Most cells periodically divide into two daughter cells, so a cell has a life cycle extending from one division to the next. This cell cycle (fig. 4.12) is divided into four main phases: G1, S, G2, and M.
G1 is the first gap phase, an interval between cell division and DNA replication. During this time, a cell synthesizes proteins, grows, and carries out its preordained tasks for the body. Almost all of the discussion in this book relates to what cells do in the G1 phase. Cells in G1 also begin to replicate their centrioles in preparation for the next cell division and accumulate the materials needed to
1 muta = change
replicate their DNA in the next phase. In cultured cells called fibroblasts, which divide every 18 to 24 hours, G1 lasts 8 to 10 hours.
S is the synthesis phase, in which a cell carries out DNA replication. This produces two identical sets of DNA molecules, which are then available, like the centrioles, to be divided up between daughter cells at the next cell division. This phase takes 6 to 8 hours in cultured fibroblasts.
G2, the second gap phase, is a relatively brief interval (4 to 6 hours) between DNA replication and cell division. In G2, a cell finishes replicating its centrioles and synthesizes enzymes that control cell division.
M is the mitotic phase, in which a cell replicates its nucleus and then pinches in two to form two new daughter cells. In cultured fibroblasts, the M phase takes 1 to 2 hours. The details of this phase are considered in the next section. Phases G1, S, and G2 are collectively called inter-phase—the time between M phases.
The length of the cell cycle varies greatly from one cell type to another. Stomach and skin cells divide rapidly, bone and cartilage cells slowly, and skeletal muscle cells and nerve cells not at all (see insight 4.3). Some cells leave the cell cycle for a "rest" and cease to divide for days, years, or the rest of one's life. Such cells are said to be in the G0 (G-zero) phase. The balance between cells that are actively cycling and those standing by in G0 is an important factor in determining the number of cells in the body. An inability to stop cycling and enter G0 is characteristic of cancer cells (see insight 4.4 at the end of the chapter).
Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill
Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.