Plant Mitosis Microviewer
Background information:
Have you ever propagated a geranium plant by cutting? You take a small stem on which a few
leaves are growing and keep it well watered. When a few tiny roots begin to appear, you plant the
stem in soil. After a few weeks the cutting grows roots which are just like the roots of the original
plant. The new plant has the hereditary traits of the parent plant.
Many plants and shrubs are propagated this way. In each case the new plants are just like the
parent plants because in each cell there is a mechanism that operates to maintain the hereditary
pattern from one cell to its daughter cells.
The process by which this occurs is known as cell division, or mitosis.
The following eight slides were photographed from a single onion root tip showing the phases
of plant mitosis in sequence.
The terms “equatorial plate” and “poles”, as they are used in the study of mitosis, refer to
certain locations in the cell. The equatorial plate in these slides runs from the left side of the slide to
the right; the poles are locations above and below the equatorial plate.
The magnification given, for example, 1000X for Slide 1—Early Prophase—means that the
microscope was set at that power when the photograph was taken.
1. Early Prophase (1,000x)
The cell A is in a so-called resting stage of interphase. Actually, it is not resting but is carrying on
all the functions of life except cell division. In the nucleus of cell A you can see the very dark
nucleoli and smaller granules of chromatin.
It is difficult to say exactly when the first part of the process of cell division begins. Recent
research with the electron microscope indicates that even in this interphase or “resting stage” the
chromosomes are already beginning to make duplicates of themselves.
In cell B the chromosomes have become shorter and thicker, and for practical purposes, we say
that cell division, or mitosis, begins with this phase. At this stage of development they are called
prophase chromosomes.
2. Prophase (1,000x)
The large cell near the center of the slide shows that the chromosomes have continued to become
thicker and shorter. They can now be seen very clearly inside the nucleus. Soon after they reach
this stage, they begin to move toward the middle of the cell.
3. Metaphase (1,000x)
The cell at C has now reached the middle stage of mitosis called metaphase. Characteristically,
the chromosomes have developed into short, thick rods. They have moved to a position in the
middle of the cell called the equatorial plate.
Examine the individual chromosomes carefully. They have become thick and at least two of them
show doubling.
4. Early Anaphase (1,000x)
In cell D each chromosome has doubled and the two parts are separating. As the split rods move
away from each other, they shape themselves into what may be described as two V’s facing each
other.
Spindle fibers are faint but visible at S in the lower part of the cell. Their function is to pull the
newly formed chromosomes towards the pole. Electron microscopes show that spindle fibers are
made of extremely small structures called microtubules.
5. Anaphase (1,000x)
In cell E every chromosome has now completed its separation. At this stage the cell has achieved
the main function of mitosis which is the production of two duplicate sets of chromosomes.
Spindle fibers in the upper part of this cell may be seen quite clearly. The two new sets of
chromosomes are beginning to move further away from each other and toward the poles.
6. Late Anaphase (1,000x)
In cell F the movement of the two complete sets of chromosomes toward the poles of the cell is
much further advanced.
As soon as the two sets of chromosomes reach the region of the poles, they will begin to organize
themselves into two complete nuclei.
The number and kind of chromosomes in each of the two sets is exactly the same as in the original
cell at the beginning of mitosis.
There is as yet no sign of a cell wall developing between the two groups of chromosomes. The V
shape of the chromosomes can still be seen in this slide.
7. Telophase (1,000x)
The telophase is the final stage in the process of mitosis. The two sets of chromosomes in cell G
have drawn in tightly to form a dense mass at each pole. Individual chromosomes can no longer
be seen. Each mass will become the nucleus of a cell. Spindle fibers are clearly visible between the
two masses.
In the middle of the space between the masses, across the spindle fibers, a faint line can be seen.
On each side of this line, each new cell will secrete its own cellulose wall.
8. Late Telophase (1,000x)
The cells H show the very last stage of mitosis. The cell walls are not quite fully developed across
the equator of the cell. The left side of the wall is a little more advanced than the right side. Some
fibers still can be seen on the right side, though not too clearly.
The cells at A have finished mitosis. They are now in the interphase or resting stage. The new
nuclei can be seen with nucleoli and some chromatin throughout. The cell walls have developed
fully and divides the original cell into two complete cells. The two cells will now increase in size
and, after a period of growth and assimilation, each will begin mitosis.
Animal Mitosis
First Cleavage in Ascaris
Background information
This is the story of the first hour of an animal’s life-of a man or a dog, or a fish or a worm. Each
begins life as a single cell.
How does an animal begin to develop from a single cell?
In order to observe animal mitosis under the microscope, we have chosen the egg sac of the
ascaris (AS-ca-ris), a parasitic worm.
