Which of the following step in cell cycle is known as restriction point?
**Core Concept**
The restriction point is a critical regulatory checkpoint in the cell cycle, allowing cells to bypass quiescence and enter the G1 phase. This concept is essential in understanding cell proliferation and growth regulation. The restriction point is often associated with the activation of cyclin-dependent kinases (CDKs) that drive cell cycle progression.
**Why the Correct Answer is Right**
The restriction point is typically reached during the G1 phase, where the cell commits to DNA replication and cell division. At this point, the cell becomes insensitive to growth-inhibitory signals and can progress through the cell cycle regardless of the presence of growth factors. This is achieved through the activation of cyclin-dependent kinases (CDKs), particularly CDK2 and CDK4, which phosphorylate and inactivate the retinoblastoma protein (Rb), releasing E2F transcription factors that promote cell cycle progression.
**Why Each Wrong Option is Incorrect**
**Option A:** G2 checkpoint is incorrect because it occurs later in the cell cycle, where cells ensure proper DNA replication and repair before entering mitosis.
**Option B:** Mitosis is incorrect because it is the stage where chromosomes are divided, and not the regulatory checkpoint that allows cells to bypass quiescence.
**Option C:** S phase is incorrect because it is the stage of DNA replication, and not the regulatory checkpoint that commits cells to cell division.
**Clinical Pearl / High-Yield Fact**
The restriction point is a critical regulatory checkpoint that allows cells to bypass quiescence and enter the cell cycle. Understanding this concept is essential in understanding the mechanisms of cancer development and the effects of growth factors on cell proliferation.
**Correct Answer: C. G1 phase**