The cell cycle and why cells divide

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. A human body cell has 46 chromosomes in G1. The drawn cell sits in the G2 segment: its nucleus shows each chromosome as an X of two joined sister chromatids.

2. 1.

   Ari studies a human body cell drawn in G2. How many chromosomes and how many chromatids are in its nucleus?

   • 92 chromosomes, 46 chromatids
   • 46 chromosomes, 46 chromatids
   • 92 chromosomes, 92 chromatids
   • 46 chromosomes, 92 chromatids

3. Use this

   Count table (model data), 4 rows. Columns: where in the cycle, chromosomes, chromatids. A cell in G1: 8, 8. The same cell after S: 8, 16. The same cell just before it divides: 8, 16. One daughter cell just after cytokinesis: ____, ____.

4. 2.

   Esme completes the count table for a model organism. Which numbers go in the last row?

   • 8 chromosomes, 8 chromatids
   • 16 chromosomes, 16 chromatids
   • 4 chromosomes, 4 chromatids
   • 8 chromosomes, 16 chromatids

5. Use this

   Cycle wheel (real data from OpenStax Biology 2e: human cells dividing rapidly in a lab dish), 24 hours around.

7. 3.

   Jun reads the wheel. Which phase takes the largest share of the 24 hours, and about what percent?

   • S, about 42 percent
   • M, since dividing is the main event of the cycle
   • S, about 10 percent
   • G1, about 38 percent
8. 4.

   Kira adds up interphase on the same wheel. How long is interphase, and what percent of the cycle is it?

   Use the picture in question 3.

   • 19 hours, about 79 percent
   • 0 hours: interphase is a rest, so it is not counted
   • 0.5 hours: mitosis is the cell cycle, so interphase is not counted
   • 23.5 hours, about 98 percent
9. 5.

   Milo reads the table. Which statement does it support?

   • The 10 µm cell has the higher ratio, because its surface area of 600 is larger
   • The 10 µm cell exchanges materials better, because 600 is more surface area than 150
   • Both cells share one ratio, because both are cubes with the same shape
   • The 10 µm cell has 4 times the area and 8 times the volume, so a lower ratio
10. 6.

    Noah's cube cell grows from a side of 4 µm to 8 µm. Which sentence best explains why growing cells divide?

    • A bigger cell has more membrane, so it exchanges better and has no need to divide
    • A bigger cell has too much surface area for its organelles to use
    • Volume rises faster than surface area, so a big cell has less membrane for its volume
    • Dividing makes surface area fall to match volume, so the ratio stays the same
11. 7.

    In Pia's model, a cube cell divides once its ratio falls to 0.5. Using the table, at which side does it first divide?

    • 12 µm
    • 3 µm
    • 15 µm
    • 6 µm
13. 8.

    At the G2 gate, Ren learns that some chromosomes were not fully copied. What does the cell do?

    • It enters mitosis, where the copying is finished
    • It passes, because the M gate repairs missing copies
    • It skips to cytokinesis and splits first
    • It is held at the gate while it finishes copying or repairs the DNA
14. 9.

    At the gate in mitosis, Tala sees one pair of sister chromatids attached to spindle fibers from only one pole. What happens?

    Use the picture in question 8.

    • The cell goes back to S to copy the DNA again
    • The chromatids separate anyway; cells keep dividing once started
    • The cell divides its cytoplasm first, then sorts the chromosomes
    • The cell waits, since pulling chromatids apart cannot be undone
15. 10.

    Zee follows a cell to the G1 gate, but nutrients are low. What can the cell do?

    • Skip S and G2 and go straight to mitosis
    • Go on into S, since cells keep dividing whatever the conditions
    • Wait at the gate, or step into G0 until conditions improve
    • Leave the cycle for good, since G0 cannot be left