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DNA Replication Determines Cell Cycle Duration

By BiotechDaily International staff writers
Posted on 10 Jul 2018
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Image: Shown here in green is IkBa, the inhibitor of NF-kappaB and regulator of the immune system. The red region highlights an ubiquitin-independent degron (Photo courtesy of Wikimedia Commons).
Image: Shown here in green is IkBa, the inhibitor of NF-kappaB and regulator of the immune system. The red region highlights an ubiquitin-independent degron (Photo courtesy of Wikimedia Commons).
An international team of cell and molecular biologists found that DNA damage and danger of cancer development were minimized by a molecular mechanism that delays cell division (mitosis) until two complete copies of the cell's genome are created.

To maintain genome stability, cells have to replicate their DNA before dividing. Upon completion of bulk DNA synthesis, the mitotic kinases CDK1 and PLK1 become active and drive entry into mitosis.

Investigators at the Karolinska Institutet (Stockholm, Sweden) and the University of Sussex (United Kingdom) tested the hypothesis that DNA replication determined the timing of mitotic kinase activation. To do this, they generated a double-degron system to rapidly deplete the essential DNA replication-initiation factor CDC6 (Cell division control protein 6 homolog). A degron is a portion of a protein that is important in regulation of protein degradation rates. Known degrons include short amino acid sequences, structural motifs and exposed amino acids (often lysine or arginine) located anywhere in the protein. Some proteins contain multiple degrons.

The investigators reported in the June 28, 2018, online edition of the journal Molecular Cell that untransformed human cells shortened the cell cycle and prematurely entered mitosis in the absence of DNA replication. Using RNAi and inhibitors to independently target DNA replication licensing or firing, they found similar results in cancer cells. They also found that abrogating CHK1 activity in transformed cells, or CHK1 and p38 activity in untransformed cells enhanced CDK activation specifically upon G1/S transition, supporting the notion of a DNA replication checkpoint in human cells.

In the presence of DNA replication, inhibition of CHK1 and p38 led to premature activation of mitotic kinases, which induced severe replication stress. These results demonstrated that, rather than merely being a cell cycle output, DNA replication was an integral signaling component that restricted activation of mitotic kinases. DNA replication thus functioned as a brake that determined cell cycle duration.

"By creating cells that cannot copy their DNA and by following protein activities over time in single cells, we found that DNA replication blocks the enzymes that trigger cell division. Immediately after DNA replication is completed, the machinery that starts cell division is activated. This fundamental mechanism contributes to determining when human cells will divide," said senior author Dr. Arne Lindqvist, senior researcher in cell and molecular biology at Karolinska Institutet.

Related Links:
Karolinska Institutet
University of Sussex

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