Download Comparison of Mitosis in Plants and Animals - District 196 e

Comparison of Mitosis in Plants and Animals
Name:___________Period:_____
Reference: Prentice Hall Biology pp 244-249 660-661, 1016-1019, (Dragon Fly book) and pp 158-171 (Elephant book)
Introduction: The Cell Theory states that all cells come from preexisting cells. What happens to a cell during its
existence is described as the Cell Cycle. Cells spend a great deal of their cycle in a stage that is collectively known as
Interphase. Although many cytologists used to describe interphase as the “resting” stage, today much more has been
discovered about cells. We now know a cell is extremely busy doing what it was intended to do (ie…performing its
functions) during interphase. We will discuss this in more detail later in the unit and as we proceed through the course.
Cells go through a period of interphase that can last for hours, days, months, or years. Cells that are coming to the end
of their life cycle often follow a process that allows them to pass on their genetic information to successive generations
of cells. We call this reproduction. [Otherwise, if the cell is damaged or not functioning properly or must stop
operating as a normal part of development, we say the cell is experiencing Apoptosis, which is programmed cell
death.]
Cells reproduce in two basic ways: Sexually and Asexually. In this unit, we are observing two processes of cell
division that together would be considered a type of asexual reproduction and are known as Mitosis and Cytokinesis.
The end result of mitosis and cytokinesis should be two identical daughter cells. In order to achieve this, the cells must
ready themselves for cell division. This occurs during interphase. For instance, the cells replicate their set of
chromosomes during the s-phase of interphase so that when mitosis is complete, one complete set will be in each cell.
This is important because the chromosomes contain the DNA which contains the genes which give the cells their
instructions.
During mitosis, the cell is attempting to evenly divide its number of chromosomes and the rest of the nuclear material.
Although this occurs in one continuous process, scientists have broken the events down into 4 separate phases the
nuclear material sequentially runs through so that each new cell gets a full set of chromosomes.
In order for equal division of a full set of chromosomes, the recently replicated chromosomes must first condense so
they can be easily moved. This means the chromatin in the nucleus is coiling up into easier to handle packages (much
like you would do if you wanted to store or move around extension cords when they are not in use). When these coiled
packages of chromatin become visible, they are said to be condensed (much like moisture in the atmosphere
condenses into a raindrop) and are now called chromosomes or “colored bodies”. The chromosomes are actually an
“X” shape now. The “X” is made up of two identical copies of the chromosome, created during the S-phase of
interphase, that are attached together at a structure called a centromere. Each ½ of the “X” is actually called a sister
chromatid while they are attached together. Once the sister chromatids are separated from each other, they are then
each described as being chromosomes. Actually each chromosome has a centromere and they are connected to each
other by a larger structure called a kinetochore.
Next, the nuclear membrane (or envelope) must dissolve so the chromosomes can move to opposite sides of the one
parent cell as it divides into two new daughter cells. The chromosomes then attach themselves to the building system
of spindle fibers (at the centromere) so that they can be moved to the middle of the cell. These spindle fibers are
produced from the central region of where the nucleus was located and which is subsequently called the centrosome.
The spindle fibers are made of contracting filaments called microtubules. In animal cells, structures called centrioles
(also made of microtubules) seem to have a function that is associated with the spindle fibers but it is not yet well
understood. Animal cells also produce an Aster at each pole of the cell near where the centrioles were located. The
aster is described as “star-shaped” due to the set of spindle fibers radiating from each one. Plant cells also produce
spindle fibers from the centrosome region yet they lack centrioles and asters. It has been suggested that these structures
are NOT needed in a plant cell because the plant cells are more rigid and are in NO danger of mishapening the spindle
fibers so the chromosomes will not equally split. All of this happens in a phase (or stage) of mitosis called Prophase.
Next, the spindle fibers help move the chromosomes to the middle of the cell and when the chromosomes reach the
end of this phase, they are said to be lined up on the equatorial plate of the cell. This moving of the chromosomes to
the middle of the cell in a single file line is known as Metaphase.
Once the chromosomes reach the middle of the cell, the kinetochore breaks apart allowing the sister chromatids to
separate and move to the opposite sides (or poles) of a cell. The spindle fibers are apparently digested by enzymes,
pulling the chromosomes away from the middle of the cell. This process that occurs “after” metaphase is called
Anaphase.
The final stage of mitosis involves reestablishing a nucleus around the 2 “new” sets of chromosomes at the opposite
sides of the cell. Once the nuclear membranes are reconstructed, each set of chromosomes uncoil to become chromatin
once again and structures like the nucleolus once again become visible. This final stage of mitosis is called Telophase.
So, mitosis in plant and animal cells involves 4 stages or phases that sequentially are given the acronym of PMAT.
