When the cell cycle goes wrong: checkpoints and cancer

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Cycle wheel (model, not to scale), clockwise from the top

   G1, S, G2, mitosis (M), cytokinesis. Three gates (checkpoints) stand on the wheel: the G1 gate near the end of G1, the G2 gate before mitosis, and the M gate in mitosis, before the sister chromatids are pulled apart.

2. 1.

   UV rays from sunlight can damage a cell's DNA. Ari reads that a cell is in G1 with damaged DNA. What does a working G1 gate do?

   • It lets the cell into S, and the damage is copied with the DNA
   • It halts the cycle and gives the cell time to repair the DNA
   • It switches off, so the cell can no longer divide
   • It waits until the sister chromatids are attached to the spindle
3. 2.

   At the G2 gate, Bo learns that one chromosome is not fully copied. What does the cell do?

   • It is held at the gate until the copying is finished or repaired
   • It passes, because G2 checks the cell's size and nothing else
   • It enters mitosis and finishes the copying there
   • It goes back to G1 and starts the whole cycle again
4. 3.

   In mitosis, one pair of sister chromatids is not yet attached to spindle fibers from both poles. Esme reads the wheel. What does the M gate do?

   • It lets the cell go on, and the chromatids are sorted out later
   • It waits, because pulling chromatids apart cannot be undone
   • It sends the cell back to S to copy the DNA again
   • It hands the job to the G1 gate, which repairs the attachment

5. Mutation card (model)

   when	what the protein of gene X does
   before the mutation	tells the cell to divide when needed
   after the mutation	stays switched on

6. 4.

   Jun reads the card. Gene X was a proto-oncogene. After the mutation, what is gene X, and what is it like?

   • An oncogene, like a gate that halts the cell for good
   • A proto-oncogene, like an accelerator that works as usual
   • An oncogene, like a stuck accelerator
   • A tumor suppressor, like a stuck accelerator

7. p53 card (from OpenStax Biology 2e)

   protein	job
   p53	halts the cycle at the G1 gate when DNA is damaged and calls in repair enzymes

8. 5.

   A mutation stops the p53 gene from making a working protein. Kira reads the card. What happens at the G1 gate?

   • It checks for copied chromosomes instead of DNA damage
   • It no longer holds cells with damaged DNA, like a brake that fails
   • It holds cells for good, like a brake stuck on
   • It pushes cells into S faster, like an accelerator

9. Use this

   Cells in a dish after each round (model data), 5 rows. Columns: round, working gate, broken gate. 0, 4, 4. 1, 4, 8. 2, 8, 16. 3, 16, 32. 4, 32, 64. In this model, a working gate holds damaged cells for 1 round of repair, then they divide each round; a broken gate holds none.

10. 6.

    Milo reads the table. After round 4, how many more cells does the broken-gate dish hold than the working-gate dish?

    • 96 cells
    • 64 cells
    • 16 cells
    • 32 cells
11. 7.

    If the broken-gate dish keeps doubling each round, how many cells will Noah count in it after round 6?

    • 256 cells
    • 128 cells
    • 24 cells
    • 28 cells
12. 8.

    If the working-gate dish keeps doubling each round, how many cells will Pia count in it after round 5?

    • 128 cells
    • 48 cells
    • 36 cells
    • 64 cells

13. Cycle wheel (model, not to scale), clockwise from the top

    G1, S, G2, mitosis (M), cytokinesis. Three gates (checkpoints) stand on the wheel: the G1 gate near the end of G1, the G2 gate before mitosis, and the M gate in mitosis, before the sister chromatids are pulled apart. A gate drawn open lets a cell pass, a gate drawn closed holds the cell, and a gate drawn broken no longer checks. The G1 gate is drawn broken; the G2 gate and the M gate are drawn open. The parent cell has a mutated p53 gene.

14. 9.

    Ren reads the wheel. The cell's p53 gene is mutated, so its G1 gate is broken. The cell divides once. What do the two daughter cells carry?

    • No mutation, since each daughter cell starts fresh
    • The mutated p53 gene, but their G1 gates work as usual
    • The mutated p53 gene, so their G1 gates are faulty too
    • A working p53 gene, since division repairs the gene

15. Cycle wheel (model, not to scale), clockwise from the top

    G1, S, G2, mitosis (M), cytokinesis. Three gates (checkpoints) stand on the wheel: the G1 gate near the end of G1, the G2 gate before mitosis, and the M gate in mitosis, before the sister chromatids are pulled apart. A gate drawn open lets a cell pass, a gate drawn closed holds the cell, and a gate drawn broken no longer checks. The G2 gate is drawn closed; the G1 gate and the M gate are drawn open. A cell in G2 has one chromosome not fully copied.

16. 10.

    Zee reads the wheel. The G2 gate is closed, and the cell is in G2. What happens next?

    • The cell stays in G2 until the gate opens, then enters mitosis
    • The cell enters mitosis at once and finishes the copying there
    • The cell returns to G1 and starts the cycle again
    • The cell moves on to cytokinesis and splits first