Ascaris is a good subject for this study because the chromosomes are large and the characteristic
number of chromosomes in this species is only four.
The female ascaris lays her eggs in small sacs. We can observe some of the eggs as they begin
their development. Although the adult worm is very different from a human, the first hours-of a
man and a worm-are quite similar.
These eight slides begin the story just after the egg has been fertilized. They show the activity
within the cell from the moment until it divides to form the two cell stage of the embryo.
The terms “equatorial plate” and “poles”, as they are used in the study of mitosis, refer to certain
locations in the cell. The equatorial plate in these slides runs from the top of the slide to the
bottom; the poles are locations to the left and right of the plate.
The magnification given, for example, 750x for Slide 1-The Zygote-means that the microscope
was set at the power when the photograph was taken.
1. The Zygote (750X)
This is the zygote - the fertilized egg- of the ascaris. Two masses of chromatin can be seen in the
cell. One mass was the nucleus of the egg. The other came from the sperm that fertilized the egg.
The sperm and the egg are very different in size and shape. But the chromatin contributed by
each is equal amount. Each parent, therefore, supplies an equal amount of hereditary material to
the off spring.
The chromosomes are not seen distinctly because they are very thin. This cell is thick and round
like a tiny ball, making it difficult to keep all the chromatin in focus.
2. Pro- Metaphase (750X)
The chromosomes have become much shorter and thicker, so that they are now easily seen.
Each parent supplied two chromosomes to the zygote.
Notice how the chromosomes begin to pair up. Study their shapes carefully. One pair looks like
two horseshoes. The other pair resembles bent rods. If you move ahead to slide 4, you can get a
better idea of the particular shapes they have assumed. In these slides, we shall refer to these
chromosomes as the “U-shaped” and the “bent”
The sperm supplied the U-shaped and one bent chromosomes. The other U-shaped and bent
chromosomes came from the egg cell. After fertilization, the two U-shaped ones pair up, as do the
two pairs begin to move towards the equatorial plate.
3. Metaphase (750X)
In this slide the chromosomes have moved to the equatorial plate is the imaginary line running
from E to E. at each pole is a centrioles (P). A starlike structure, called an aster, radiates from
each centrioles.
Fine spindle fibers go from the centrioles at P to each chromosomes. Some of these can be seen
in this slide.
At the same time, each of the four chromosomes is getting ready to divide lengthwise.
4. Metaphase- Polar View (750X)
This is the same stage in the development of the egg as shown in slide 3, but we are now looking
at the cell from the side, that is, from one pole, through the equatorial plate, to the other pole.
The four chromosomes are seen as they lie flat on the equatorial plate.
Look back at slide 3. You can see the chromosomes from the side.
5. Early Anaphase (750X)
Each chromosome has completed duplication. There are now four U’s and four bents.
In these eight chromosomes there is enough hereditary material for two cells.
At this stage you can also see that the duplicated chromosomes are beginning to form two groups.
The groups are starting to move toward the poles and, as they progress, the separations between
them will be clearly seen.
6. Anaphase (750x)
The eight chromosomes have now separated to form two groups containing four chromosomes
each. The original nucleus has now torn itself in two.
The centrioles and asters at the two poles seem to be drawing the chromosomes groups toward
them. Scientists do not as yet fully understand the forces of separated at work here.
Look closely at the chromosomes. Some of them appear to be beaded in places. Authorities on
this subject believe that the long, thing chromosomes become short and thick by twisting into a
tight coil. Sometimes the coil loosens up a bi, making the chromosomes look beaded.
7. Telephone (750x)
The two groups of chromosomes have drawn completely apart from each other. At the same time
the rest of the cell is beginning to divide. The cell membrane pinches inward and the cytoplasm is
dividing into two masses. If you go back to slide 6 and examine the cell membrane carefully, you
will actually see the beginning of these pinching processes.
As the cell approaches its division into two cells, the individual chromosomes is the become less
distinct.
8. Late Telephone
The separation is now complete. The original zygote of the ascarists has divided to form two
daughter cells. Each of these two cells has four chromosomes.
The chromosomes in each cell will soon lose their clarity. They will become long and thin and not
so distinct. Then the process of mitosis will start again in each of the two daughter cells, The result
will be four cells, and these in turn will start the process of cell division or mitosis again. You can
thus see how mitosis in the ascaris achieved its function. One cell with two pairs of chromosomes
formed two cells each a duplicate of the original.
In the human being the process of mitosis is the same. But, the number of chromosomes forty-six
as compared with the four in the ascaris. Until 1958, scientists believe that the characteristic
number of chromosomes in man was forty-eight. Improvements in instruments and techniques
have since made an accurate count possible.