LIFE09200604002 Lalit Sehgal - Homi Bhabha National Institute

the cyclin-cdk complex is held in an inactive state in the nucleus and cytoplasm.
Activation of cyclinB1/cdk1 complex requires the dephosphorylation of cdk1 on T14 and
Y15 by the cdc25 family of dual specificity phosphatases (86, 112, 237, 345). A cdk1
mutant (cdk1AF mutant) in which T14 and Y15 are mutated to alanine and phenylalanine
respectively does not get phosphorylated and induces premature entry into mitosis (27,
125, 187). Prior to mitosis the phosphate residues on T14 and Y15 are removed by the
action of the cdc25 family of dual specificity phosphatases (86, 112, 192, 237, 321, 344,
345) which are discussed below.
1.2 Cell cycle checkpoint pathways.
Checkpoint pathways are signal transduction pathways that are activated by stress to
DNA. The stress, which can be incomplete S-phase, DNA damage or aberrant attachment
to the mitotic spindle, is detected by sensor proteins that transduce the signal to
downstream effector kinases leading to multiple responses of which one is cell cycle
arrest (reviewed in (91, 142, 405)) (Figure 1.2). Checkpoint pathways can be activated by
a variety of agents. e.g. agents such as ultraviolet radiation (UV) and hydroxyurea
interfere with DNA replication and different types of DNA damage can be induced by
exposure to physical agents such as UV or ionizing radiation (IR) or by chemical agents
such as methyl-methane sulphonate (MMS), cisplatin, adriamycin (Doxorubicin), etc. In
addition to these agents, highly reactive chemical species, such as free radicals or reactive
oxygen species, which arise as a result of cellular metabolism, also cause DNA damage
(reviewed in (72, 405)). Spindle inhibitors such as colchicine and nocodazole disrupt the
mitotic spindle resulting in activation of the spindle assembly checkpoint (reviewed in
(186)). Activation of the checkpoint pathways result in a variety of responses such as cell
cycle arrest, apoptosis, repair of damaged DNA and transcription of repair genes
(reviewed in (405))
Checkpoint dependent cell cycle arrest is important because it gives the cell time to repair
the damage (reviewed in (91)). Loss of checkpoint pathways allows cells to continue
cycling in presence of incompletely replicated or damaged DNA or in presence of an
incomplete spindle assembly (reviewed in (262)). Loss of checkpoint function may result
in accumulation of mutations, deletions, amplifications, translocationsand aneuploidy, all
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