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CD
CO
B
The Cell
O The Cell
Cycle
O Interphase
A
F
B
O Cytokinesis
G
O Mitosis
O H Phase
(Cell
Division)
Cycle
O G? Phase
.. . .C
Phases
of
H
Mitosis
O Gj Phase
D
O Prophase
H,
O S Phase
E
O Metaphase
H?
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O Anaphase
H3
O Telophase
H4
O Mother Cell
a
O Daughter Cells
b
CHAPTER
TWO
Chanter 2-16:
MitOSl'S
During the cell division phase of the cell cycle, the cell under
goes mitosis and then cytokinesis. Mitosis is the process in which
the duplicated chromosomal pairs separate and, in cytokinesis,
the cell splits to form two new cells. This plate will explore the
As mitosis continues, the chromatids separate into
chromosomes. Continue your reading and color the
structures as you encounter them in the plate.
process of mitosis.
The next phase of mitosis is metaphase. Here the chromatid
pairs align themselves along the equator of the cell, at an area
called the metaphase, or equatorial plate. The chromatids (G,)
are linked near the middle of the chromosome at a development
called the kinetochore (I). There is one kinetochore located on
each sister chromatid, and their compositions are unknown. At
this stage, the spindle fibers (H) are distinct, and they extend out
from the centrioles. The remainder of the cytoplasm (A) should be
This plate shows six diagrams that portray the phas
es of a cell undergoing mitosis. This process is a
continuous one, and is generally described in terms
of four phases; interphase, prophase, metaphase,
and telophase. Use the same colors throughout the
six diagrams. These diagrams tend to be fairly
detailed, so light colors are recommended.
colored in a light color.
In anaphase, the DNA at the kinetochore (I) has duplicated,
As you may remember, the DNA in the nucleus of the cell repli
cated during the S phase of the cell cycle, but is not distinguished
and the chromatids have separated. Each chromatid is now a
phase. The nucleus (B) contains the DNA in a diffuse mass called
chromatin. The nucleolus (C) is seen clearly in the interphase cell,
and the nuclear membrane |D) encloses the nucleus. Color the
them by their centrioles. An equal number of chromosomes move
as distinct chromosomes during the first phase of mitosis, inter
cytoplasm (A) a light color.
Two submicroscopic bodies (also duplicated prior to mitosis)
that participate in mitosis are the centrosomes. Each of the centrosomes contains two cylindrical structures that are arranged at
right angles to each other, called centrioles (E), which are
involved in the organization of microtubules during cell division.
We now begin the process of mitosis; our cell is in
early prophase. The same colors that were used for
the interphase cell should be used here.
Prophase is the longest phase of mitosis. It begins when the
chromatin of the cell nucleus condenses to form distinct chromo
somes. Because DNA replication has taken place during
interphase, each chromosome is composed of two identical
strands, known as chromatids (G,). Notice that, in early
prophase, the centrioles (E) are surrounded by a series of micro
tubules that radiate outward; these are called asters |F).
In late prophase, the centrioles (E) have moved to opposite
poles of the cell and the asters (F) are still visible. Spindle fibers
(H) can be seen extending between the centrioles and should be
traced with a light color such as yellow. Spindle fibers are com
posed of microtubules and associated proteins. Notice that the
chromatids (G,) have continued to compact, becoming shorter
and thicker. The nuclear membrane begins to break apart and
disappear as the cell proceeds through late prophase.
chromosome (G2). Four chromosomes are seen moving to the
bottom of the diagram, and four to the top of the diagram. The
chromosomes resemble "V's" because the spindle fibers lead
to the opposite poles of the cell. In a human cell, for example,
forty-six chromosomes move to one pole and forty-six chromo
somes move to the opposite pole.
We will now examine a cell in telophase; the phase
that signals the end of mitosis and immediately pre
cedes cytokinesis.
As the dividing cell enters telophase, you can see that the chro
mosomes (G2) arrive at opposite ends of the cell, where they
become thinner and less distinct. The spindle fibers (H) begin to
break down in this phase, the nuclear membrane |D) begins to
form around the chromosomal material, and the nucleolus (C)
reappears.
As telophase comes to an end, the cytoplasm (A) is divided
between the two new daughter cells. At the center of the cell in
animal cells a cleavage furrow (J) begins to form as the mem
brane pinches in from both sides. The appearance of the
cleavage furrow signals the end of telophase and the beginning
of cytokinesis. The furrow pushes inward from opposite sides of
the cell until two cells are created. These cells are referred to as
the daughter cells.
