CHI SEQUENCE

MOLECULAR BIOLOGY

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.

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.

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.

CHI SITE MEDIATED HOMOLOGOUS RECOMBINATION BY RecBCD PATHWAYA

STEP 1:

STEP 2:

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.

ROLE OF CHI SITE

Generating new gene/allelli. E.g: immunoglobulin rearrangement.

Intigration of a specific DNA element.

DNA repair.

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