molecular biology

1. MOLECULAR BIOLOGY

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