The cell cycle

Abstract

The cell cycle is the sequence of events that take place to enable DNA replication and cell division. It can be divided into two phases: interphase and mitosis. Interphase is further divided into the G1 (gap 1), S (synthesis), and G2 (gap 2) phases, which prepare the cell for division. In mitosis, the single cell is dividing into two identical daughter cells. The cell cycle is subject to strict controls that prevent cells with damaged or faulty DNA from further dividing and passing on defects to daughter cells. Controlled cell death (apoptosis) is initiated if the DNA damage is irreparable. Disorders of these regulatory mechanisms play an important role in carcinogenesis.

The mechanisms for apoptosis are discussed in the “cellular changes and adaptive responses” learning card.

Abnormalities of the cell cycle that lead to the development of cancer are discussed in detail in the “general oncology” learning card.

Cell cycle phases

Cell cycle

The sequence of events that take place to enable DNA replication and cell division. The cell cycle is first divided into two phases: interphase and mitosis. Interphase is further divided into the G1 (gap 1), S (synthesis), and G2 (gap 2) phases, which prepare the cell for division.

G stands for Gap/Growth and S for Synthesis!

Interphase

G0 phase (Resting phase)

  • Resting period of a cell after exiting the cell cycle.
  • Cell is differentiated; and has a specific respective function but is no longer undergoing cell division.
  • Only proliferating cells pass through the cell cycle. Most mature tissue cells are in the G0 phase.
  • Certain cell types can enter the G1 phase from the G0 phase if stimulated

Most cells in mature tissue are in the resting phase (G0 phase). They are differentiated and have a specific respective function. Mitosis is rare in mature differentiated tissue!

One of the features of malignant tumors is the dedifferentiation of tumor cells, i.e., reversal to less differentiated cells with a high mitotic rate. Multiple mitotic figures seen on microscopy are indicative of a malignant process.

Mitosis

Proper functioning of the mitotic spindle is a prerequisite for chromosome transportation. Inhibition with spindle poison leads to arrest of mitosis and cessation of cell division. Spindle poisons include colchicine, which inhibits microtubule polymerization, as well as vinca alkaloids and taxanes.

Cell cycle regulation

Basic principles of cell cycle regulation

The p53 protein prevents cells with genetic errors from entering the S phase of the cell cycle. Therefore it is regarded as a tumor suppressor!

Important checkpoints and transition points

A mutation in the Rb gene leads to dysregulation of the cell cycle, resulting in the unrestrained growth of retinal tumor cells, or a retinoblastoma.

Cell types

Types Characteristics Examples
Labile cells
  • Never go to G0, divide rapidly with a short G1
Quiescent (stable) cells
  • Enter G1 from G0 when stimulated.
Permanent cells

Cell cycle and cancer

Abnormalities of the cell cycle that lead to the development of cancer are discussed in detail in the “general oncology” learning card.

last updated 08/20/2018
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