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 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 accumulate the materials needed to 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 makes a duplicate copy of its centrioles and nuclear DNA. This is the point at which the cell carries out the semiconservative replication described earlier.

The two identical sets of DNA molecules are then available 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 the interval (4-6 hours in fibroblasts) between DNA replication and cell division. In G2, a cell exhibits further growth,

makes more organelles, finishes replicating its centrioles, and synthesizes enzymes that control cell division. It also checks the fidelity of DNA replication and usually repairs any errors that are detected.

M is the mitotic phase, in which a cell replicates its nucleus and then pinches in two to form 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.

The length of the cell cycle varies greatly from one cell type to another. Stomach and skin cells divide rapidly, whereas bone and cartilage cells divide slowly.

Some cells leave the cell cycle for a "rest" and cease to divide for days, years, or the rest of one's life-mature neurons, skeletal muscle cells, and adipocytes, for example.

Such cells are said to be in the Go (G-zero) phase. The balance between cells that are actively cycling and those standing by in Go is an important factor in determining the number of cells in the body.

An inability to stop cycling and enter Go is characteristic of cancer cells (see Deeper Insight 4.3).

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Question	Answer
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 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 accumulate the materials needed to 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 makes a duplicate copy of its centrioles and nuclear DNA. This is the point at which the cell carries out the semiconservative replication described earlier.	The two identical sets of DNA molecules are then available 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 the interval (4-6 hours in fibroblasts) between DNA replication and cell division. In G2, a cell exhibits further growth,	makes more organelles, finishes replicating its centrioles, and synthesizes enzymes that control cell division. It also checks the fidelity of DNA replication and usually repairs any errors that are detected.
M is the mitotic phase, in which a cell replicates its nucleus and then pinches in two to form 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.
The length of the cell cycle varies greatly from one cell type to another. Stomach and skin cells divide rapidly, whereas bone and cartilage cells divide slowly.	Some cells leave the cell cycle for a "rest" and cease to divide for days, years, or the rest of one's life-mature neurons, skeletal muscle cells, and adipocytes, for example.
Such cells are said to be in the Go (G-zero) phase. The balance between cells that are actively cycling and those standing by in Go is an important factor in determining the number of cells in the body.	An inability to stop cycling and enter Go is characteristic of cancer cells (see Deeper Insight 4.3).

Created by: Russells3709

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