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Yeast Artificial Chromosomes (YACs)

Yeast Artificial Chromosomes (YACs)

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Yeast Artificial Chromosomes (YACs)

  1. 1. YACs are plasmid shuttle vectors capable of replicating and being selected in common bacterial hosts such as Escherichia coli, as well as in the budding yeast Saccharomyces cerevisiae. Yeast artificial chromosome (YAC) is a human-engineered DNA molecule used to clone DNA sequences in yeast cells
  2. 2. YEAST ARTIFICIAL CHROMOSOMES  YAC is an artificially constructed chromosome that contains a  Centromere  Telomeres  Autonomous replicating sequence (ARS) element required for replication and preservation of YAC in yeast cells  ARS elements are thought to act as replication origins First described in 1983 by Murray and Szostak
  3. 3. YACs behave like naturally existing chromosomes, provided that they are of the proper size, showing comparable stability.
  4. 4. Purpose:  Cloning vehicles that propogate in eukaryotic cell hosts as eukaryotic Chromosomes  Clone very large inserts of DNA: 100 kb - 10 Mb Features:  YAC cloning vehicles are plasmids  Final chimeric DNA is a linear DNA molecule with telomeric ends: Artificial Chromosome
  5. 5. Relatively small size (approaximat ely 12 kb) Circular form Amplified in E. coli Very large size (several hundreds of kilobases) Linear Amplified in yeast
  6. 6. Yeast Artificial Chromosomes (pYACs) Plasmids  Many different yeast artificial chromosomes plasmids exist, such as pYAC3 and pYAC4 plasmids.  The basic structural features of YACs were developed from the yeast centromere shuttle- plasmids YCp series.  These are composed of:  double-stranded circular DNA sequences carrying the b-lactamase gene bla and the bacterial pMB1 origin of replication  Include yeast ARS1 with its associated CEN4 DNA sequence, as well as the URA3 selectable
  7. 7.  Yeast HIS3 is flanked by a telomere-like DNA sequence that are adjacent to two recognition sites for the BamHI restriction enzyme.  Most of these YACs also contain the cloning site in the middle of the SUP4 suppressor of an ochre allele of a tyrosine transfer RNA (tRNA) gene.
  8. 8. Circular map of plasmid vector pYAC3
  9. 9. Construction of Yeast Artificial Chromosomes Plasmid DNA purification Treatment with restriction enzymes Ligation and yeast transformation
  10. 10. CONSTRUCTION OF YAC  A YAC is built using an initial circular plasmid typically broken into two linear molecules using restriction enzymes  DNA ligase is then used to ligate a sequence or gene of interest between the two linear molecules  forming a single large linear piece of DNA
  11. 11.  This inserted gene compensates for a mutation in the yeast host cell that causes the accumulation of red pigment  The host cells are normally red, and those transformed with YAC only, will form colourless colonies  Cloning of a foreign DNA fragment into the YAC causes insertional inactivation, restoring the red colour  Therefore the colonies that contain the foreign DNA fragment are red. The yeast artificial chromosome (YAC) vector is capable of carrying a large DNA fragment (up to 2 Mb)  Transformation efficiency is very low.
  12. 12. Homologous Recombination  In recombinationally-targeted YAC cloning, YACs are assembled in vivo, by recombination, and not by ligation in vitro  Recombination takes place between a target segment of the exogenous DNA, and the YAC vector that contains sequences homologous to these targets
  13. 13.  Firstly two YAC vectors arms and the exogenous segment(flanked by desired sequences) are transformed into the yeast cell  Then followed by recombination  Results in formation of desired stable YACs.
  14. 14. Figure.Recombinati onal targeted cloning with YAC vectors. A yeast strain is transformed with a mixture of the two YAC vector arms and large fragments of DNA. Recombination in vivo results in the formation of a specific YAC clone. The two YAC vector arms are derived from linearized plasmids that contain targeting segments that are homologous to the termini of the DNA segment that is to be cloned.
  15. 15. Existing YAC clones can be modified by homologous recombination in yeast •‘Retrofitting’ Modifying YACs by Homologous Recombination
  16. 16. Applications of YACs include generating whole DNA libraries of the genomes of higher organisms to identifying essential mammalian chromosomal sequences necessary for the future construction of specialized mammalian artificial chromosomes (MACs) Use of Yeast Artificial Chromosomes
  17. 17. Another major application of YACs is in the study of regulation of gene expression by cis-acting, controlling DNA elements That are present either upstream or downstream of large eukaryotic genes, after the transfer of these YACs from yeast to mammalian cells
  18. 18.  It is possible to construct YACs with megabase-long inserts using the precise homologous recombination  original DNA sequence of a eukaryotic genome fragment more than 2Mb in size can be maintained in a single YAC vector YAC Genomic Libraries
  19. 19. COMPARISON BETWEEN YAC AND BAC SYSTEMS FEATURE YAC BAC Configuration Linear Circular Host Yeast Bacteria Copy Number / Cell 1 1-2 Cloning Capacity Unlimited None to low Chimerism Up to 40% Insert Stability Unstable Stable

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