LCPS Core Experience | Cell Cycle Investigation TWO
Cell Cycle
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LINK
1. Why must organisms produce new cells?
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2. How do cells make more cells?
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Background
During childhood the human body grows at a very fast rate. This fast rate is due to a constant
increase in the total number of cells making up the body. New cells within the body are produced
from existing cells in a process called cell division. This process requires that the original (parent)
cell copies all of its contents, including DNA and organelles, and splits these materials evenly into
two new (daughter) cells. In this way the cell has made an exact copy of itself.
DNA is found inside almost all cells in the body. DNA is contained in the nucleus of the cells and
makes up an organism’s chromosomes. The DNA of a chromosome contains many genes. Each
body cell has two copies of each chromosome in its nucleus. The copies are called homologous
chromosomes. In sexual reproduction, the cell receives one of these copies from the father and the
other from the mother.
EXPERIMENT
Lesson One: Modeling Mitosis in Body Cells
1. Build a model of a body cell with two chromosome pairs in the nucleus.
a. Make your first chromosome by attaching 5 blue pop beads together in a straight line.
b. Make the homologous chromosome by attaching 4 green pop beads and 1 blue pop
bead together in a straight line.
c. Make a second pair of homologous chromosomes. Attach 3 red pop beads together in
a straight line. Then, attach 2 red pop beads and 1 orange pop bead together in a
straight line. This is your second pair of homologous chromosomes.
d. Place a large rubber band on the lab table so that it is in the shape of a circle. This
represents the nuclear membrane.
e. Place a circular piece of string outside the rubber band. This outer string represents
the cell membrane.
f. Put both pairs of homologous chromosomes into the nucleus.
LCPS Core Experience | Cell Cycle Investigation TWO
2. Question: Draw, color and label your model in the space below. Label the following parts
of your drawing:
a. The nuclear membrane.
b. The 2 pairs of homologous chromosomes.
3. Question: Are both chromosomes in the homologous pair exactly the same? Explain.
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Background
Why aren’t homologous chromosomes exactly the same?
In body cells, each chromosome in a homologous chromosome pair came from a different
parent (one from the father and one from the mother). Homologous chromosomes contain the
same genes, but they are not identical. They may contain different alleles, or versions, of the
genes. For example, if the homologous chromosomes carry the gene for chin shape, one of
the homologous chromosomes may contain the allele for cleft chin and the other may contain
the allele for smooth chin.
Cells in the body can divide and produce new cells. They do this through a process called the
cell cycle. The cell cycle is divided into different parts. The first part of the cell cycle is
called interphase. During interphase, the cells grow larger and each chromosome is copied.
The two identical copies of each chromosome are called sister chromatids. They are
attached together by a structure called the centromere.
4. Interphase (INCREASE)-Duplicate each of the chromosomes in the model cell.
a. Make one new blue chromosome by connecting 5 blue pop beads together in a straight
line. Attach the new chromosome to the first blue chromosome with the tiny rubber
band centromere. The two blue copies represent sister chromatids. The rubber band
represents the centromere.
b. Repeat with the remaining three chromosomes.
c. Put all of the duplicated chromosomes in the nucleus.
d. The model cell is now ready for mitosis.
LCPS Core Experience | Cell Cycle Investigation TWO
5. Question: Look at the drawing below. Your model should look similar. Label the following
parts of the drawing:
a. Any other sister chromatids
b. One centromere
Homologous Chromosome Pair
c. The nuclear membrane
d. The cell membrane
Homologous Chromosome Pair
Sister Chromatids
Background
The second part of the cell cycle is called mitosis. Mitosis is divided into four steps called
phases. The four phases of mitosis are: prophase, metaphase, anaphase, and telophase.
6. In prophase (PAIR), the nuclear membrane dissolves. The chromosomes condense into rodlike structures that can be seen with a microscope. Model prophase by removing the rubber
band.
7. Question: Why did you remove the rubber band to model prophase?
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8. In metaphase (MIDDLE), spindle fibers have formed in the cell. Model spindle fibers by
laying a piece of string under each centromere.
9. In metaphase, the chromosomes line up in the center of the cell.
Model metaphase by lining up the chromatids as shown in the
picture at the right. They should still be inside the cell membrane.
10. In anaphase (APART), the chromatids separate as the spindle
fibers shorten and move to opposite ends of the cell. Model
anaphase:
a. Gently take the centromeres (rubber bands) off to start
separating the sister chromatids.
b. Starting with the chromosome at the top, separate each of
the paired sister chromatids.
c. Repeat the last step with the 3 remaining chromosomes.
LCPS Core Experience | Cell Cycle Investigation TWO
11. In telophase (TWINS/TWO), the chromatids have moved to the ends of the cell and the
spindle fibers disappear. Nuclear membranes form around each set of chromosomes. The
cell is now prepared to divide. Model telophase:
a. Move all of the chromatids to the ends of the cell (the chromatids on the left move
left, and the chromatids on the right move right).
b. Remove the strings under the centromeres.
c. Put a rubber band around the chromosomes on the left. Put a second rubber band
around the chromosomes on the right. The rubber bands represent the new nuclear
membranes.
d. Represent the remaining step in cell division, cytokinesis (CUT), by removing the
piece of string that surrounds both nuclei. Place a piece of string around each new
nucleus Cell division (cytokinesis) is complete.
12. Question: Draw, color and label your model in the space below to the right. Label the
following parts on your drawing:
a. Each homologous chromosome pair
b. Two nuclear membranes
c. Two cell membranes
13. Question: In the model of mitosis, how many cells were present at the start?
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14. Question: In the model of mitosis, how many cells were present at the end?
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15. Question: How did the chromosomes in the first cell compare with the chromosomes in the
cells at the end of mitosis? (Hint: Compare your drawings for question #2 and #12).
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16. Question: When body cells divide, how do the new cells get the same amount and type of
DNA as the original cells?
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EVALUATION
1. What is the purpose of mitosis? Explain.
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