BackThe Discovery of S Phase and the Cell Cycle in Eukaryotic Cells
Study Guide - Smart Notes
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The Cell Cycle
Discovery and Definition of the Cell Cycle
The cell cycle is the orderly sequence of events that leads a eukaryotic cell through the duplication of its chromosomes to the time it divides. This concept was first described by Alma Howard and Stephen Pelc, who used autoradiography to visualize DNA synthesis in cells.
Autoradiography: A technique where radioactive isotopes (such as phosphorus or thymidine) are incorporated into newly synthesized DNA, and their emissions are visualized as black dots on X-ray film.
Key Finding: DNA synthesis occurs during a specific period in interphase, not during M phase (mitosis).
Historical Context: Howard and Pelc's experiments in 1951 demonstrated that DNA replication is temporally separated from chromosome partitioning.
S Phase (Synthesis Phase)
The S phase is a distinct stage within interphase where DNA replication occurs. This stage was identified by observing cells that incorporated radioactive thymidine into their DNA, indicating active synthesis.
Definition: S phase is the period in interphase when the cell's genetic material (DNA) is copied.
Separation of Processes: DNA replication (S phase) is separated from chromosome partitioning (M phase).
Experimental Evidence: Only interphase cells showed black dots (active DNA synthesis) in autoradiography, confirming the existence of S phase.
Experimental Techniques
Autoradiography was crucial in identifying the timing of DNA synthesis in the cell cycle.
Radioactive Phosphorus: Used to label newly synthesized DNA, visualized as black dots on X-ray film.
Radioactive Thymidine: Incorporated into DNA but not RNA, providing specific evidence for DNA replication.
Summary Table: Cell Cycle Stages
Stage | Main Event | Experimental Evidence |
|---|---|---|
Interphase (S phase) | DNA synthesis (replication) | Autoradiography: black dots in interphase cells |
M phase | Chromosome partitioning (mitosis) | No DNA synthesis detected |
Key Terms
Interphase: The phase of the cell cycle between cell divisions, including G1, S, and G2 phases.
M phase: The phase where mitosis and cytokinesis occur.
Autoradiography: Visualization of radioactive molecules in cells using X-ray film.
Example
In a typical experiment, cells are exposed to radioactive thymidine. Only those in S phase incorporate the label, which is then visualized as black dots on X-ray film, confirming the timing of DNA replication.
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