The crucian carp (Carassius carassius) inhabits lakes and ponds in Northern Europe, where it is able to survive anoxia that often occurs for months during the long winter. Several physiological adaptations, including the exotic ability of ethanol production, allows the crucian carp to cover its energy needs by anaerobic metabolism, making the survival dependent on its glycogen stores.There are several oxygen-dependent processes in animals that are not directly related to the generation of ATP. One of these is the production of DNA precursors, deoxyribonucleotides, where the radical-based enzyme ribonucleotide reductase (RNR) catalyzes the rate-limiting step of synthesis. The generation of the radical required for the catalytic mechanism is oxygen-dependent in all eukaryotes, implying that this process cannot occur in anoxic crucian carp. Nevertheless, previous studies have shown that DNA synthesis continues in crucian carp exposed to seven days of anoxia.In this study, genes coding for the crucian carp RNR were cloned, and the expression of the subunits that constitutes the enzyme was quantified by real-time RT-PCR. Results showed that all subunits needed for RNR activity in proliferating cells were expressed in both the heart and the liver of normoxic and anoxic crucian carp. This supports the previous observations of cell proliferation in anoxic crucian carp, and the presence of the S-phase specific R2 subunit also suggests that there is mitotic activity in the crucian carp heart and liver during anoxia.In the heart there was a significant decrease in the transcription of the R1 and p53R2 subunits in the anoxic crucian carp. As these subunits are believed to be responsible for providing deoxyribonucleotides for DNA repair and mitochondrial DNA replication, this reduction probably reflects a depressed mitochondrial activity due to the inability to maintain respiration.The crucian carp subunits responsible for generating and storing the radical, R2 and p53R2, were purified from over-expressing Escherichia coli cells and investigated by electron paramagnetic resonance (EPR) spectroscopy. These experiments concluded that the chemical environment of the radical in crucian carp RNR is similar of mammalian RNR. At present, it is therefore not possible to conclude if, and explain how, this enzyme is able to function in anoxic crucian carp.