|Mitosis: Chromosome Replication & Division|
|Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.|
|Exercises 1 & 2 take approximately 2 1/2 hours.|
|1.|| How does a human being grow from a single
fertilized cell into an individual containing billions of cells?|
Cells increase their number through a process called cytokinesis or cell division. Cell division is preceded by nuclear division or mitosis. The genetic information of the parent cell is reproduced precisely in each daughter cell, whereas division of the other cell components is more approximate.
|2.|| Do all the cells of the body look like one another? Do they perform the same jobs?|
There are many different types of cells in the body which serve many different functions. A nerve cell, which conducts electrical messages, looks very different from a muscle cell, which is used to move some part of our body.
|3.|| Do all the cells of the body contain the same
All cells in the body with the exception of egg and sperm have identical copies of an individualšs genetic information. Different genes are activated in different cell types.
|4.|| How is the genetic blueprint
that makes you who you are transmitted faithfully from one cell to the next?|
The faithful transmission of genetic material from one cell generation to the next is accomplished through DNA replication (in interphase) and division (mitosis). This nuclear replication and division occurs billions of times as a human being grows and develops, with great fidelity.
|5.|| How long does it take for one
parent cell to become two daughter
In humans, rapidly dividing cells, such as skin and gut divide as often as once per day. Other cells such as brain and nerve tissue divide rarely in an adult.
|6.|| Are cells alive?|
Yes, cells are the smallest units of life. They are judged to be living because they are capable of respiration, nutrient intake, release of waste materials, faithful reproduction of themselves, movement, responsiveness, and other processes characteristic of living things.
|7.|| What is a cell,
Cells, sometimes referred to as the basic units of life, are the small compartments in your body which house your DNA and perform all the essential tasks required to sustain life. They are surrounded by a nuclear membrane, and contain a nucleus and many other organelles and subcellular structures.
| 2 sets of white and 2 sets of red plastic
knives, forks and spoons per group for chromosomes|
1 large (3 ft) length and two smaller lengths (1.5 ft) of yarn for nuclear membrane
white or brown paper per group
string for spindle fibers
small rubber bands for centromeres
yarn that is longer and a different color to represent cell membrane
|Once you have completed these exercises you
should be able to:|
|1.||Describe how cells reproduce themselves.|
|2.||Explain how chromosomes are copied and distributed to each daughter cell in a precise way.|
|3.||Describe the need for, and the mechanism of, conservation of hereditary material.|
|4.||Be able to define and correctly use the following terms: allele, anaphase, chromosome replication, cytokinesis, diploid, DNA synthesis, gene, homologous chromosome, interphase, life cycle, metaphase, mitosis, prometaphase, prophase, replicated chromosomes, sister chromatids, spindle fibers, telophase, unreplicated chromosomes.|
Cell DivisionYour body is composed of more than a billion cells. Cells are continually dying, and new cells are continually being formed. An identical copy of your hereditary material is found in the nucleus of each and every somatic cell. A somatic cell is any cell in the body except for the reproductive cells in the reproductive system.
This genetic blueprint is organized into 46 chapters or parts known as chromosomes. It is estimated that, on average, each chromosome contains between one and two thousand genes. A gene contains the information for making a single protein or RNA product.
Every time a cell divides, each chromosome must be carefully replicated (copied) and then distributed to assure that each daughter cell gets a complete and accurate set of information. Thus, nuclear division includes successive processes of chromosome replication, separation, and distribution (Figure 1).
Adapted from Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc. Figure 7.8, page 173.
DNA synthesis occurs in the nucleus, producing an exact replica of every chromosome. A chromosome can be thought of as a very long DNA double helix. During replication, the double helix opens up and a new complementary strand is synthesized along each parent strand (Figure 2). This results in two identical DNA helices, each containing one original parent strand and one newly synthesized strand.
DNA synthesis occurs during the S phase of interphase. Each cell goes through a regular life cycle, similar to the cycle of life in humans. Where we might call our stages infancy, childhood, adolescence, young adult, adult, and senior, the major cell stages are interphase, mitosis, and cytokinesis. Interphase is subdivided into G1 (growth 1), S (synthesis), and G2 (growth 2), and mitosis is divided into P (prophase), PM (prometaphase), M (metaphase), A (anaphase), and T (telophase). This is shown in Figure 3.
Adapted from Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc. Figure 7.6, page 171.
Figure 4: Cell Division
Adapted from Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc. Figure 7.7, page 172.
Exploring the Process of Mitotic Cell Division
|To Do||1.||You will study mitosis in the Triffle, a mythical creature
with six chromosomes that look
like knives, forks, and spoons. You will work out each step of the process using
paper for cells, yarn for membranes, string for spindle fibers, and plastic knives, forks
and spoons for |
|2.||Go through the entire process (1.1 through 1.8) several times, with each group member taking a turn as the "explainer". Follow along with the procedure below for the first one or two turns, and perform the subsequent repetitions from memory. Answer the questions about each stage as you go along, and answer them each time you go through the process. Explain your answers in your own words and your own way -- don't recite them by rote memory.|
|3.||Take one large piece of paper for your cell, and use one color yarn to show the nuclear membrane and a different color yarn to show the cell membrane.|
|4.||Begin with a cell and nucleus containing six chromosomes represented by two forks (one red & one white), two knives (one red & one white), and two spoons (one red & one white). This represents a diploid cell with three pairs of chromosomes (Figure 5).|
1.2 Interphase and Chromosome Replication
|To Do||1.||Throughout interphase, the chromosomes are extended and are not visible in the light microscope (Figure 6). That is, the DNA is uncoiled. We cannot simulate this extended condition with the knives, forks, and spoons, so please imagine it. Replicate each of the chromosomes in your Triffle nucleus, pretending they are extended at the time. Do this by obtaining six more chromosomes that match the set you already have. Attach a red fork to your red fork, a white fork to your white fork, and so on with an elastic band (which will represent the centromere). In this process, each chromosome has essentially made an identical copy of itself.|
|2.||Your nucleus initially contained six unreplicated chromosomes, and now it contains six replicated chromosomes. The two identical copies of each chromosome, sister chromatids, remain attached at a point called the centromere (Figure 7).|
Adapted from Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc. Figure 7.4C, page 170.
