Cell division is an important process in living
organisms. It is involved in the multiplication of cells
for growth and development of an organism.
Moreover, cell division plays a key role in repairing
injured body parts of animals. Hence, processes such
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as regeneration and wound healing are effected by
cell division.
Cell division can be viewed as a cycle, which
can be arbitrarily, categorized into two stages namely
interphase and mitosis. The interphase, wherein the
cell undergoes growth and prepares for cell division,
might comprise as much as 90% of the cycle. All the
necessary materials needed for cell divisions, which
include microtubule proteins among others, are
CELL synthesized at this stage. Furthermore, to ensure that
each daughter cell would have a complete set of
DIVISION - genetic material, actual replication of genetic material
MITOSIS of DNA occurs in this phase. Once the preparatory
stages are completed the cell undergoes the actual
cell division or mitosis. Although, mitosis comprises
10% of the cycle, cell biologists consider several
phases in mitosis based on the different events
occurring in the nucleus and cytoplasm of the cell.
In this exercise, the different stages of the cell
cycle will be identified using a whitefish embryo at the
blastula stage. Cells making up this embryo are
actively undergoing cell divisions. Thus, such
specimen can readily show the different stages of the
cell cycle.
At the end of the activity, the students should be able
to:
1. identify the phases of the cell cycle
2. identify the structures associated with each phase
of the cycle using the whitefish blastula as
specimen
3. differentiate the different stages of mitosis
Materials
prepared slide of whitefish blastula
light microscope
Procedure
1. Obtain a prepared slide of cross section of whitefish blastula.
2. Focus under LPO. Observe that the section of the blastula contain several cells
revealing different phases of cell divisions.
3. Switch to HPO. Locate and identify the different stages of the cell cycle using the
descriptions below. Mitosis is a continuous process so that it is likely to see
intermediate stages
3.1 Interphase: Look for a cell with a distinct nucleus, that is with an intact
nuclear membrane. The genetic materials inside the nuclear appear as thin,
thread-like structures appropriately called chromatin. Within the nucleus, is
1 to 2 nucleoli, these are dense, darkly stained bodies formed by several
chromosomal materials that code for certain RNA’s. Near the nucleus is the
centrosome which contains centrioles. The centrioles may not readily be
seen using the light microscope.
3.2 M- phase: phase during which the cell divides. It is divided into four stages:
3.2.1 Prophase: Locate a cell in which the nuclear membrane starts to
disintegrate or disappear. The chromatin fibers start to appear as coiled,
thickened, condensed structure forming chromosomes that are visible under
the light microscope. Centrioles are seen in the opposite poles. Locate ray-
like microtubule bodies called asters radiating around each centrioles and
mitotic spindles forming between centrioles. The nucleoli disappear at this
stage.
3.2.2 Metaphase: The cell at this stage can readily be identified from the
rest of the phases of mitosis in that the chromosomes are aligned at the
equatorial region of the nucleus. At this stage, the replicated chromosomes,
which are much shorter and more condensed, are readily recognized. The
chromosomes are made up of chromatids joined together at their central
region called centromere. A protein coat called kinetochore is found in each
of the sister chromatid. The spindle fibers are attached at the centromere of
each chromosome.
3.2.3 Anaphase: At this stage, look for a cell where the chromatids are
pulled apart toward the opposite poles. This stage is differentiated into early
and late stages depending on the distance traveled by the chromatids. Once
the chromatids separate, these are considered as chromosomes reaching
the opposite poles.
3.2.4 Telophase : Look for a cell where a cleavage furrow, a constriction of
the plasma membrane at the region of the equatorial plate starts to appear.
This is the start of early telophase. At this stage the nuclear membrane and
the nucleolus start to reappear. The chromosomes begin to uncoil and
assume a threadlike appearance. The asters and mitotic spindles disappear.
At late telophase, the cleavage furrow becomes more constricted leading to
the formation of 2 daughter cells.
Cytokinesis, which is the division of the cytoplasm usually, occurs along
with telophase. Completely separated daughter cells are formed soon after
mitosis.
4. Be sure that the different stages of the cell cycle are observed and identified. Do
not rely so much on the photomicrograph and diagrams provided, there is still no
real substitute to an actual observation of the specimen.
References
Burns, R.D. and K.A. Stiles. 1997. Laboratory Explorations in General Zoology.
MacMillan Publishing Co., Inc. pp. 15-19.
Campbell, N. 1966. Biology 4th Edition. Benjamin Cummings Publishing House. pp.
207-212.
Dolphin, W.D. 1997. Biology Laboratory Manual 4th Edition. WCB McGraw Hill. pp.
79-90.
Hickman, F.M. and C.P. Hickman. 1984. Laboratory Studies in Integrated Zoology.
Times Mirror/College Publishing. pp. 26-30.
II. Guide Questions:
1. Why is fish embryo used to demonstrate the different stages of the cell cycle?
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2. As you scan through the different cells of the whitefish blastula, which of the
stages of cell cycle is frequently observed? Why is this so?
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3. Differentiate the following terms: chromatin, chromosomes, chromatids
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