Annaluru, N. et al. Science 344, 55–58 (2014).

Following viruses and bacteria, the yeast is the latest organism to be equipped with a fully functional synthetic chromosome. Researchers in the Synthetic Yeast Genome Consortium (Annaluru et al.) engineered a leaner chromosome III of Saccharomyces cerevisiae with only 273 kilobase pairs instead of the native 317 kilobase pairs. The first step in the assembly, going from small nucleotides to 750-base-pair building blocks, was done by undergraduate students at Johns Hopkins University. The researchers then transformed the building blocks into yeast, where they successively replaced native sequence by homologous recombination. After 11 rounds, the entire chromosome III was made up of the synthetic sequence that no longer carried sequences such as subtelomeres and introns but instead had non-essential genes flanked with loxP sites to allow subsequent scrambling of the genome. Strains carrying the synthetic chromosome showed no reduction in fitness.