Mitotic cell cycle progression is accomplished through a reproducible sequence of events, DNA replication (S phase) and mitosis (M phase) separated temporally by gaps known as G1 and G2 phases. Cyclin-dependent kinases (CDKs) are key regulatory enzymes, each consisting of a catalytic CDK subunit and an activating cyclin subunit. CDKs regulate the cell's progression through the phases of the cell cycle by modulating the activity of key substrates. Downstream targets of CDKs include transcription factor E2F and its regulator Rb. Precise activation and inactivation of CDKs at specific points in the cell cycle are required for orderly cell division. Cyclin-CDK inhibitors (CKIs), such as p16Ink4a, p15Ink4b, p27Kip1, and p21Cip1, are involved in the negative regulation of CDK activities, thus providing a pathway through which the cell cycle is negatively regulated.
 Eukaryotic cells respond to DNA damage by activating signaling pathways that promote cell cycle arrest and DNA repair. In response to DNA damage, the checkpoint kinase ATM phosphorylates and activates Chk2, which in turn directly phosphorylates and activates p53 tumor suppressor protein. p53 and its transcriptional targets play an important role in both G1 and G2 checkpoints. ATR-Chk1-mediated protein degradation of Cdc25A protein phosphatase is also a mechanism conferring intra-S-phase checkpoint activation.

Cellular Processes; Cell growth and death
BRITE hierarchy

nt06130  Cell cycle (viruses)
nt06160  Human T-cell leukemia virus 1 (HTLV-1)
nt06161  Human immunodeficiency virus type 1 (HIV-1)
nt06162  Hepatitis B virus (HBV)
nt06163  Hepatitis C virus (HCV)
nt06164  Kaposi sarcoma-associated herpesvirus (KSHV)
nt06165  Epstein-Barr virus (EBV)
nt06166  Human papillomavirus (HPV)
nt06167  Human cytomegalovirus (HCMV)
nt06170  Influenza A virus (IAV)
nt06230  Cell cycle G1/S
nt06261  Gastric cancer
nt06262  Pancreatic cancer
nt06263  Hepatocellular carcinoma
nt06265  Bladder cancer
nt06266  Non-small cell lung cancer
nt06267  Small cell lung cancer
nt06268  Melanoma
nt06270  Breast cancer
nt06272  Prostate cancer
nt06273  Glioma
nt06276  Chronic myeloid leukemia

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