When the cell cycle goes wrong: checkpoints and cancer

Grade 9 Science Worksheet - Building Confidence

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.

   Ari reads the wheel. Which gate checks that all the chromosomes have been copied before mitosis starts?

   • The G2 gate, before mitosis
   • The G1 gate, near the end of G1
   • No gate: the cell goes on whether or not the copying is finished
   • The M gate, in mitosis
3. 2.

   Bo reads the wheel. Before the cell moves on into S, what does the G1 gate check?

   • That the chromosomes have been copied
   • The cell's size and reserves, and whether its DNA is damaged
   • Nothing: the cell goes on into S whatever its condition
   • That each pair of sister chromatids is attached to the spindle
4. 3.

   Pulling sister chromatids apart cannot be undone. Esme reads the wheel. What does the M gate wait for?

   • The cell to reach twice its starting size
   • Each pair of sister chromatids to be attached to the spindle
   • The DNA to be copied for the first time
   • Nothing: the chromatids are pulled apart whether or not they are attached

5. Regulator card (model)

   kind of regulator	job
   positive regulator	moves the cell on past a gate
   negative regulator	halts the cycle at a gate until a problem is fixed

6. 4.

   Jun reads the regulator card. Cyclins and Cdks are positive regulators. What is their job?

   • They copy the DNA during S phase
   • They halt the cycle until a problem is fixed
   • They split the cell's cytoplasm in two
   • They move the cell on past a gate

7. Use this

   model, not to scale. A gate drawn open lets a cell pass, a gate drawn closed holds the cell, and a gate drawn broken no longer checks.

8. 5.

   Milo reads the wheel. The G1 gate is closed and the cell's DNA is damaged. What is the gate doing?

   • Holding the cell while the DNA is repaired
   • Broken, so the cell can no longer divide
   • Letting the cell into S, so the damage is copied
   • Moving the cell on to mitosis
9. 6.

   Noah reads the wheel. The cell is in G2, its chromosomes are all copied and undamaged, and the G2 gate is open. What happens next?

   • The cell goes back to G1 to start again
   • The cell splits in two before mitosis begins
   • The cell is held at the gate while it copies more DNA
   • The cell passes the gate and enters mitosis

10. Use this

    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. A cell at the G1 gate has damaged DNA and no working p53.

11. 7.

    Pia reads the wheel. The G1 gate is broken because p53 does not work, and the DNA is damaged. What does the cell do?

    • It skips S and goes straight to mitosis
    • It stops dividing for good, because a broken gate halts the cell
    • It stays at G1, because the gate still holds it
    • It goes on into S, because the gate no longer checks

12. Cells in a dish where every cell divides each round (model data)

    round	cells in the dish
    0	1
    1	2
    2	4
    3	8
    4	16
    5	32

    Every cell divides once in each round.

13. 8.

    Ren reads the table. How many cells are in the dish after round 4?

    • 8 cells
    • 32 cells
    • 5 cells
    • 16 cells
14. 9.

    Tala reads the table. In which round does the dish first hold 32 cells?

    • Round 4
    • Round 3
    • Round 6
    • Round 5
15. 10.

    Zee reads the table. What happens to the number of cells in each round?

    • It doubles
    • It goes up by 2 cells
    • It stays the same
    • It halves