|There are a few basic tools commonly used in the world
of molecular biology. All together and in various combinations, they make
up the bulk of our toolbox for studying genes and gene expression.
One of these tools is a vector. A vector can be called a "carrier". We use these to carry and transmit pieces of DNA. There are several different types of vectors commonly used in recombinant DNA work. The difference is based mainly on the size of the DNA pieces that can be carried.
A plasmid can transmit a piece of DNA up to about 10 kilobases (10,000 bases) in size. Plasmids are circular pieces of DNA normally carried in bacteria. We can use the recombinant bacterial plasmid to transmit DNA that we wish to study by cutting the plasmid with restriction enzymes to produce sticky ends complementary to our piece of DNA and ligating them together to form a recombinant plasmid. How do we get the bacteria to accept this foreign DNA without activating its restriction defense? By using special laboratory strains where these defenses have been disabled.
A bacteriophage is a kind of virus that infects bacteria. The virus reproduces, or replicates, itself in the bacterial host. We can insert DNA into regions of the phage DNA to produce recombinant bacteriophage that allow for replication of both the phage and insert DNA. Bacteriophage can transmit pieces of DNA 15 to 20 kilobases in size.
A "cosmid" is yet another type of vector that can be used to transmit pieces of recombinant DNA up to 40 kilobases in size. An "artificial chromosome" is a vector capable of transmitting even larger fragments of recombinant DNA.