Chap 12

The Cell Cycle

Kangaroo rat epithelial cells

Phases of the Cell Cycle

G1 – gap phase 1•Primary growth phase•Longest phase for most cells

S – Synthesis•DNA replicate is made via transcription

G2 – gap phase 2•Second growth phase•Preparation for separation of replicated genome•Mitochondria and other organelles replicate now•Microtubules assemble at a spindle

M – Mitosis•Cell nucleus proceeds through PMAT

Cytokinesis – cytoplasmic division•Cell plate forms in plant cells•microfilaments constrict around middle of cell resulting in cell cleavage.

Cell Cycle Regulation

• The timing and rate of division in cells is crucial to normal growth and development.

• The frequency of division varies with the type of cell

Cell Cycle Control System

• This control system both triggers and coordinates key events in the cell cycle.

• This system proceeds on its own, driven by a built-in clock, yet regulated at certain checkpoints by both internal and external controls.

• Checkpoint – critical point where “stop and go” signals are relayed.

• Three major checkpoints are found: G1 checkpoint - primary point where

external signals can alter events. Linked to growth and prep for DNA synthesis.

G2 checkpoint – linked to growth and prep for division.

M checkpoint - assures that all chromosomes are attached to spindle fibers prior to anaphase.

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