Download Reading on Mitosis

Eukaryotic Cells and Their DNA
Prokaryotic cells have a single chromosome, but eukaryotic
cells have many chromosomes. Eukaryotes have more DNA
than prokaryotes do. Different kinds of eukaryotes have different numbers of chromosomes. More-complex eukaryotes do
not always have more chromosomes than simpler eukaryotes
do. For example, fruit flies have 8 chromosomes, potatoes have
48, and humans have 46. Figure 2 shows the 46 chromosomes
of a human body cell lined up in pairs. These pairs are made
up of similar chromosomes known as homologous chromosomes
(hoh MAHL uh guhs KROH muh SOHMZ). Although chromosomes vary in size, the homologous chromosomes in each pair
will be very similar.
How many chromosomes are in the cells of the
human body?
7.2.e
Making More Eukaryotic Cells
The eukaryotic cell cycle has three stages. In the first stage,
called interphase, the cell grows and copies its organelles and
chromosomes. After each chromosome is duplicated, the two
copies are called chromatids. Chromatids are held together at a
region called the centromere. The joined chromatids condense
into an X shape, as shown in Figure 3. After this step, the cell
enters the second stage of the cell cycle.
In the second stage, the chromatids separate. The process by
which chromosomes separate is called mitosis. During mitosis,
each new cell receives a copy of each chromosome. Mitosis
is divided into four phases: prophase, metaphase, anaphase,
and telophase.
In the third stage, cytokinesis, the cell splits into two cells
called daughter cells. Cytokinesis is the division of cytoplasm and
all of the materials in the cytoplasm. The new daughter cells
are exactly the same as each other and as the original cell. For
example, each daughter cell receives exactly the same number
of chromosomes.
Figure 3 This duplicated
chromosome consists of two
chromatids. The chromatids
are joined at the centromere.
Chromatids
Centromere
Figure 2 Human body cells
have 46 chromosomes, or 23
pairs of chromosomes.
Mitosis Adventure
How does a cell change during a cell cycle? Describe cell
division from inside the cell.
Go to go.hrw.com, and type
in the keyword HY7ACTW.
7.1.b Students know the characteristics that
distinguish plant cells from animal cells, including
chloroplasts and cell walls.
7.1.e Students know cells divide to increase their
numbers through a process of mitosis, which
results in two daughter cells with identical sets of
chromosomes.
7.2.e Students know DNA (deoxyribonucleic acid)
is the genetic material of living organisms and is
located in the chromosomes of each cell.
Section 2
The Cell Cycle
153
Figure 4
The Cell Cycle
Interphase Before mitosis
begins, chromosomes are
copied. Each chromosome
becomes two chromatids.
Mitosis Phase 1 (Prophase)
Mitosis begins. Chromosomes
condense from long strands
into rodlike structures.
Mitosis Phase 2 (Metaphase)
The nuclear membrane is
dissolved. Paired chromatids
align at the cell’s equator.
Mitosis and the Cell Cycle
Figure 4 shows the cell cycle and the phases of mitosis in
an animal cell. Mitosis has four phases, as shown and described
above. This diagram shows only four chromosomes to make
it easy to see what’s happening inside the cell.
Cytokinesis
Cell plate
Figure 5 When a plant cell
divides, a cell plate forms and
the cell splits into two cells.
154
Chapter 5
In animal cells and other eukaryotes that do not have cell
walls, division of the cytoplasm begins at the cell membrane.
The cell membrane begins to pinch inward to form a groove.
Eventually, the cell is pinched in half, and two daughter cells
form. Cytokinesis is shown at the last step of Figure 4.
Eukaryotic cells that have a cell wall—such as the cells of
plants, algae, and fungi—go through cytokinesis differently. In
this kind of cell, a cell plate forms in the middle of the cell.
The cell plate contains the materials for the new cell membranes and for the new cell walls that will separate the new
cells. After the cell splits in two, a new cell wall forms where
the cell plate was. The cell plate and a late stage of cytokinesis
in a plant cell are shown in Figure 5.
What is the difference between cytokinesis in an
animal cell and cytokinesis in a plant cell?
7.1.b
The Cell in Action
Mitosis Phase 3 (Anaphase)
The paired chromatids separate and move to opposite
sides of the cell.
Mitosis Phase 4 (Telophase)
A nuclear membrane forms
around each set of chromosomes, and the chromosomes
decondense. Mitosis is
complete.
Cytokinesis In a cell that
lacks a cell wall, the cell
pinches in two. In a cell that
has a cell wall, a cell plate
forms and separates the cells
into two new cells.
Quick Lab
The Mitosis Flipbook
The process of mitosis involves four phases,
but the transition from one phase to the next
involves many intermediate stages. In this
activity, you will illustrate and assemble a flipbook. The flipbook will show the phases and
intermediate stages of mitosis.
1. Punch two holes near the upper edge of 20
index cards.
2. Take four cards from your stack. List a different phase of mitosis on each card.
3. Review the images above. Draw a diagram
of each phase of mitosis on the correct card.
4. Use colored pencils to make each
chromosome a different color. On each card
where the cell part is present, label one
example of the chromosome, centromere,
and nuclear membrane.
5. On the blank cards, draw the intermediate
stages between each phase. Show the gradual transitions of mitosis by making small
changes on each card.
6. Put your cards in order. Place a brad in each
hole to assemble your flipbook.
7. In what phase of mitosis does the nuclear
membrane dissolve?
8. This flipbook shows the actions of chromosomes, centromeres, and the nuclear membrane during mitosis. Is it acceptable to
leave out what is happening to the rest of
the cell’s organelles? Explain your answer.
9. Why is it important for eukaryotic cells to
undergo mitosis?
7.1.e
7.7.d
30 min
Section 2
The Cell Cycle
155