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

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

   • It enters mitosis and finishes the copying there
   • It goes back to G1 and starts the whole cycle again
   • It passes, because G2 checks the cell's size and nothing else
   • It is held at the gate until the copying is finished or repaired
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 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
   • It lets the cell go on, and the chromatids are sorted out later

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 stuck accelerator
   • An oncogene, like a gate that halts the cell for good
   • A proto-oncogene, like an accelerator that works as usual
   • 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 holds cells for good, like a brake stuck on
   • It no longer holds cells with damaged DNA, like a brake that fails
   • It checks for copied chromosomes instead of DNA damage
   • It pushes cells into S faster, like an accelerator

9. Mutation list (model)

   change	what is different
   1	a Cdk is active without its cyclin partner
   2	an enzyme that digests sugar works less well
   3	p53 is made at higher levels
   4	cyclin levels rise and fall on schedule

10. 6.

    Luz reads the card. Which change is most likely to push a cell past a gate before the conditions are met?

    • Change 4, because cyclins that rise on schedule skip the checks
    • Change 1, because a Cdk acting alone pushes the cycle ahead
    • Change 3, because extra p53 pushes cells past the G1 gate
    • Change 2, because sugar-digesting enzymes decide when cells divide

11. 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.

12. 7.

    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
13. 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

14. 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.

15. 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?

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

16. 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 closed; the G2 gate and the M gate are drawn open. A cell waits at the G1 gate with low reserves.

17. 10.

    Tala reads the wheel. The cell is at the G1 gate and its reserves are low. What can the cell do?

    • Go on into S, since reserves are checked after S
    • Skip ahead to mitosis and divide
    • Stop dividing for good, since a closed gate means the cell is broken
    • Wait at the gate, or rest in G0 until conditions improve