After completing this unit you should be able to:

1. Create a representation of a cell with a defined number of sets of chromosomes. Then explain and illustrate how chromosomes change, replicate, and move through all stages of the cell cycle for mitosis and meiosis.

2. Explain how sister chromotids are produced from a single chromosome strand and distinguish the difference between a chromosome and a chromotid in each stage of the cell cycle for mitosis and meiosis.

3. Explain how the process of DNA replication insures that daughter cells are genetically identical to the parent cell in mitosis.

4. Explain how independent assortment and crossing over during meiosis provide genetic variability to the gametes.

5. Explain why genetic variability is important to populations in terms of evolution.

6. Explain how the processes of mitosis and meiosis play a role in the conception of twins versus a clone.

7. In this lab you will be using computer simulations and hands-on material to model the cell cycle for both asexual reproduction or 'cloning' of a cell (mitosis) and for gamete production (meiosis) which is involved in sexual reproduction of the organism.

Pre-Lab Assignment

Construct a concept map or visual representation showing the relationship between the list of terms below. You may use additional concepts if it helps you link the concepts in a more logical fashion.

dominant	Sister chromotids	Allele	Homologous chromosomes
recessive	Gamete	Independent assortment	meiosis
Diploid cell	DNA replication	Haploid cell	Gene Locus

I. Cell Reproduction: Mitosis and the duplication of cells

Where did these cells come from?

It is well established that cells do not generate spontaneously, therefore cells must come from division of pre-existing cells. The new cells that are produced are identical to the pre-existing cells. In order to do this everything inside the cell, including all the organelles and (most importantly) the genetic material contained in the chromosomes, must be copied. When cells were first observed, late in the 19th century, it wasn't fully appreciated how cells divided. But with the improvement of microscopes the internal structure of the cell emerged, and scientists could see short strands appear at the beginning of cell division. These strands that we now know as chromosomes got thicker, were separated, and then disappeared.

This process is called MITOSIS and it is extremely accurate (in a biological sense). It ensures that each of the two "daughter cells" produced gets a genetically identical set of chromosomes copied from the parent cell.

Activity 1:

Using common objects (pens, paper clips, strips of paper) model the movement of chromosomes through each phase of the cell cycle. Describe what must happen to the cell in the phases below in order for the parent cell to generate two genetically identical daughter cells.

In addition, use the DNA models to illustrate the semi-conservative replication of DNA at the appropriate phase of the cell cycle.

G1 _________________________________________________________________________

S___________________________________________________________________________

G2 _________________________________________________________________________

M (mitosis) __________________________________________________________________

Use the following web-resources for guidance.

• Visit cellsalive.com and select Cell Cycle
• The DNA in each chromosome must replicate itself in the S phase of the cell cycle (http://www.accessexcellence.org/AB/GG/dna_replication.html).
• Mitosis from The On-line Biology Book
• Visit cellsalive.com. Select Mitosis.
Model MITOSIS with common objects for your TA and be ready to answer questions about it.

T.A. signature __________________

II. Cell Reproduction: Meiosis and the production of gametes (Eggs and Sperm)

Sperm and egg are genetically distinct cells that contain only half the number of chromosomes of other body cells.

Why do eggs and sperm only contain half the number of chromosomes?

How are sperm and eggs produced?

Activity 2:

Team up with a classmate and open the computer package called 'Plant Bio 100 MEIOSIS'. This is a computer simulation that models the process of meiosis. As you work through the exercise, emulate the process of meiosis, crossing over, and linkage with your "common objects". (Meiosis and Cross Over computer simulation developed in Hypercard by Ed Dole, Univ. of Illinois).

1. Press on "B. Modeling Meiosis".

2. Before starting go through each of the "Concepts to Consider:" on the right side of your screen.

3. Press return to spore mother cell and type in letters to represent alleles for one, two or three gene locations.

4. Go through "Meiosis without Crossing Over". Continue to "go forward one card" making sure to check the corresponding card at each stage.
   • How do the corresponding cards differ from their counterpart? ______________________

   • Why do they differ? ________________________________________________________

   • What is independent assortment? ______________________________________________

   • When does independent assortment happen? _____________________________________

   • Why is it important for an individual to be able to produce genetically variable gametes? How is genetic variability important to long term evolution of the species? ________________________________________________________________________

5. Now go through "Meiosis with Crossing Over".
   • What does crossing over mean? _______________________________________________

   • When does crossing over happen? _____________________________________________

   • How do the final daughter cells (gametes) differ from a cell that did not have crossing over occur?___________________________________________________________________

   • What is meant by genetic linkage? ____________________________________________

   • Why is it more difficult to separate some linked genes by crossing over than others? _________________________________________________________________________

   • Give an example of heritable traits that are likely to be closely linked. ___________________________________________________________

   In order to answer these questions you may want to explore the "Cross Over" stack.
Model MEIOSIS with common objects for your TA and be ready to answer questions about it.

T.A. signature __________________

III. When Twins Marry Twins (Allen & Duch, Thinking Towards Solutions, 1998)

Sally Thompson meets Harry Branaugh in her junior year at a small liberal arts college in Massachusetts. It's a case of love at first sight. In the spring of their senior year, they both have been lucky enough to find jobs in the Boston area, so they plan to get married in the June following graduation.

At their wedding rehearsal dinner Sally's twin sister Emma meets Harry's twin brother Ken for the first time. It's a case of love at first sight. As Sally and Harry have their first serious argument about who should have told whom about having a twin, Emma and Ken make plans for the evening that don't include the rest of the family. Three months later, they also decide to get married.

The couples keep in touch, and three years later Sally and Emma are delighted to discover that they are both expecting (twins?). Emma's due date is in October, and Sally's in December. On December 12th , seventeen hours into labor, Sally is no longer sure she's delighted about the prospect of motherhood, and begins to worry about the child she's about to deliver.

"Why didn't you think of it sooner," she says to Harry, gripping his arm rather severely. "Identical twins should never marry identical twins. Our child's going to look just like Emma and Ken's little boy." Her first impression of Kenneth Jr., she recalls, was that he had the sort of face that only a mother and father could love. Two hours later, Sally is scared to take a look as the obstetrical nurse puts her first child into her arms.

Activity 3: After reading the scenario above team up with a classmate and research the following questions. Prepare a visual diagram that answers these questions. Be sure to consult the web resources to guide your inquiry.

Questions:

1. How does a couple conceive twins?

2. What is the difference between identical (monozygotic) twins and fraternal (dizygotic) twins?

3. How was Dolly the sheep clone "made"?

4. How do clones and twins differ?

5. Will Sally and Harry's child look just like his or her "double cousin", Ken Jr.? Why or why not?

6. Assuming that Sally is right and the children will look identical, will they also have similar personalities, behavior, and attitudes?

7. What is the maximum percent of the two children's genetic composition that could consist of identical genes? The minimum percentage?

Use the following web resources for guidance

• University of MN, Dept . of Psychology (http://www.psych.umn.edu/psylabs/mtfs/special.htm)
• an excellent (although rather lengthy) article by Berkowitz, Ari from BioScience 46:42-51, 1996 (http://serendip.brynmawr.edu/gen_beh/Berkowitz.html).
• Cloning 1-2-3, from Time Magazine (http://www.pathfinder.com/time/cloning/cloning.html)
• Explanation of cloning from the Association of Reproductive Health care Professionals
Give your TA a tour through your diagram.

T.A. signature____________________________