Mitosis: Chromosome Replication & Division
Grade Level		Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.
Time		Exercises 1 & 2 take approximately 2 1/2 hours.
To Ponder		1.	How does a human being grow from a single fertilized cell into an individual containing billions of cells?
		2.	Do all the cells of the body look like one another? Do they perform the same jobs?
		3.	Do all the cells of the body contain the same genetic information?
		4.	How is the genetic blueprint that makes you who you are transmitted faithfully from one cell to the next?
		5.	How long does it take for one parent cell to become two daughter cells?
		6.	Are cells alive?
		7.	What is a cell, anyway?
Supplies		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 scissors string for spindle fibers small rubber bands for centromeres yarn that is longer and a different color to represent cell membrane
Objectives		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.
Background Information		Cell Division Your 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).
Figure 1: Chromosome Replication & Division 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. Figure 2: DNA Replicating 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. Figure 3: Cell Cycle 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. Another way to illustrate this cycle is shown in Figure 4. 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. a. How many rounds of chromosome replication occur in the cell prior to mitosis? b. How many times does a cell divide in mitosis?
Exercise 1		Exploring the Process of Mitotic Cell Division 1.1 Introduction
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 chromosomes.
		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).
Figure 5. Triffle Diploid Chromosome Set   a. What does diploid mean? b. Are most human cells diploid? c. How many pairs of homologous chromosome are present in the picture of a Triffle cell above? d. Draw a circle around each homologous pair of chromosomes in the picture above. e. Are the homologues, (a short name for homologous chromosomes) above paired with one another in the cell, or are they independent from one another? f. What is the best description of homologous chromosomes? (choose the best response) (1) they are the same size and shape (2) they contain the same types of genes in the same order (3) they generally contain different versions (alleles) of many of their genes (4) all of the above g. Define homologous chromosome. h. Contrast gene and allele.
		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.
Figure 6: Interphase
		2.	Your nucleus initially contained six
Figure 7: Chromosome Centromere 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.   a. What is a chromatid made of (protein, carbohydrate, lipid, and/or DNA)? b. How does a sister chromatids differ from a ? c. What is the centromere? d. Contrast extended and condensed chromosomes.
		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.
Figure 8. Prophase   a. Are the two sister chromatids that are connected by a centromere identical to one another or do they contain different alleles? Explain. b. As noted above, these structures are called replicated chromosomes (or, in many books, simply chromosomes ). Replicated chromosomes are quite different from the unreplicated chromosomes seen earlier. Compare replicated chromosomes to unreplicated ones (by filling in the blanks below). (1) the amount of DNA in a replicated chromosome is_____ times the amount of DNA in an unreplicated chomosome (2) the number of copies of each gene in a replicated chromosome is _____ times the number of copies in an unreplicated chromosome (3) each replicated chromosome contains _____ (insert number) complete copies of genetic inf (4) the copies of genetic information in each chromosome are ________________ (identical, homologous, or complementary) c. Do you think that the homologous replicated chromosomes (the two pairs of knives, the two pairs of forks, and the two pairs of spoons) will pair with one another during mitosis ? Explain. d. How many sister chromatids are in your Triffle nucleus in prophase? e. A diploid human cell contains 46 unreplicated chromosomes in early interphase . How many sister chromatids will be present in the human cell during prophase 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).
Figure 9: Prometaphase
		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.
Figure 10. Metaphase
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.   a. How many replicated chromosomes are on the metaphase plane in the Triffle? b. How many replicated chromosomes would be on the metaphase plane in a human cell undergoing mitosis?
		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.
Figure 11. Anaphase
		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 the cytoplasm. a. Are the daughter chromosomes replicated or unreplicated? b. Are the two sets of daughter chromosomes, the one moving toward the left and the other toward the right, identical or non-identical? c. Are the two sets of daughter chromosomes identical to those in the parent cell? d. What is accomplished by this process?
		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 membranes 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.
Figure 12. Telophase
		1.8. Cytokinesis
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.
