Porth's Essentials of Pathophysiology, 4e

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Cell and Tissue Function

U N I T 1

A cell reproduces by performing an orderly sequence of events called the cell cycle. The cell cycle is divided into four phases of unequal duration that include the (1) synthesis (S) and mitosis (M) phases that are separated by (2) two gaps (G 1 and G 2 ). There is also (3) a dormant phase (G 0 ) during which the cell may leave the cell cycle. Movement through each of these phases is mediated at (4) specific checkpoints that are controlled by specific enzymes and proteins called cyclins. U N D E R S T A N D I N G The Cell Cycle

Synthesis and Mitosis. Synthesis (S) and mitosis (M) rep- resent the two major phases of the cell cycle. The S phase, which takes about 10 to 12 hours, is the period of DNA synthesis and repli- cation of the chromosomes. The M phase, which usually takes less than an hour, involves formation of the mitotic spindle and cell division with formation of two daughter cells. 1

S

G 1

G 2

M

G 0

2

Gaps 1 and 2. Because most cells require time to grow and double their mass of proteins and organ- elles, extra gaps (G) are inserted into the cell cycle. G 1 is the stage during which the cell is starting to prepare for DNA replication and mitosis through protein synthesis and an increase in organelle and cytoskel- etal elements. G 2 is the premitotic phase. During this phase, enzymes and other proteins needed for cell division are synthesized and moved to their proper sites.

S

G 1

G 2

G 0

M