During cytokinesis, the cell is attempting to split the entire cell into two equal halves. At the end of these two
processes, if performed as intended, the two new daughter cells produced should have an equal complement of
genetic material and a nearly equal volume of cytoplasm and organelles. Cytokinesis actually starts sometime between
the end of anaphase and the beginning of telophase.
Cytokinesis also has differences between plant and animal cells due to the fact that plants have a cell wall and animals
don’t. In plant cells, a new cell wall is built between the emerging two new daughter cells. As it is being built, it is
called a cell plate. Since they lack a cell wall, animal cells split by having their cytoplasm pinched off in the middle.
This narrowing is called a cleavage furrow and it eventually pinches off completely and as the new cell membranes
are built around the exposed cytoplasm, cytokinesis has produced two new daughter cells.
These two processes (mitosis and cytokinesis) of cell division that occur in most eukaryotic cells take much longer
(several hours compared to about every 20 minutes) than the type of cell division that occurs in prokaryotic cells,
which is known as binary fission. While each major group of organisms has slight differences in how these processes
proceed, they are very similar in plants and animals. We can easily learn the similarities plant and animal cells have in
common with respect to sequences of events that occur during nuclear and cell division and then learn the subtle
nuances that make each process unique.
It might also be of interest to find out that certain cells do not undergo cell division. These cells are said to be in the G0
phase which is outside the normal cell cycle of most eukaryotic cells. Cells like erythrocytes (Red Blood Cells) are
produced in the bone marrow from stem cells. When RBC’s mature, they lack a nucleus and so can’t divide. They last
for about 90-120 days before being filtered out of the blood stream by the spleen. Other cells, like neurons in the brain
are thought not to divide. However, in recent years, scientists have discovered stem cells in the brain and this is
causing them to rethink what we know about how the brain develops and what the life cycle of brain cells truly is.
Stem Cells are cells that remain in a constant state of division and youth. Stem cells don’t seem to grow old and this
might be because the chromosomes never lose any pieces of their ends called telomeres when they divide like other
cells do. Most cells have a limited number of (25-50) divisions that they can experience before they are forced to die
by apotosis. Stem cells are said to be immortal. Scientists are discovering how to manipulate stem cells so they can
cause them to become cells of their choosing. They have discovered chemicals, such as cyclins, which trigger the cell
to differentiate which means mature into a specific type of cell with a specific duty. Cancer cells are also immortal
but they seem to be stuck in the cell cycle continuously dividing at a rapid rate with very little time spent in interphase.
Cancer is a result of cell division gone wrong. We will soon be studying cancer and stem cells as well as the normal
pathway cells take from zygote to full grown organism.
Lab materials: In addition to the computer work you have done or will do, we will have available the following
equipment:
Microscopes and lens paper.
Textbook pictures of animal and plant cells undergoing cell division. (Try both our current text and older versions.)
Prepared slides of plant cells (onion (allium) root tip cells from the Meristem region of the root).
Prepared slides of animal cells (whitefish egg cells from a blastula – a fertilized but as yet undifferentiated set of cells).
(some slides are much better than others so if you are struggling finding good images, it might be the slide and not
your lack of ability to focus on the cells. See me if you are struggling and need help.)
Don’t forget to clean both the slide and the lens with lens paper. Please treat the cells with care as they are expensive.
Directions
1. Carefully observe a prepared onion root tip plant slide. Use low power to locate the Meristem region. The region
grows downward into the soil from here and so it is actively dividing. It is just behind the dead epidermal cells of the
root cap that protect the newly dividing cells and just below the elongating cells above, some of which have
differentiated are growing root hairs. The cells you are looking at are mostly in interphase but you will see several that
are in various stages of PMAT and cytokinesis.
2. As practice for you understanding of the different stages and the events that take place in them, sketch a cell in each
of the PMAT phases. Then use your text and/or the information above to help identify the major events occurring in
each phase. Make your sketches large enough to see the details of what is happening in the nucleus.
3. Do the same for the whitefish animal cells.
4. Answer the analysis questions.
Plant Cells
Animal Cells
Summary of events for Interphase
Summary of events for Interphase
1. Nucleolus and nuclear membrane are present.
2. Distinct chromosomes are NOT visible.
3. Cell is metabolically active.
4. Chromosomes are duplicated.
1.
2.
3.
4.
Interphase Drawings
Summary of events for Prophase
1.
2.
3.
4.
Summary of events for Prophase
1.
2.
3.
4.
Prophase Drawings
Summary of events for Metaphase
Summary of events for Metaphase
1.
2.
3.
4.
1.
2.
3.
4.
Metaphase Drawings
Summary of events for Anaphase (& start of cytokinesis)
1.
2.
3.
4.
Summary of events for Anaphase (& cyto start)
1.
2.
3.
4.
Anaphase Drawings
Summary of events for Telophase (& end of cytokinesis)
1.
2.
3.
4.
Summary of events for Telophase (& end of cyto)
1.
