This paper provides detailed insight into the mechanisms of repair of different types of DNA damage and the direct molecular players (enzymes repairing the damage or tagging the damaged site for further processing; damage sensor molecules; other signalling and effector molecules). The genetic bases of diseases and conditions associated with defective DNA repair are comprehensively reviewed, from the ''classic'' severe diseases such as xeroderma pigmentosum and Cockayne syndrome to the much more subtle UV sensitivity syndromes. The review analyses the basic molecular mechanisms underlying the relatively rare monogenic diseases of DNA repair and management of genome integrity as well as the common multifactorial diseases and conditions with late onset that are associated with increased levels of oxidative stress (metabolic syndrome, diabetes type 2, cardiovascular disease) and with accumulation of ''errors'' in DNA (normal and pathological ageing phenotypes, various cancers). The role of cell cycle checkpoints in dividing cells and the mechanisms of decision-making for the fate of a damaged cell are discussed with regards to the cell homeostasis in normal and cancerous tissues. The role of major DNA damage-associated signalling and effector molecules (p53, ATM, poly-(ADP-ribose)-polymerase, DNA-dependent protein kinase, BRCA proteins, retinoblastoma protein, and others) is discussed and illustrated with examples in the context of health and disease. DNA repair and programmed cell death are viewed together as a unified mechanism for limiting the presence of damaged cells and cells with potentially oncogenic transformation in multicellular organisms. Special attention is paid to ageing as a natural phenomenon and an adaptive evolutionary mechanism, with a brief outline of ''successful ageing''. The differential rates of repair of DNA in transcribed and nontranscribed regions of the genome and the specificities of DNA repair profile in some types of cells (terminally differentiated cells, pluripotent stem cells, etc.) and in certain taxonomic groups (e.g. ''the rodent repairadox'') are discussed with regards to replicative ageing and the evolutionary processes on micro- and macroscale. The role of mutagenesis as a ''hit and miss'' mechanism and the ''leakiness'' of DNA repair for increasing genetic diversity in the course of individual life and on evolutionary scale and the phenomenology of ongoing molecular evolution are extensively reviewed.
