This PhD project investigated the role of the main Schizosaccharomyceps ornbe cell cycle regulator Cdc2 (CDKI in Homo sapiens, Cdc28 in Saccharomyces cerevisiae) in the cell cycle-dependent regulation of homologous recombination. Increasing evidence in the literature suggests that cells suppress homologous recombination in S phase, whereas this process is promoted in G2 to repair ssDNA gaps or DNA breaks that arose during DNA replication. Suppression of recombination in S phase appears to be important because ssDNA gaps are a very good substrate for DNA recombination and the latter process could interfere with DNA replication. Although recombination proteins travel with replication forks and perfonn repair functions during S phase, cells do not engage full homologous recombination until they enter G2. The underlying mechanisms of this regulation are still enigmatic, but it is known that DNA helicases play a crucial role. The work presented in this thesis proposes two novel functions for S. pombe Cdc2 in the cell cycle-dependent coordination of DNA recombination: (i) during unperturbed cell cycle and (ii) in response to camptothecin (CPT)-induced DNA breaks. In unperturbed cells, in vivo elevated Cdc2 activity causes problems during DNA replication that lead to an increase in spontaneous gene conversion between sister chromatids and enhanced loss of a non-essential minichromosome. Data presented here suggest that Cdc2 regulates the anti-recombinogenic activity of the Srs2 DNA helicase to prevent such spontaneous gene conversion events in S phase. Both proteins associate with PCNA in distinct protein complexes, which may allow them to regulate DNA repair in S phase. Elevated Cdc2 activity leads to constitutive phosphorylation of the checkpoint kinase Chk I, indicating that the inability to regulate Srs2 DNA helicase causes DNA replication lesions, which engage the G2-M checkpoint. Cells with elevated Cdc2 activity are specifically sensitive to the Topoisomerase I (Top 1) poison CPT. The camptothecin sensitivity of cdc2. Iw mutant cells increases in the absence of Tyrosyl-DNA-phosphodiesterase (Tdpl), which cleaves immobilised Topl releasing it from the 3'-end of DNA in S phase. As during the unperturbed cell cycle, Cdc2 appears to regulate Srs2 DNA helicase under these circumstances. Srs2 may unwind the blocked Y-strand in the absence of Up I to allow a nuclease to access the damaged site. Both Mus8l and Radl 6 are potential candidates since both nucleases act in the Cdc2-dependent pathway in response to CPT. Although all "wee" mutants with elevated Cdc2 activity are defective in this CPT repair pathway, both Weel and Mikl kinase may perform independent repair functions. Weel kinase is required for homologous recombination upstream of Rqhl DNA helicase and for the repair of UV induced DNA damage when nitrogen- starved cells exit the cell cycle in GI. MikI kinase, which regulates Cdc2 activity in S phase, appears to act in a novel repair pathway depending on the Ku80-Ku7O heterodimer but not on DNA ligase IV.