Cell cycle phases and checkpoints illustrated in a circular diagram.

This worksheet is about the cell cycle, which is the series of events that take place in a cell leading to its division and duplication. The diagram shows the main phases of the cell cycle, including interphase (G1, S, G2), mitosis (M), and the G0 phase.

Let’s go through each labeled part of the diagram and explain what's happening at each stage:

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1) G1 Phase (Gap 1)

- What’s happening now?
The cell grows in size, carries out normal functions, and synthesizes proteins and organelles in preparation for DNA replication. It also checks if conditions are favorable for division.
- Key activity: Cell growth and preparation for DNA synthesis.

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2) Checkpoint #1 (G1 Checkpoint)

- What’s happening now?
This is a critical checkpoint where the cell evaluates whether it has grown enough, has sufficient nutrients, and whether the DNA is undamaged. If all is good, the cell proceeds to the S phase. Otherwise, it may enter G0 or repair damage.
- Purpose: Prevents damaged or underdeveloped cells from replicating DNA.

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3) S Phase (Synthesis)

- What’s happening now?
DNA replication occurs. Each chromosome is duplicated, resulting in two sister chromatids joined at the centromere.
- Key event: Chromosomal DNA is copied.

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4) Checkpoint #2 (G2 Checkpoint)

- What’s happening now?
The cell checks that DNA replication was completed accurately and that the cell is large enough to divide. It ensures no DNA damage remains before entering mitosis.
- Purpose: Ensures the cell is ready for mitosis.

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5) G2 Phase (Gap 2)

- What’s happening now?
The cell continues to grow and produces proteins and organelles needed for mitosis (e.g., microtubules for the spindle apparatus).
- Key activity: Preparation for cell division.

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6) Mitosis (M Phase)

- What’s happening now?
The nucleus divides. This includes four stages: prophase, metaphase, anaphase, and telophase. After mitosis, cytokinesis usually follows (division of the cytoplasm).
- Result: Two genetically identical daughter nuclei.

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7) Cytokinesis

- What’s happening now?
The cytoplasm divides, resulting in two separate daughter cells. This typically happens after mitosis.
- Note: While not always shown separately, cytokinesis is the physical separation of the cell.

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8) Interphase

- What’s happening now?
This is the combined period of G1, S, and G2 phases when the cell grows and prepares for division. Most of the cell’s life is spent in interphase.
- Includes: Growth, DNA replication, and preparation for mitosis.

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9) G0 Phase

- What’s happening now?
Some cells exit the active cell cycle and enter a resting state called G0. These cells may remain inactive permanently (like nerve cells) or re-enter the cycle later (like liver cells).
- Example: Mature neurons, muscle cells.

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10) M Phase (Mitosis)

- What’s happening now?
Nuclear division occurs. Chromosomes condense, align, separate, and move to opposite poles of the cell.
- Sub-stages: Prophase → Metaphase → Anaphase → Telophase.

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Summary Table:

| Number | Phase/Checkpoint | What’s Happening Now? |
|--------|------------------------|------------------------|
| 1 | G1 Phase | Cell grows; prepares for DNA replication |
| 2 | G1 Checkpoint | Checks if cell is ready to replicate DNA |
| 3 | S Phase | DNA replication occurs |
| 4 | G2 Checkpoint | Checks DNA integrity and readiness for mitosis |
| 5 | G2 Phase | Final preparations for division |
| 6 | Mitosis (M Phase) | Nucleus divides into two |
| 7 | Cytokinesis | Cytoplasm divides; two daughter cells form |
| 8 | Interphase | G1 + S + G2; growth and DNA prep |
| 9 | G0 Phase | Resting phase; cells exit active cycle |
| 10 | M Phase (Mitosis) | Chromosomes separate and nuclei form |

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✔ Final Answer (Fill-in-the-blanks):

1) G1 Phase – Cell grows and prepares for DNA synthesis
2) G1 Checkpoint – Cell checks size, nutrients, and DNA integrity before S phase
3) S Phase – DNA replication occurs
4) G2 Checkpoint – Ensures DNA is fully replicated and undamaged before mitosis
5) G2 Phase – Cell grows and prepares for mitosis
6) Mitosis (M Phase) – Nuclear division occurs
7) Cytokinesis – Cytoplasm divides into two daughter cells
8) Interphase – Period of cell growth and DNA replication (G1, S, G2)
9) G0 Phase – Resting phase; cell exits the cycle temporarily or permanently
10) M Phase – Division of the nucleus (mitosis)

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