Introduction to Biology Biology Diagrams The cell-cycle control system was simply a black box inside the cell. It was not even clear whether there was a separate control system, or whether the processes of DNA synthesis, mitosis, and cytokinesis somehow controlled themselves. A major breakthrough came in the late 1980s with the identification of the key proteins of the control system When the cell cycle control system malfunctions, the consequences can be severe, leading to uncontrolled proliferation and genomic instability. Regulatory disruptions may result from mutations in checkpoint proteins, overexpression of cyclins, or loss of tumor suppressor function, allowing unchecked division and tumor formation.
Cyclin-dependent kinases (CDKs) lie at the heart of eukaryotic cell cycle control, with different cyclin-CDK complexes initiating DNA replication (S-CDKs) and mitosis (M-CDKs)1,2. However, the
Components of the Cell Biology Diagrams
To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell cycle checkpoints at which the cell cycle can be stopped until conditions are favorable. Figure 1 The cell cycle is controlled at three checkpoints. Integrity of the DNA is assessed at the G1 checkpoint. The length of the cell cycle is highly variable, even within the cells of a single organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for epithelial cells, and to an entire human lifetime spent in G 0 by specialized cells, such as cortical neurons or cardiac muscle cells. The cell cycle is a series of events that cells go through to grow, replicate their DNA, and divide. This process is vital for the growth, development, repair, and maintenance of living organisms. The Cell Cycle: Principles of Control. London: Published by New Science Press in association with Oxford University Press. ISBN 978--87893-508-6.
To achieve cell-cycle-dependent regulation of its activity, the SCF usually targets proteins only when they are phosphorylated at one or multiple sites (343, 371). The SCF is important for cell cycle control because of its role in the degradation of cyclin E. Cyclin E levels peak at the G1/S boundary and then decline as DNA is replicated (125 Cell cycle control is focused mainly on two events, the replication of genomic DNA and its subsequent segregation between daughter cells, which in eukaryotic cells occur during distinct cell cycle Cell cycle checkpoints secure ordered progression from one cell cycle phase to the next. They are important to signal cell stress and DNA lesions and to stop cell cycle progression when severe problems occur. Recent work suggests, however, that the cell cycle control machinery responds in more subtle and sophisticated ways when cells are faced with naturally occurring challenges, such as
