To introduce individual genes into bacteria or eukaryotic cells, the most widely used vector is a plasmid, according to Scitable. Plasmids consist of a circular piece of double-stranded DNA that replicates one or more times during each reproductive cycle of its host cell.
Plasmids can carry from several hundred DNA base pairs to a maximum of 100 kilobase pairs, according to "Molecular Cell Biology." This size range covers virtually every gene detected to date in the human genome.
By the 1980s, genetic engineers developed a variant of a plasmid called a "cosmid." This vector consists of a plasmid with special inserts called "cos sites" that allow the DNA to be packaged into a virus called a "bacteriophage." Although the idea worked well on paper, scientists discovered that bacteria often responded to cosmids as they would to a bacteriophage virus, which means that they cleaved onto the foreign DNA with restriction enzymes. This aspect of the cosmid defeats the point of using it to introduce genes into the bacteria.
In some experiments, biologists wanted to clone entire pieces of a chromosome or see the effects of multiple genes on a cell line. Moving stretches of DNA longer than 100 kB requires the use of a different vector, either a bacterial artificial chromosome or a yeast artificial chromosome. A BAC can carry up to 300 kB of DNA, while a YAC can carry up to 1000 kB of DNA, which is 10 times the amount that any plasmid can carry.