Download THE CELL CYCLE AND MITOSIS

THE CELL CYCLE AND MITOSIS
Mitosis is the name for the kind of cell division that produces a greater number of cells = cell
multiplication; after division, the daughter cells are about half the size of their parent, and they grow
before division occurs again. A cell divides into two daughter cells tht are genetically identical to the
original cell and to each other. Cells multiply to make an organism bigger, to repair damage, or to
multiply the number of organisms of that kind.
The cell cycle refers to the continuing series of divisions alternating with cell growth: interphase—
mitosis—interphase—mitosis—interphase. An acronym for the cell cycle is
...IPMATIPMATIPMATI... Most of the time a cell is in interphase, the growth and preparation
stage of the cycle.
Mitosis, the actual process of dividing has four defined phases: Prophase, Metaphase, Anaphase,
and Telophase; then the daughter cells enter interphase. Mitosis is a continuous process, and the
phases blend into one another; it can often be hard to tell if an image is in the late part of one phase
or the early part of another. We see them shown in books as snapshots of a particular instant in the
process; you have to judge what was happening “when the music stopped.”
Note: Plant cells are often shaped like boxes because they are surrounded by a cell wall; at the end of
mitosis, the cell plate divides the two daughter cells. Animal cells take a variety of different shapes;
at the end of mitosis, a neck forms to separate the two daughter cells.
Slides—PHASES OF THE CELL CYCLE:
PLANT CELL DIVISION—root meristem of Allium cepa, the garden onion
Allium root tip:
Examine the square cells just inside the root
cap. This is the root meristem (embryonic
tissue) where mitosis is occurring. Farther up
the root is the elongation zone, where cells
are long rectangles; these cells are not
undergoing mitosis.
INTERPHASE
The nucleus of the cell is clearly stained and
appears to have tiny dots and one or more
dark nucleoli inside. What you can't see are
the phases of interphase:
G1: Period of cell growth before (G = gap);
ribosomes and organelles are being
duplicated.
S: DNA replication (S = synthesis; new
copies of DNA are synthesized.
G2: DNA has been replicated; cell is
preparing for mitosis.
MITOSIS (4 stages make up mitosis: Prophase, Metaphase, Anaphase and Telophase)
Prophase:
Chromosomes become visible and nucleoli
disappear (DNA + associated proteins
become tightly organized).
Chromosomes consist of two sister
chromatids (DNA replicas + associated
proteins) attached together at a specialized
region called the centromere.
Nuclear membrane breaks down and the
chromosomes spread out.
Spindle fibers (microtubules) appear. They
radiate out to the plasma membrane at the
poles in animal cells. These radiations, called
asters, are absent in plant cells.
Metaphase:
The chromosomes assemble on the
equatorial plate (an imaginary disc that
crosses the center of the 3-dimensional cell).
Some part of each chromosome is on this
plate. Metaphase ends when the centromeres
of each pair of chromatids split apart.
Anaphase:
Sister chromatids separate from each other
and move to opposite poles of the cell. The
kinetochore, a part of the centromere of each
new chromosome, moves along a spindle
fiber. The free ends of each chromosome
trail back toward the equatorial plate and
indicate movement.
Telophase & Cytokinesis:
This is like the reverse of prophase—the cell
is returning to interphase. Chromosomes
(now single molecules of DNA with
associated proteins) have reached opposite
poles of the cell. Spindle fibers disappear.
Nuclear membrane forms around the
chromosome clusters. Chromosomes
disappear from view as DNA re-extends, and
nucleoli appear. The cell divides into two
daughter cells (cytokinesis). In plant cells
vesicles deposit new cell wall material along
the equator to form the cell plate.
INTERPHASE: Daughter cells grow in size and prepare for renewed mitosis.
ANIMAL CELL DIVISION—Embryonic blastula stage of whitefish
Whitefish blastula:
The developing embryo of any organism is a
good tissue to examine for mitosis, since cells
must divide at a high rate to transform a
fertilized egg (single cell) into the trillions of
cells of a viable organism.
