Accurate duplication of eukaryotic genome is a challenging task, given that environment of cell growth and division is rarely ideal. Cells are constantly under the stress of intrinsic and extrinsic agents that cause DNA damage or interference with DNA replication.
To cope with these assaults, cells are equipped with DNA maintenance checkpoints 3 to arrest cell cycle and facilitate DNA repair pathways. DNA maintenance checkpoints include (a) the DNA damage checkpoints that recognize and respond to DNA damage, and (b) the DNA replication checkpoint that monitors the fidelity of copying DNA.
DNA damage checkpoint
DNA damage checkpoints ensure the fidelity of genetic information both by arresting cell cycle progression and facilitating DNA repair pathways. Studies on many different species have uncovered a network of proteins that form the DNA damage checkpoints. Central to this network are protein kinases of ATM/ATR family known as Tel1/Mec1 in budding yeast and Tel1/Rad3 in fission yeast. These kinases sense DNA damages and phosphorylate number of proteins that regulate cell cycle progression and DNA repair pathways.
DNA replication checkpoint
Accurate replication of the millions or billions of DNA base pairs in a eukaryotic genome is a remarkable achievement. This accomplishment is even more astonishing when one considers for DNA synthesis are rarely ideal. Damaged template, protein complexes bound to DNA, and poor supply of dNTPs are among the many obstacles that must be overcome to replicate genome. All of these situations can stall replication forks.
Stalled forks pose grave threats to genome integrity because they can rearrange, break, or collapse through disassembly of the replication complex. The pathways that respond to replication stress are signal transduction pathways that are conserved across evolution.
Atop the pathways are also ATM/ATR family kinases. These kinases together with a trimeric checkpoint clamp (termed 9-1-1 complex) and five-subunit checkpoint clamp loader (Rad17-RFC2-RFC3-RFC4-RFC5) senses stalled replication forks and transmit a checkpoint signal.
One of major functions of replication checkpoint is to prevent the onset of mitosis by regulating mitotic control proteins such as Cdc25. But perhaps the most important activity of replication checkpoint is to stabilize and protect replication forks.
The protein kinase Cds1 (human Chk2 homolog; in human, Chk1 is a functional Cds1 homolog) is a critical effector of the replication checkpoint in the fission yeast Schizosaccharomyces pombe.
Cds1 is required to prevent stabilization of replication fork in cells treated with hydroxyurea (HU), a ribonucleotide reductase inhibitor that stalls replication by depleting dNTPs. In the budding yeast Saccharomyces cerevisiae, a failure to activate Rad53 (Chk2 homolog) is associated with collapse and regression of replication forks and gross chromosomal rearrangements in cells treated with HU.