Serine/threonine-protein kinase which is required for checkpoint-mediated cell cycle arrest and activation of DNA repair in response to the presence of DNA damage or unreplicated DNA. May also negatively regulate cell cycle progression during unperturbed cell cycles. This regulation is achieved by a number of mechanisms that together help to preserve the integrity of the genome. Recognizes the substrate consensus sequence [R-X-X-S/T]. Phosphorylates wee1 at 'Ser-549' and cdc25c at 'Ser-287', which creates binding sites for 14-3-3 proteins which activate wee1 and inhibit cdc25c. Phosphorylates cdc25a at 'Ser-504' which prevents the interaction of cdc25a with CDK2-cyclin E1, CDC2-cyclin A1 and CDC2-cyclin B1. This inhibitory effect does not require 14-3-3 protein binding. Activation of wee1 and inhibition of CDC25 results in increased inhibitory tyrosine phosphorylation of CDK-cyclin complexes and consequent inhibition of cell cycle progression. May promote DNA repair, regulate chromatin assembly and the transcription of genes that regulate cell-cycle progression. May also play a role in replication fork maintenance. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated through phosphorylation by atr or atm in response to DNA damage or inhibition of DNA replication. Interacts with and phosphorylates clspn, an adapter protein that regulates the ATR-dependent phosphorylation of chek1. Expressed throughout development. Transiently activated by ATR-mediated phosphorylation from the midblastula transition (MBT) to the initial gastrula stage. Developmentally regulated activation of the DNA replication checkpoint may occur as the nucleo-cytoplasmic ratio increases and maternal replication factors are depleted. Elongation of the embryonic cell cycle may allow time for the transcription of genes that initiate the switch from maternal to zygotic control of embryogenesis. The autoinhibitory region (AIR) binds to the kinase domain and inhibits its activity. Phosphorylated by atm in response to ionizing irradiation (By similarity). Phosphorylated by atr at Thr-314, Ser-344, Ser-356 and Ser-365 in response to various stimuli that cause checkpoint activation. Phosphorylation impairs binding of the C-terminal autoinhibitory region (AIR) to the kinase domain and thereby enhances kinase activity. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. NIM1 subfamily.