INTERPHASE
Chromosomes are extended and not visible;
regions where rRNA is being transcribed
stain dark and are termed nucleoli.
MITOSIS
Prophase:
Replicated chromosomes, consisting of two
sister chromatids held together at the
centromere, have condensed and can be seen
as stained bodies (chromo-somes).
Metaphase:
Some part of every chromosome lies inside
the equatorial zone.
Anaphase:
Sister chromatids have separated from each
other, and they, as new chromosomes, are
moving to opposite poles of the cell.
Telophase & Cytokinesis:
Chromosomes have arrived at opposite poles
of the cell, and cells are pinching off from
each other (cytokinesis); compare this with
cell plate formation in plant, above.
INTERPHASE
Daughter cells are returning to interphase.
FIRMING UP WHAT YOU LEARNED ABOUT THE STEPS IN MITOSIS
Modeling mitosis with pipe cleaners and beads: The pipe cleaners represent chromosomes and the
beads represent centromeres.
Use these models to show in detail the behavior of chromosomes through one round of the cell cycle,
i.e., interphase (G1, S, G2) through cell division (steps of mitosis [prophase, metaphasee, anaphase
and telophase] and cytokinesis) back to interphase. You will review the stages for both haploid and
diploid cells.
BRING AT LEAST FOUR DIFFERENT COLORED PENCILS TO CLASS.
Haploid Cells Undergoing Mitosis
Materials. Obtain from the front desk:
•
•
4 pipe cleaners: 2 long pipe cleaners of the same color and 2 short pipe cleaners of the same
color.
4 beads of the same color.
(Note: Initially, you will not use all the materials you obtained. However, you will need them to
complete the stages that follow).
Procedure:
As you work with the pipe cleaners, use your colored pencils to diagram each stage in the circles of
the diagram.
Imagine that a haploid cell with 2 chromosomes (1 long and 1 short) has just undergone mitosis and
is in Interphase G1.
1. Set up the pipe cleaners and beads to represent Interphase G1.
2. Squeeze an identical pipe cleaner through each centromere to represent the result of
replication in Interphase S.
3. Continue through G2
Modify the configuration of pipe cleaners to illustrate:
4.
5.
6.
7.
8.
prophase
metaphase
anaphase (add centromeres as required)
telophase
cytokinesis
Additional terms you should have applied/learned: replication, double stranded chromosome, sister
chromatid, single stranded chromosome, centromere, spindle apparatus, equatorial plane, cell plate
(for plants only) and cleavage furrow (for animals only).
Diploid Cells Undergoing Mitosis
Materials. Obtain from the front desk:
•
•
•
4 long pipe cleaners: 2 long pipe cleaners of one color and 2 long pipe cleaners of a different
color.
4 short pipe cleaners: 2 short pipe cleaners of the one color 2 short pipe cleaners of a different
color.
8 beads of the same color.
(Note: Initially, you will not use all the materials you obtained. However, you will need them to
complete the subsequent stages.)
Procedure:
As you work with the pipe cleaners, use your colored pencils to diagram each stage in the circles of
the diagram.
Imagine that a cell with 4 chromosomes (2 long each a different color, and 2 short each a different
color) is in Interphase G1.
1. Set up the pipe cleaners and beads to represent Interphase G1.
2. Squeeze an identical pipe cleaner through each centromere to represent the result of
replication in Interphase S.
3. Continue through G2
Modify the configuration of pipe cleaners to illustrate:
4.
5.
6.
7.
8.
prophase
metaphase
anaphase (add centromeres as required)
telophase
cytokinesis
Additional Terms: homologous chromosomes
Explain to your neighbor and to your instructor what you have done. Relate your model to the
images of mitosis in Allium root tip meristem and whitefish blastula.
Some examples of pipe cleaners and beads as chromosomes:
one chromosome,
single stranded in
one chromosome,
double stranded after
a pair of homologous
chromosomes,
Interpahse G1
Interphase S
Return to index.
Last updated 3 October 2010 (JHW & VS), images © John H. Wahlert, 9/18/10
double stranded