LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
nitroquinoline oxide [150, 151]. About 30 polypeptides are involved in NER, and the
NER process has been reconstituted with purified components [150].
Key steps of NER include: (i) recognition of a DNA defect; (ii) recruitment of a repair
complex; (iii) preparation of the DNA for repair through action of helicases; (iv) incision
of the damaged strand on each side of the damage, with release of the damage in a singlestrand
fragment about 24–32 nucleotides long; (v) filling in of the gap by repair
synthesis; (vi) ligation to form the final phosphodiester bond [151, 152].
Base excision repair (BER) is a major DNA repair pathway protecting mammalian cells
against single-base DNA damage caused by methylating and oxidizing agents, other
genotoxicants, and a large number (about 10,000 per cell per day) of spontaneous
depurinations [153]. BER is mediated through at least two subpathways, one involving
single nucleotide BER and the other involving longer patch BER of 2–15 nucleotides.
A highly conserved set of MMR proteins in humans is primarily responsible for the postreplication
correction of nucleotide mispairs and extra-helical loops. Mutational defects
in MMR genes in humans give rise to a mutator phenotype, microsatellite instability, and
a predisposition to cancer. Mouse embryonic fibroblast and human epithelial cell lines
lacking the MMR protein, MLH1, are more resistant than wild-type cells to two inducers
of oxidative stress, hydrogen peroxide and tert-butyl hydroperoxide [154]. Analysis of
this resistance indicates that it results from a defect in apoptosis, as the consequence of a
requirement for wild-type MLH1 in the transduction of apoptotic signals by a
mitochondrial pathway, although the details of this pathway are currently unknown.
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