- YACs carry DNA fragments up to one million base pairs.
- They provided the first physical maps of the human genome.
- Chimerism rates of 10-60% led to their replacement by BACs.
A yeast artificial chromosome (YAC) is a laboratory-built DNA molecule that replicates inside the yeast Saccharomyces cerevisiae and carries large fragments of foreign DNA, up to about one million base pairs, for cloning and analysis.
Key figure
~1,000 kb
Maximum DNA insert a single YAC can carry
Why It Matters
YACs opened a door in genomics that no earlier tool could. Before their development, the largest cloning vectors available to molecular biologists, cosmids and bacteriophage lambda, topped out at roughly 40 kilobases. Entire human genes, some spanning hundreds of thousands of base pairs, simply would not fit.
The Human Genome Project relied on YAC libraries in its early mapping phase during the 1990s. Researchers at the Centre d'Etude du Polymorphisme Humain (CEPH) in Paris assembled some of the first physical maps of human chromosomes using overlapping YAC clones. Those maps guided later sequencing efforts, even after the project shifted to bacterial artificial chromosomes and other more stable vectors.
YACs also proved valuable in plant genomics, where genomes can dwarf the human one. Arabidopsis thaliana, rice, and wheat were all mapped in part using YAC libraries. In each case, the ability to carry very large inserts made YACs the only practical option at the time.
How It Works
A YAC contains the minimum elements a chromosome needs to survive in yeast: a centromere for proper segregation during cell division, telomeres to protect the ends from degradation, and an autonomously replicating sequence (ARS) that lets the DNA copy itself. These components come from Saccharomyces cerevisiae.
Key figure
1987
Year Burke, Carle, and Olson built the first practical YAC cloning vector
To build a YAC, researchers assemble these elements into a linear vector, then cut it at a cloning site and ligate in the target DNA. The construct is transformed into yeast cells, which maintain it alongside their own chromosomes. Selectable markers, typically genes that allow growth on specific media, let scientists identify which yeast cells carry the YAC.
Andrew Murray and Jack Szostak at Harvard Medical School demonstrated in 1983 that yeast cells could maintain artificial linear chromosomes built from these minimal elements. Four years later, David Burke, Georges Carle, and Maynard Olson at Washington University refined the system into a practical cloning vector capable of accepting inserts up to several hundred kilobases, publishing their results in the journal Science.
Key Context
YACs carry a significant technical limitation: chimerism. Studies found that 10 to 60 percent of clones in typical YAC libraries contain DNA from two or more unrelated genomic regions joined together. This happens when multiple DNA fragments ligate into a single vector, or when recombination inside the yeast cell merges separate YACs.
The Human Genome Project encountered this problem at scale and shifted its primary cloning strategy to bacterial artificial chromosomes (BACs), which proved far more stable.
Despite that limitation, YACs remain the only vector system that can carry inserts approaching one megabase. No BAC or fosmid comes close. This capacity keeps YACs relevant for specialized applications, including the construction of mammalian artificial chromosomes and the study of large-scale chromosome structure.
FAQ
What is the difference between a YAC and a BAC?
A YAC replicates in yeast and can carry inserts up to about 1,000 kilobases. A BAC replicates in E. coli and handles inserts up to roughly 300 kilobases. BACs are more stable and produce fewer chimeric artifacts, which is why the Human Genome Project ultimately favored them over YACs.
Why were YACs important for the Human Genome Project?
YACs provided the first physical maps of human chromosomes because they could carry DNA fragments large enough to span entire genes and gene clusters. The CEPH YAC library, assembled in the early 1990s, was a foundational resource for the mapping phase of the project.
Are YACs still used in research today?
Yes, though less commonly than in the 1990s. YACs remain useful when researchers need to clone very large DNA segments that exceed BAC capacity, such as in constructing mammalian artificial chromosomes or studying long-range regulatory elements.
What causes chimerism in YAC libraries?
Chimerism occurs when two unrelated DNA fragments are ligated into the same YAC vector, or when separate YACs recombine inside the host yeast cell. The result is a clone that appears to contain a single continuous stretch of DNA but actually joins pieces from different genomic locations.
Sources
- Primary Research:
- Construction of artificial chromosomes in yeast (Murray & Szostak, Nature, 1983)
- Cloning of large segments of exogenous DNA into yeast by means of artificial chromosome vectors (Burke, Carle & Olson, Science, 1987)
- Additional Context:
- Yeast Artificial Chromosome (YAC) (National Human Genome Research Institute)
- Yeast Artificial Chromosome overview (ScienceDirect)
