G1/S checkpoint

Monitors nutritional state, biosynthetic capacity, and cell adhesion and imposes G1 arrest until cell is prepared for S phase.

G2/M checkpoint

Monitors completion of DNA synthesis (S phase) and imposes G2 arrest until cell is prepared for M phase.

Metaphase/anaphase checkpoint

Monitors attachment of chromosomes to the spindle and imposes metaphase arrest until cell is ready for anaphase.

DNA damage (genotoxic stress)

Kinases are activated at DNA damage site by single stranded DNA and 5′, 3′ ends, leading to p53 activation and transcription of genes encoding p21 inhibitors of cyclin-dependent kinases, and hence G1/S or G2/M arrest until repair is complete.

Cytosolic unfolded protein pathway

Activated by heat, alcohol, anaerobiosis, and amino acid analogs, leading to activated transcription of genes encoding chaperone proteins until protein refolding is complete.

Endoplasmic reticulum (ER) unfolded protein pathway

Unfolded proteins activate receptor thre/seri kinase, leading to release of a nuclease that degrades some mRNAs and reduces translation (G1 arrest), and splices some mRNAs leading to transcription of genes encoding chaperone proteins until protein refolding is complete.

Apoptosis (cell death)

Triggered by intracellular damage or extracellular signals, leading to caspase protease activation and cell destruction.

Ultraviolet, hyperosmotic shock, free-radical oxidation pathways

Mediated by MAP kinases, leading to transcription, until damage is reversed.