Figure 13. Cytokinesis Completed
		2.	These 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 again.   a. Does the parent cell still exist? b. How are these daughter cells related to one another? c. How are these daughter cells related to the parent cell? d. Overall, what has been accomplished by mitosis? e. You have used your materials to model mitosis (nuclear division) and cellular division. Explain some ways in which a model differs from the actual things and processes it represents.
		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.
Table 1. Cell Cycle Summary
			Interphase G1 stage Growth & development of the cell Protein synthesis S-phase Chromosome replication via DNA synthesis G2 stage Growth & development Organelle Replication Mitosis Prophase Replicated chromosomes condense Prometaphase Nuclear membrane dissolves Spindle fibers form Metaphase Replicated chromosomes align at center Anaphase Sister chromatids separate Daughter chromosomes move to poles Telophase New nuclear membranes form Spindle fibers disappear Cytokinesis Cell divides into two daughter cells
Exercise 2		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?
Figure 14. Human Chromosome Spread and Karyotype Photograph from Goodenough, U. & Levine, R. P. (1974). Genetics. San Francisco: Holt, Rinehart, & Winston. Figure 2-16, page 57.
		2.	Then examine the human karyotype in the bottom half of Figure 14. How do you think such a picture is obtained?
Describe		3.	Relate what you have learned in this lab to:   a. the growth and differentiation of tissues in babies, b. the use of a somatic cell rather than sperm and egg to create a new organism such as a sheep or frog, c. another related phenomenon of your own choosing.

Supplementary Resources		Goodenough, U. & Levine, R. P. (1974). Genetics. San Francisco: Holt, Rinehart, & Winston. Figure 2-16, page 57. 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. Interactive Mitosis Tutorial (created with Director) What The Heck is a Gene? Dictionary of Cell Biology The Biology Project, University of Arizona The Cell Cycle Graphic Illustrations Oklahoma State Univerity Access Excellence University of Texas
Related AAAS Benchmarks		Chapter 5: THE LIVING ENVIRONMENT Section B: Heredity Grade 6-8 (Benchmark 1 of 3) In some kinds of organisms, all the genes come from a single parent, whereas in organisms that have sexes, typically half of the genes come from each parent. Section B: Heredity Grade 9-12 (Benchmark 1 of 6) Some new gene combinations make little difference, some can produce organisms with new and perhaps enhanced capabilities, and some can be deleterious. Section F: Evolution of Life Grade 9-12 (Benchmark 4 of 9) Heritable characteristics can be observed at molecular and whole-organism levels--in structure, chemistry, or behavior. These characteristics strongly influence what capabilities an organism will have and how it will react, and therefore influence how likely it is to survive and reproduce. Section B: Heredity Grade 9-12 (Benchmark 6 of 6) The many body cells in an individual can be very different from one another, even though they are all descended from a single cell and thus have essentially identical genetic instructions. Different parts of the instructions are used in different types of cells, influenced by the cell's environment and past history. Section C: Cells Grade 3-5 (Benchmark 2 of 2) Microscopes make it possible to see that living things are made mostly of cells. Some organisms are made of a collection of similar cells that benefit from cooperating. Some organisms' cells vary greatly in appearance and perform very different roles in the organism. Section C: Cells Grade 6-8 (Benchmark 1 of 4) All living things are composed of cells, from just one to many millions, whose details usually are visible only through a microscope. Different body tissues and organs are made up of different kinds of cells. The cells in similar tissues and organs in other animals are similar to those in human beings but differ somewhat from cells found in plants. Section C: Cells Grade 9-12 (Benchmark 4 of 8) The genetic information in DNA molecules provides instructions for assembling protein molecules. The [genetic] code used is virtually the same for all life forms. Chapter 6: THE HUMAN ORGANISM Section B: Human Development Grade 9-12 (Benchmark 1 of 4) As successive generations of an embryo's cells form by division, small differences in their immediate environments cause them to develop slightly differently, by activating or inactivating different parts of the DNA information.