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S-phase (Synthesis Phase)
Definition: The S-phase is the part of interphase where DNA synthesis (replication) takes place.
Key event: The amount of DNA doubles while the chromosome number remains the same.
Example: If DNA content was 2C in G1, it becomes 4C after S-phase.
However, if the chromosome number was 2n, it remains 2n.
Other processes:
In animal cells, DNA replication occurs in the nucleus.
Centriole duplication takes place in the cytoplasm.
Special feature: S-phase is also called the invisible phase because replicated chromosomes are not yet visible under the microscope.
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G2-phase (Gap 2 Phase)
Definition: The G2-phase is the final stage of interphase before the cell enters mitosis.
Key events:
Cytoplasmic organelles such as mitochondria, chloroplasts, and Golgi complex are duplicated.
RNA and protein synthesis continues actively.
The cell contains double the DNA compared to the original diploid cell.
Preparation for mitosis:
The cell checks and prepares itself for M-phase.
Synthesis of regulatory proteins and kinases ensures proper control of cell division.
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M-phase (Mitosis Phase)
Definition: The M-phase is the stage of the cell cycle when actual cell division occurs.
Key steps:
Nuclear division (mitosis): The replicated chromosomes are separated.
Cytoplasmic division (cytokinesis): The cytoplasm divides, forming two daughter cells.
End result: Two genetically identical daughter cells, each with the same chromosome number as the parent cell.
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Quick Recap of Flow
1. S-phase → DNA replicates (2C → 4C, chromosome number unchanged).
2. G2-phase → Organelles double, proteins and RNA synthesized, cell prepares for mitosis.
3. M-phase → Nuclear division + cytokinesis → two daughter cells.
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Final Words
The S-phase, G2-phase, and M-phase are tightly regulated to maintain genetic stability. Any error in these stages can lead to mutations, abnormal cell division, or diseases such as cancer. Understanding these phases is not only important for students but also for fields like genetics, biotechnology, and medicine.