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H
Interphase
Prophase (Early)
Prophase (Late)
D
Metaphase
Anaphase
Telophase
Mitosis
O Cytoplasm
A
O Centrioles
O Nucleus (Chromatin)
B
O Asters
O Nucleolus
C
O Chromatids
E
F
G,
O Nuclear Membrane
D
O Chromosomes
G,
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O Spindle Fibers
O Kinetochore
O Cleavage Furrow
H
I
J
CH.A PTER
TUO
Chapter 2-17:
The process of mitosis, discussed in the previous plate, occurs
in cells that are reproducing during growth and wound healing,
and the replacement of dead cells. The two cells that arise from
mitosis are genetically identical to their parent cell. Certain cells
undergo another form of cell division known as meiosis. In this
process, a single parent cell produces four cells, each of which
has half the number of parental chromosomes. The parent has
two sets of chromosomes and is said to be diploid (2N), while the
cells that result from meiosis each have a single set of chromo
somes and are said to be haploid (N).
Meiosis takes place in the reproductive organs and results in
cells that are used during reproduction. These cells, which are
sperm and egg cells, are called gametes. At fertilization, the
fusion of two haploid gametes forms a single cell, called the
zygote, which is diploid.
Mei'OSl'S
At the end of meiosis I, the chromosome pair has sep
arated and a chromosome that consists of two sister
chromatids has moved to each daughter cell. The sister
chromatids are held together at the centromere. Each of
the two daughter cells will now enter meiosis II.
The two daughter cells now enter meiosis II, shown at the top
of the second column. View 2a shows prophase. Again, we see
the centrioles (F) and the cytoplasm (E), which should be colored
in a pale color. Sister chromatids 1 (H,) are in the left cell, and
sister chromatids 2 (H2) are in the right cell. In view 2b, the sister
chromatids line up along the equator of each cell. Then the kinetochores separate the sister chromatids.
Now, in view 2c, anaphase is in process, and the sister chro
matids are considered chromosomes. In the left cell, chromosome
In this plate, we trace the two main phases of meio
sis. Many of these processes are similar to those of
mitosis, and you should refer to the previous plate
whenever necessary. We will follow a single pair of
chromosomes through the process of meiosis and will
note how they are distributed to four cells.
The process of meiosis involves two rounds of cell division,
known as meiosis I (A) and meiosis II (B). The bars that indicate
these two rounds should be colored. The first round results in
daughter cells that have reduced numbers of chromatids. In the
second round, these chromatids are distributed to the gametes.
Each round of meiosis contains a prophase, metaphase,
anaphase, and telophase, as is the case in mitosis.
We will begin with meiosis I. Here we see a parent cell with a
distinctive nucleus (C) and nucleolus (D). The cytoplasm (E)
should be colored in a pale color. The centriole (F) functions in
meiosis as it does in mitosis. The phase designated 1 a represents
prophase.
Prophase continues in view 1 b. Here a single pair of chromo
somes is considered (remember that humans have twenty-three
pairs of chromosomes per cell). We see homologous chromosome
1 (G,) and homologous chromosome 2 (G2). The DNA in each
chromosome has replicated. Here, the chromosomes have come
together, and crossing over (chapter 3) may take place.
View 1 c represents metaphase. The homologous chromosomes
line up along the equator of the cell, and we see that each consists
1 (I,) moves to one side of the cell, while chromosome 2 (l2)
moves to the other. Chromosome 3 (l3) and chromosome 4 (l4)
separate in the second cell. As telophase commences, in view 2d,
the chromosomes are situated at the poles, and the nuclei are tak
ing shape once again. Cell division (cytokinesis) begins.
In the final view, 2e, we see the four cells that result from
cytokinesis. Each cell is haploid, meaning that it contains a single
chromosome from the original chromosome pair. Recall that we
began with two chromosomes. Now in the final view, each cell
has one chromosome from that original pair. In the human male,
these cells will undergo further development to become sperm
cells, and in the human female, one of these cells will become an
egg cell.
Meiosis is linked to sexual reproduction in plants and animals
because haploid cells join to form a fertilized diploid cell. In ani
mals, the haploid stage is very brief, but in simple plants, the
haploid stage predominates over the diploid stage, as you will
see in the plates on plant biology.
of sister chromatids. Chromosome 1 has sister chromatids 1 (H,),
and chromosome 2 consists of sister chromatids 2 (H2).
Anaphase is shown in view 1 d. Sister chromatids 1 move to
the left pole of the cell while sister chromatids 2 move to the right.
In telophase, sister chromatids 1 are contained in the left daugh
ter cell, and sister chromatids 2 are in the right daughter cell. This
marks the end of meiosis I.
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Meiosis
O Mei osi s
I
A
O Meiosis
II
B
O Nucleus
C
O
Nucleolus
D
O
Cytoplasm
E
O Centrioles
O Homologous
F
1
G,
O Homologous Chromosome 2
Chromosome
G,
O Sister Chromatids
1
H,
O Sister Chromatids 2
H,
CD
O
Go
00
B
to
to
8
8
O Chromosome
1
O Chromosome 3
I3
O Chromosome
2
O Chromosome 4
I4
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