טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOfir Yaara
SubjectRegulation of Premeiotic DNA Replication in Budding Yeast
by Cdc28,Ime2 and Cdc6
DepartmentDepartment of Biology
Supervisor Professor Emeritus Yona Kassir


Abstract

DNA replication is a crucial process essential for cell proliferation. However, this process is potentially dangerous to the cell. Incomplete or over-replication may result in malignancy or cell death. Therefore, in all eukaryotes, the initiation of DNA replication is regulated by a precise mechanism that ensures that DNA replication will occur only once in each cell cycle, and that entry into mitosis will take place only upon completion of DNA replication. This regulation is accomplished by an ordered assembly of distinct DNA/proteins complexes, formed on origins of DNA replication. 

In this thesis, we examined the role of Cdc6, a major component of the pre-replication complex, in the initiation of premeiotic DNA replication. We first showed that Cdc6 is required for the initiation of premeiotic DNA replication. We then focused on the regulation of Cdc6, showing that upon entry into the meiotic S-phase Cdc6 is degraded. However, the origin-bound Cdc6 fraction is protected from degradation throughout the meiotic cycle. By contrast, in the mitotic cell cycle, both the origin-bound and the soluble Cdc6 are similarly degraded. Premature activation of Cdc28, which leads to early entry into meiotic S-phase, does not eliminate the protection of the origin-bound Cdc6, suggesting that meiotic changes in chromatin structure might cause this protection. In the mitotic cell cycle degradation of Cdc6 depends on phosphorylation by the cyclin-dependent kinase Cdc28. On the other hand, we show that in the meiotic cycle the degradation of Cdc6 is independent of Cdc28. We suggest that in meiosis, Ime2 phosphorylates Cdc6, because preliminary data show physical association between Cdc6 and Ime2.