BackThe Discovery of S Phase and the Cell Cycle
Study Guide - Smart Notes
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The Cell Cycle
Discovery and Definition
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 coined by Alma Howard and Stephen Pelc, who used autoradiography to visualize DNA synthesis in cells.
Autoradiography: A technique where radioactive phosphorus is incorporated into newly synthesized DNA, and emissions are visualized as black dots on X-ray film.
Key Finding: DNA synthesis occurs during interphase, not during M phase.
Historical Context: In 1951, Howard and Pelc demonstrated that DNA replication is restricted to a specific period in interphase.
S Phase (Synthesis Phase)
The S phase is a distinct stage within interphase where DNA replication occurs. This stage was identified by observing the incorporation of radioactive thymidine into DNA, which is not incorporated into RNA.
Definition: S phase is the period during interphase when the cell synthesizes new DNA, preparing for cell division.
Separation of Processes: DNA replication (S phase) is temporally separated from chromosome partitioning (M phase).
Experimental Evidence: Only interphase cells showed active DNA synthesis, as indicated by black dots in autoradiography experiments.
Example: Radioactive thymidine labeling is used to track DNA synthesis in living cells.
Interphase and M Phase
Interphase is the phase of the cell cycle where the cell grows, replicates its DNA, and prepares for division. M phase is when the cell actually divides, partitioning the replicated chromosomes.
Interphase: Includes G1, S, and G2 phases.
M phase: Includes mitosis and cytokinesis.
Key Point: DNA replication occurs only during S phase of interphase, not during M phase.
Summary Table: Cell Cycle Phases
Phase | Main Event |
|---|---|
G1 | Cell growth and preparation for DNA synthesis |
S | DNA replication (synthesis) |
G2 | Preparation for mitosis |
M | Chromosome partitioning and cell division |
Key Terms
Autoradiography: Visualization of radioactive molecules in cells.
S phase: Period of DNA synthesis in the cell cycle.
Cell cycle: Sequence of events from chromosome duplication to cell division.
Additional info: The image shows an autoradiography experiment, which is directly relevant to the discovery of S phase and the cell cycle.
