Losing Control of a Car Relates to Unregulated Cell Division

Written by Kelly A. Hogan, University of North Carolina at Chapel Hill

When discussing the cell cycle and cancer causing genes, I often use an analogy to cars. There are two ways to lose control of a car: the gas pedal can get stuck down or the brakes will not work when engaged. In either case, the car speeds along without driver control. In this anology, tumor supressors are like brakes, which normally prevent the cell cycle from losing control (preventing cancer). When mutated, the brakes are lost and the cell divides out of control. Proto-oncogenes are like the gas pedal, in that they promote cell division. When mutated, like a gas pedal stuck down, they cause unregulated cell division.

Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models

Written by Kelly A. Hogan, University of North Carolina at Chapel Hill

Learning Outcomes:

– To compare mitosis and meiosis

– To demonstrate the meaning of the words: haploid, diploid, homologous chromosomes, and sister chromatids

– To demonstrate how independent orientation during meiosis leads to variation

Activity Description: Students will use cut-out chromosome models to demonstrate the stages of the cell cycle at their individual desks and/or by taping the cut-outs to a large board in the classroom as part of a whole class activity. Using big models as a large class activity can be used after smaller groups have tried this or can be used as a standalone activity, inviting a few students at a time to answer questions. Students will be asked to show specific phases of the cell cycle and to define words via demonstration by moving around the chromosome models.

Time Needed: 10-15 minutes as a whole class demonstration or up to 45 minutes in student groups with discussion

Materials Needed: Scissors, tape, and large chromosome cut-outs or individual worksheets students use to cut out chromosomes by themselves

Activity Instructions:

1. Have the students pick up two chromosomes that are homologous. Have them pick up two chromosomes that are sister chromatids. Ask them to explain the difference in definitions between sister chromatids and homologous chromosomes.

2. Ask the students, “Do you need all the pieces above for both mitosis and meiosis?” (Yes.)

3. Have students use the chromosomes to demonstrate the stages of mitosis by moving the chromosomes around on a whiteboard or on their desk. (Have them begin in G1, prior to DNA replication.) Use this time to ask them why 2n = 6.

4. Have the student demonstrate meiosis stages starting with G1 and stopping with metaphase I (You can choose to ignore crossing over at this point to simplify). Use this time to stop and ask how metaphase I is different from metaphase of mitosis (they should point out homologous chromosome pairing in metaphase).

5. Students should be able to demonstrate different independent orientations that can randomly occur at metaphase I. You might get them to figure out how many different alignments there are. (There are four possible alignments when n = 3.)

6. Have students complete meiosis I with one alignment of metaphase I, writing down the combinations of alleles they would wind up with in the gametes. Have them go back and choose another alignment to see that different gametes can form. Discuss Mendel’s Law of Independent Assortment and how this creates variation in gametes.

7. Have students define haploid and diploid using the chromosomes to demonstrate.

8. Ask them what their chromosomes might look like if there was crossing over with the AB homologous chromosomes (you may choose to cut and tape the chromosomes to demonstrate the recombination that occurs). Ask students when crossing over between homologous chromosomes occurs (prophase I) and be sure they understand that there is no crossing over in mitosis.

9. Ask the students various questions, such as:

“Can a gamete form that has alleles: A, B, H, R, d? Explain.” (Yes, crossing over between the AB genes, plus one of the HR chromosomes [no crossing over] and one of the d chromosomes.)

“Can a gamete form that has alleles: A, A, b, b, a, a, B, B? Explain.” (No, each gamete only gets one copy of each gene. Use an example like this to explain the term haploid again.)

10. Ask students to use the chromosome models to show you a cell in which 2n = 4 in G1.

11. Have students make a list of differences between mitosis and meiosis. Have them list similarities.

12. Ask students to reflect on the value of using models of chromosomes rather than looking at static images from the book or PowerPoint. Ask them to reflect on the value of this activity vs. watching an animation.