Introduction about cell cycle by emphasizing DNA replication - Research Paper Example

The Cell Cycle Introduction The eukaryotic cell, like all cells, has a need to replicate in a precise manner as part of an extended cell cycle. DNA replication, one of the most important elements of the cell cycle, occurs during the S phase, relying on a precise assembly of pre-replicated complexes (RCs) during the previous G1 phase (Perkins, Drury &Diffley, 2001). DNA replication occurs in cells to ensure that the daughter cells contain genetic material from the parent cell through a process known as semi-conservative replication (Kurth & Gautier, 2010).
DNA replication relies on several checkpoints controlled by various molecules and enzymes (Byun et al, 2005, Lou et al, 2008). Evidently, there is a need to ensure that DNA replication occurs only once per cell cycle to prevent daughter cells having inappropriate copy numbers and other potentially fatal errors (Byun et al, 2005, Lou et al, 2008, Kurth& Gautier, 2010). This means that DNA replication is tightly controlled by a number of different cell elements, including RCs and the cyclin-dependent kinase (CdK) known as Cdc28p (Drury, Perkins &Diffley, 2000). The CdK family is important in cell cycle regulation as they phosphorylate proteins involved in cell cycle checkpoints.
During the assembly of RCs at an origin, a family of proteins (Mcm2-7) becomes associated with the DNA (Randell et al, 2010, Lydeard et al, 2010), a mechanism that requires the origin-recognition complex (ORC) and Cdc6 (cell division cycle 6) (Randall et al, 2006). The loading of Mcm2-7 helicase onto origin-proximal DNA for replication is directed by the ORC and Cdc6, initiated by sequential ATP hydrolysis (Randall et al, 2006). This occurs at a precise time during G1 in part due to the mechanisms of Cdc28p (Drury et al, 2000), which plays the dual function of preventing RC formation and initiating the replication of DNA within the cell cycle (Drury et al, 2010). Cdc6p is also important in the cell cycle because it prevents DNA re-replication, as without it there may be multiple rounds of DNA re-replication without the mitosis event (Drury et al, 2010). It is possible to judge which phase of the cell cycle a cell is at through levels of Cdc6p: high during G1 and then gets phosphorylated during the S phase (Rizzardi& Cook, 2013).
Perkins, Drury &Diffley (2001) showed the importance of CDC6 by introducing mutant alleles of the CDC6 gene. Two major transformants of CDC6 were chosen. CDC6-d1 ensured that the cell could not progress onto S phase, as there was abnormal assembly of the pre-RC complex. CDC6-d2 ensured that cells could not complete mitosis as the cell cycle was arrested after S phase (Perkins et al, 2001, Boronat & Campbell, 2007). These results support an important role for CDC6 in DNA replication and emphasize the necessity of cell cycle checkpoints. CDC6 itself is targeted for degradation when levels require reduction for replication control by the SCF complex.
The SCF complex is a multi-protein complex, having a role in targeting cell cycle regulators to tag them for ubiquitin-mediated proteasomal degradation (Drury, Perkins &Diffley, 2000) as well as ubiquitinating and degrading the cyclin-dependant kinase inhibitors (CKIs). Separate parts of this complex act as recognition elements which target CDC6 for proteolysis (Perkins, Drury &Diffley, 2001). Drury, Perkins &Diffley (2000) showed that, whilst the Cdc6p proteolysis before Start does not require the SCF complex, it is required for the degradation of Cdc6p in the late G1 phase. Without this degradation, levels of Cdc6p in the cell become unstable and the cell is unable to complete mitosis and DNA replication (Boronat& Campbell, 2007).
Works Cited
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