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Types of artificial chromosomes

YAC – Yeast artificial chromosome

YAC is the first artificial chromosome produced and it was the major breakthrough when compared to the previous vector molecules as it could contain up to 1000 kb of foreign DNA (which is about 20 times more than the hybrid vectors like cosmids and phagemids, and hundreds of times more than some regular plasmid vectors). As the name suggests, this chromosome was produced using the Yeast DNA.

YAC contains both parts of the chromosome and parts of the vector molecule. These include centromere, telomeres, origins of replication and selectable markers. The main problem with the YAC is the fact that it is unstable (this is the main reason why it was replaced with BAC – bacterial artificial chromosome during the Human Genome Project). However, its stability was improved to a certain degree by the group of scientists during the 1980s. They have accomplished this through the discovery of autonomously replicating sequences (ARS).

Autonomously replicating sequences are basically origins of replication which can also affect the plasmid stability. They are found in Yeast and the interesting thing about them is the fact that they don’t have to be used in every cell cycle.

Advantages and applications of YAC

One of the best applications of yeast artificial chromosome is its usage to express eukaryotic proteins. Regular bacterial plasmids or phages cannot be used as vectors for eukaryotic genes since eukaryotes possess specific posttranslational modifications not found in prokaryotes.

Moreover, since YACs can contain a lot of foreign DNA, they can be used in the creation of genomic libraries (genomic library contains the whole genome of one organism). This can be achieved in two ways: random fragmentation (which is better for the creation of gene bank) and partial restriction enzyme digestion (which is better for the creation of genome bank of an organism). Once the genomic library is generated, it should be maintained in the stable form.