either by cleaving the foreign DNA and vector DNA with the same restriction enzyme or by adding adaptors/ linkers to both the ends of the insert DNA.

1-2.2.4 Preparation of DNA to be cloned:

DNA to be cloned can be obtained by:

1. Cutting using restriction enzyme from genomic or organellar DNA,

2. PCR based amplification,

3. Chemical synthesis.

The DNA to be cloned is isolated and treated with restriction enzymes to generate random fragments with ends capable of being linked to those of the vector. While choosing the restriction enzyme to cut the desired gene, care should be taken so that the restriction enzyme does not cut in the middle of the gene, but only at the ends. PCR based methods are used to obtain DNA segments, using either genomic DNA or mRNA as template sequences through reverse transcription. Short length sequences can be artificially synthesized in vitro. If necessary, linkers or adapters containing desired restriction sites are added to create the ends which are compatible with the vector. The complementary sticky ends result in an efficient ligation due to the formation a stable structure.