Chi sequence
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The document discusses crossover hotspot instigator (Chi) sites, which are short DNA sequences near where homologous recombination occurs in bacterial genomes. Chi sites were originally discovered in bacteriophage lambda but are now known to occur about 1000 times in the E. coli genome. The Chi sequence serves as a signal to the RecBCD helicase-exonuclease complex, triggering a change in its activities. RecBCD has endonuclease and helicase activities and cuts one DNA strand near the Chi sequence. RecA then catalyzes strand exchange in the homologous recombination process. Chi sites play roles in generating genetic variation, integrating DNA elements, and DNA repair.
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Chi sequence
- 2. INTRODUCTION Crossover hotspot instigator(CHI). Chi sites are referred to as "recombination hot spots“. Chi sites was originally discovered in bacteriophage lambda, a virus that infects E. coli, but is now known to occur about 1000 times in the E. coli genome.
- 3. A Chi site or Chi sequence is a short stretch of DNA in the genome of a bacterium near which homologous recombination occur. Homologous recombination occurs between sequences that are nearly identical.
- 4. The Chi site is unique to each group of closely related organisms. Eg: 5'-GCTGGTGG-3‘(in salmonella). The Chi sequence serves as a signal to the RecBCD helicase-exonuclease that triggers a major change in the activities of this enzyme.
- 5. CHI SITE MEDIATED HOMOLOGOUS RECOMBINATION BY RecBCD PATHWAYA STEP 1:
- 6. STEP 2:
- 7. RecBCD : A COMPLEX ENZYME It has endonuclease subunit(RecBCD) which cuts one DNA strand close to Chi sequence. And DNA helicase activity (recD subunit). RecA: Catalyzes strand exchange.
- 8. ROLE OF CHI SITE Generating new gene/allelli. E.g: immunoglobulin rearrangement. Intigration of a specific DNA element. DNA repair.
- 9. Thank you