Cell proliferation and the cell cycle

Appropriate cell proliferation enables growth and development, the replacement of damaged, old or dead cells, tissue repair and the maintenance of tissue homeostasis. Cell replication is achieved via the cell cycle, a carefully orchestrated process that leads to the division of one cell into two daughter cells. This involves a tightly regulated series of steps that undertakes the accurate transcribing of the cell’s genetic information so as to provide

The cell cycle

Figure 5.1 The cell cycle.

each new cell with a complete set of chromosomes/genes as well as the requisite cellular components for normal cell function. The cell cycle is characterised by a number of distinct steps, shown in Fig. 5.1.

Normal cell replication is initiated when there is a sufficient increase in the concentration of growth factors in the cell’s microenvironment followed by their binding to receptors on the cell membrane. Once bound, these factors activate a signalling cascade within the cell, leading to the initiation of the steps that ultimately end in cell division. The signalling cascade progresses through the cell’s cytoplasm via molecules referred to as signal transducers, or second messengers, eventually passing into the nucleus via specific nuclear proteins, called transcription factors, involved in gene expression. Transcription factors mediate the access and translation of genetic information contained within DNA.

The cell replication process involves four distinct phases. The first phase is known as Gap1 or G1. During this phase the cell grows in size and manufactures proteins, nucleic acids and other necessary cellular building blocks and prepares for DNA replication. The S (synthesis) phase involves the faithful copying of the cell’s genetic information. As this process is not without some errors, mechanisms are initiated to identify and undertake any necessary repairs to the genome and its associated structures. Phase G2 enables final repairs to the DNA and the chromatin condenses into chromosomes. The final phase, mitosis (M), ensures the appropriate segregation of chromosomes to each daughter cell prior to the final separation process (cytokinesis). The cell will then either enter into a noncycling phase (G0) or, if required, will start the cycling process once again.

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