Cell - Cycle Assignment | PDF - Free Printable

You’ve uploaded a “CELLS alive! - Cell Cycle Worksheet” diagram that shows the eukaryotic cell cycle with labeled phases and checkpoints. The worksheet asks: “What’s happening now?” — meaning, you’re expected to fill in brief descriptions for each numbered step (1–10) based on what occurs during that part of the cell cycle.

Here’s a complete, accurate solution with explanations:

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✔ Solution: Filling in “What’s happening now?” for each numbered step

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1) G1 – Cells increase in size, produce RNA and synthesize protein
→ *Explanation:* This is the first gap phase after mitosis. The cell grows physically, synthesizes mRNA and proteins needed for DNA replication, and prepares for the S phase. It’s a critical growth and preparation stage.

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2) G0 – Cells can be very active metabolically, but can quit dividing cells. This stage can be temporary or permanent.
→ *Explanation:* G0 is a resting state where cells exit the active cycle. Some cells (like neurons or muscle cells) stay here permanently; others (like liver cells) can re-enter the cycle if needed. Metabolism continues, but no division occurs.

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3) G1 Checkpoint – Is the cell big enough and the environment suitable?
→ *Explanation:* Also called the restriction point. The cell checks for adequate size, nutrients, growth factors, and DNA integrity before committing to replication. If conditions aren’t met, it may enter G0 or pause.

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4) G1 continued
→ *Explanation:* After passing the G1 checkpoint, the cell continues growing and preparing for DNA synthesis. It’s still in G1 phase but moving toward S phase.

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5) Synthesis or S Phase – Period of DNA Replication, going from 2N to 4N in preparation for mitosis.
→ *Explanation:* DNA is duplicated during S phase. Each chromosome becomes two sister chromatids. Ploidy changes from 2N (diploid) to 4N (tetraploid in terms of DNA content), though chromosome number remains 2N until mitosis.

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6) G2 – After DNA Replication, the cell continues to grow and produce proteins.
→ *Explanation:* Second gap phase. The cell grows more, synthesizes proteins (especially tubulin for spindle formation), and prepares for mitosis. Organelles may also duplicate.

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7) G2 Checkpoint – Has DNA replicated? Is the cell big enough, and the environment more suitable?
→ *Explanation:* Final checkpoint before mitosis. Ensures DNA replication is complete and undamaged, cell size is sufficient, and environmental signals are favorable. If not, the cell halts for repair or apoptosis.

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8) Mitosis – Cell and protein production stop at this point in the cell cycle.
→ *Explanation:* During mitosis, the cell focuses entirely on dividing its nucleus. Protein synthesis and growth largely halt because energy and resources are directed toward chromosome segregation and cytokinesis.

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9) Mitosis Checkpoint – Are chromosomes aligned on the spindle?
→ *Explanation:* Also called the metaphase checkpoint or spindle assembly checkpoint. Ensures all chromosomes are properly attached to spindle fibers at the metaphase plate before anaphase begins. Prevents unequal division.

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10) Mitosis continued
→ *Explanation:* Refers to the later stages of mitosis — anaphase (chromosomes pulled apart), telophase (nuclear envelopes reform), and cytokinesis (cytoplasm divides). The cell is actively splitting into two daughter cells.

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🔄 Summary of the Cell Cycle Phases:

- Interphase = G1 → S → G2 (cell growth, DNA replication, prep for division)
- Mitotic Phase (M) = Mitosis (nuclear division) + Cytokinesis (cytoplasmic division)

Checkpoints (G1, G2, M) act as quality control points to ensure fidelity before proceeding.

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✔ Final Answer Boxed (for quick reference):

| # | What’s happening now? |
|----|------------------------|
| 1 | Cells grow, make RNA & proteins in G1 phase. |
| 2 | Cells in G0 rest — may divide later or never. |
| 3 | G1 Checkpoint: Checks size & environment. |
| 4 | G1 continues toward S phase. |
| 5 | S Phase: DNA replicates (2N → 4N). |
| 6 | G2: Cell grows more, makes proteins for mitosis. |
| 7 | G2 Checkpoint: Confirms DNA replication & readiness. |
| 8 | Mitosis begins — growth stops, focus on division. |
| 9 | Mitosis Checkpoint: Chromosomes must align on spindle. |
| 10 | Mitosis continues — chromosomes separate, cells divide. |

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Let me know if you’d like a printable version or a diagram explanation with visuals!