1.3. Prophase of Mitosis
|To Do||1.||In prophase, the replicated chromosomes condense and become visible (Figure 8). This is the first stage of mitosis.|
1.4. Prometaphase of Mitosis
|To Do||1.||In prometaphase, the nuclear membrane literally "disappears", which allows the rest of the mitotic events to occur. Remove the nuclear membrane from around the chromosomes in the nucleus of your cell.|
|2.||Spindle fibers form, emanating from two structures called centrioles that have migrated to opposite poles (ends) of the cell. Spindle fibers are assembled from protein microtubules. Put spindle fibers in your cell using pieces of string and draw the centrioles on the paper at the appropriate points.|
|3.||Some of the spindle fibers attach to the replicated chromosomes at their centromeres (Figure 9).|
1.5. Metaphase of Mitosis
|1.||In metaphase, replicated chromosomes are lined up on the metaphase plane (across the center of the cell) by the spindle fibers(Figure 10). Homologous chromosomes are independent of one another. That is, homologous replicated chromosomes such as the two sets of replicated spoons ARE NOT PAIRED.|
|To Do||2.||Arrange your Triffle
chromosomes across the center of
the cell. The specific order of chromosomes and their orientation (right
side up, upside down) is completely random.|
1.6. Anaphase of Mitosis
|To Do||1.||In anaphase, sister chromatids separate to become daughter chromosomes (Figure 11). Separate your sister chromatids to form daughter chromosomes.|
|2.||Daughter chromosomes are moved toward
opposite poles by the spindle fibers.
Chromatids are flexible. They do
not remain rigid, but rather bend on each side of the centromere as they are dragged through
a. Are the daughter chromosomes replicated or unreplicated?
Daughter chromosomes are now unreplicated. Each contains a single DNA double helix.
b. Are the two sets of daughter chromosomes, the one moving toward the left and the other toward the right, identical or non-identical?
With the exception of rare mutation events, mitosis leads to the formation of identical daughter chromosomes. Many of the same questions are asked several times in several different circumstances because these seem to be difficult ideas for students to grasp.
c. Are the two sets of daughter chromosomes identical to those in the parent cell?
Yes! In fact, each daughter chromosome contains one parent strand of DNA with a newly synthesized complementary strand.
d. What is accomplished by this process?
In mitosis, the genetic information in the chromosomes of a cell is first reproduced precisely and then the duplicate sets of information are distributed precisely to two daughter cells preserving the original genetic blueprint.
1.7. Telophase of Mitosis
|To Do||1.||Daughter chromosomes reach the poles of the cell and become extended (relaxed). The spindle fibers disappear actually, the microtubulin subunits are disassembled. You can remove your spindle fibers from your cells and pretend your chromosomes are going into the extended state.|
|2.||Two new nuclear membrane form, one around each set of daughter chromosomes. Use the nuclear membrane yarn to create two new nuclear membranes in your cell (Figure 12). Pinch in the yarn representing the cell membrane.|
|To Do||1.||An animal cell pinches in half at the center (Figure 12), from the outside in, until it has produced two separate daughter cells (Figure 13). Divide your cell in half in this manner by replacing the long yarn representing the parent cell membrane with two shorter pieces of yarn representing the membranes of the two daughter cells.|
daughter cells are now entering
the early interphase stage.
Pretend that your Triffle chromosomes are becoming extended. The
cells will grow to full size and, if continuing to divide, will replicate their
chromosomes, and repeat the cycle
1.9. Practice through Repetition
|To Do||1.||As noted above, you can go through the entire process several times, with each group member taking a turn as the "explainer". Follow along with the procedures outlined above for the first one or two turns, and then perform the subsequent repetitions from memory. You may refer to Table 1 for a rough guide, and your team mates can assist you by asking questions and giving hints.|
Chromosomes in Humans
|To Do||1.||Examine the
chromosome spread in the top half of Figure 14. How do you think such a
picture is obtained? |
A chromosome spread is made by:
Photograph from Goodenough, U. & Levine, R. P. (1974). Genetics. San Francisco: Holt, Rinehart, & Winston. Figure 2-16, page 57.
examine the human karyotype in the bottom half of Figure 14.|
A karyotype is made by:
|Describe||3.||Relate what you have learned in this lab to:|
|Goodenough, U. & Levine, R. P. (1974).
Genetics. San Francisco: Holt, Rinehart, & Winston. Figure 2-16, page
Klug, W. S. & Cummings, M. R. (1997). Concepts of Genetics, Fifth Edition. Upper Saddle River, NJ: Prentice Hall.
Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc.
|Chapter 5: THE
Section B: Heredity
Grade 6-8 (Benchmark 1 of 3)
Section B: Heredity
Grade 9-12 (Benchmark 1 of 6)
Section F: Evolution of Life
Grade 9-12 (Benchmark 4 of 9)
Section B: Heredity
Grade 9-12 (Benchmark 6 of 6)
Section C: Cells
Grade 3-5 (Benchmark 2 of 2)
Section C: Cells
Grade 6-8 (Benchmark 1 of 4)
Section C: Cells
Grade 9-12 (Benchmark 4 of 8)
Chapter 6: THE HUMAN ORGANISM
Section B: Human Development
Grade 9-12 (Benchmark 1 of 4)