Is an orderly sequence of events that occur as a cell duplicates its contents and divides (Fig. 8.1). During the cell cycle, genetic information is duplicated and the duplicated chromosomes are appropriately aligned for distribution between two genetically identical daughter cells.
The cell cycle is divided into four phases, referred to as G1 , S, G2 , and M. G1 (gap 1) occurs after the postmitosis phase when DNA synthesis stops and ribonucleic acid (RNA) and protein synthesis and cell growth take place. During the S phase, DNA synthesis occurs, causing two separate sets of chromosomes to develop, one for each daughter cell. G2 (gap 2) is the premitotic phase and is similar to G1 in that DNA synthesis stops, but RNA and protein synthesis continue. The phases, G1 , S, and G2 are referred to as interphase. The M phase is the phase of nuclear division, or mitosis, and cytoplasmic division. Continually dividing cells, such as the skin’s stratified squamous epithelium, continue to cycle from one mitotic division to the next. When environmental conditions are adverse, such as nutrient or growth factor unavailability, or when cells are highly specialized, cells may leave the cell cycle, becoming mitotically quiescent, and reside in a resting state known as G0 . Cells in G0 may reenter the cell cycle in response to extracellular nutrients, growth factors, hormones, and other signals such as blood loss or tissue injury that trigger cell growth. Highly specialized and terminally differentiated cells, such as neurons, may permanently stay in G0.