2.
3.
4.
Telophase Drawings
Putting it all together
Pretend you can see chromosomes during Interphase. Draw a cell that has 4 chromosomes before and after they
replicate in the S-phase of interphase. Then draw to show how the cell would change as it undergoes cell division.
You decide if you want to draw an animal or plant cell. (Label the cells with each phase and use arrows to show the
sequence of events in the correct order).
Analysis Questions (answer questions 1-12 for sure, do Q. 13 only if you decide to use the microscope slides)
1. At which stage does replication of chromosomes occur?
2. At which stage does equal division of chromosomes occur?
3. What does metaphase insure with regards to the division of information (DNA)
4. How does the information in the daughter cells compare to each other?
5. How does the information in the daughter cells compare with that of the parent cell?
6. What critical molecular event occurs in interphase of mitosis so an equal set of chromosomes can be divided into the
two new daughter cells?
7. What do you think would happen if the spindle fibers were not functioning properly before mitosis was completed?
8. How are plant and animal cell division different?
9. How are plant and animal cell division similar?
10. Briefly explain how the role of cell division in single-celled organisms likely differs from the role of cell division
in multicelluar organisms. (in otherwords – why do single cell organisms do cell division as compared to multicelled
organisms).
11. Assuming exponential growth, you can calculate the population of cells growing for a specific time period. Where
"a" is the starting amount, "b" is the rate at which that amount is growing, "x" is the amount of intervals of time for
such growth to occur, and "y" is the total amount.
Example - If a species of bacteria double every ten minutes, starting out with only one bacterium, how many bacteria
would be present after one hour.
After six(6) intervals of growth (equivalent to 1 hour), there would be 64 bacteria.
11a) Many bacteria populations double every 20 minutes. How many bacteria would there be expected in the
population after one hour? Since 20 minutes is 1/3 of an hour, there would be only 3 intervals of doubling growth
during this 1 hour. 11b) After 1 day (24 hrs/day x 3 intervals/ hr = 72 intervals) of growth how many bacteria cells
would result?
11a)
11b)
NOTE: You are not a single-celled organism so you can’t use this method to reproduce but you end up using it to
become an adult with between 50 and 100 trillion cells in your body. That is amazing since you started out as 1 single
cell called a zygote.
12. Explain the roles of the following cell structures:
Centrioles and asters –
Centrosome –
Spindle fibers –
Kinetochores –
Centromeres –
Cell Plate -
Cleavage furrow –
Chromatin –
Chromosome –
13. Were all cells you observed with the microscope undergoing cell division simultaneously and at the same stages?
14. Which stage were most cells in at the time the slide was made?
15. What might be the explanation for this being phase being so dominant when a “snap shot” is taken of the cells?
Using pages 1016-1019 and 660-661in the text to explain production of and development of the zygote
16. In sexual reproduction, a sperm unites with an egg. What is name of the cell that is the result of this union?
17. What is the name of the process that unites a sperm and egg?
18. The zygote, once formed, begins asexual cell division to increase the number of cells. The cell mass is described as
the morula up to about 4 days. How many cells is the morula (approximately)?
19. The morula was a solid mass of cells. The next phase of the developing embryo is called a ___________________
(or blastula) because it is now a hollow ball of cells.
20. The blastocyst contains an inner mass of cells that will become the embryo and an outer ring of cells called the
trophoblast. The trophoblast will produce two membranes, the amnion and placenta, which will allow the implanted
embryo to be protected and nourished by the host mother. The embryo will implant into the endometrial lining of the
uterus in humans by about day ___________________.
21. In the blastocyst, the cluster of inner mass cells will form the actual embryo. The inner cell mass forms two layers.
The inner layer of cells is called the endoderm while the outer layer is called the ______________________.
22. Soon, the blastocyst undergoes major changes (In humans, by about 13-17 days). It begins to fold in on itself
forming a blastopore. This blastopore will become the mouth or anus of the developing embryo. An animal whose
mouth is formed from the blastopore is called a ________________________. An animal whose mouth is formed from
the second opening (hence, the anus is formed from the first opening) is called a ____________________________.
23. Humans would be which classified as which one, a protostome or a deuterostome?
24. The cell, while folding in on itself, forms an inside and an outside tissue layer. As described above the inner layer
is called the ____________________________. The outer layer is called the _______________________________ .
25. A third layer forms as the folding completes itself and cells rearrange their location. This third tissue layer is called
_____________________.
26. What is the name of this folding stage?_______________________________________?
27. Gastrulation result in differentiation for the embryo? What are the names of the resulting 3 _________________
layers:
28. What will the 3 germs layers become in the adult human?
a. ectoderm
b. endoderm
c. mesoderm
29.After about 8 weeks, the embryo is now classified as a fetus. What has happened to the human fetus:
By 3 months –
By 6 months –
After 6 months -