The Genome as the Harry Potter Series

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

The genetic code is often described as being analogous to the written language. I expand this analogy to help students understand the hierarchy that exists in genetics, since I find many students don’t understand the relationship between a gene and a chromosome. Imagine a set of books, perhaps the Harry Potter series. The entire series on the shelf is analogous to the genome. Each book can be thought of as a chromosome. Within each book are chapters, these can be thought of as genes. Lastly, the 26 letters of the alphabet are arranged to make the variation of words within the genes. The genetic code has 4 letters to make unique arrangements/sequences. What would be the consequence if a few sentences or a chapter or an entire book was lost from the series? Would the story still make sense? (This would be analogous to mutations and chromosomal abnormalities.)

Pairs of Shoes and Pairs of Chromosomes

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

When discussing homologous chromosomes and sister chromatids, I often use analogies to shoes or socks. For example, I may have two pairs of the same cute flats, one pair in yellow and one in turquoise. These flats are the same size, same brand, exact same style. The yellow shoes are like sister chromatids to each other, just as the turquoise shoes are sister chomatids with each other. The yellow and turquoise are like homologs to each other. To carry the analogy further, I ask them what a pair of running sneakers might be analogous to. (These would be a completely different chromosome.)

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.


Applying the Concept of Non-Disjunction to Trisomy

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

Learning Outcomes:

– To understand how non-disjunction is abnormal meiosis

– To see how non-disjunction leads to trisomy

– To show students how to think through an application-based question

Activity Description: Students are given a question via PowerPoint as a clicker question and asked to do the problem alone. After collecting initial answers (and not telling them the correct one), students are then directed to work through the same problem using a helpful worksheet and neighbors. Students are then asked the same clicker question.

Time Needed: Approximately 15-20 minutes

Materials Needed: Worksheet or blank paper for students (if showing everything via PowerPoint)

Activity Instructions: You can insert this question into PowerPoint or use as a worksheet with the skeleton images. Consider using it as a clicker question if you are using clickers. This will likely be a tough one that students would benefit from a discussion with neighbors after trying it on their own and before being asked the same question again via clicker.

Question 1:

Do you really UNDERSTAND meiosis and non-disjunction? Try this question:

If an individual has the genotype XXY, did non-disjunction occur in their mother or their father (or both) and at which division(s), meiosis I or meiosis II?

A. Mother in meiosis I or II, Father in meiosis I or II

B. Mother in meiosis I or II, cannot be a non-disjunction in father

C. Cannot be a non-disjunction in mother, Father in meiosis I or II

D. Mother in meiosis I or II, Father only meiosis I

E. Mother only in meiosis I, Father only in meiosis I

*Choice D is correct. Students will have the most trouble with what happens after non-disjunction at Meiosis I. What they fail to understand is that the sister chromatids will still line up at meiosis II and separate. This is especially easy to spot with the father’s nondisjunction at Meiosis I. If they don’t understand this concept they will have a XX and a YY cell forming, instead of two XY cells.

Question 2:

If an individual has a genotype XYY, did non-disjunction occur in their mother or their father (or both) and at which division(s), meiosis I or meiosis II?

A. Mother in meiosis I or II, Father in meiosis I or II

B. Cannot be a non-disjunction in mother, Father in meiosis I or II

C. Mother in meiosis I or II, Father only meiosis I

D. Cannot be a non-disjunction in mother, Father in meiosis I only

E. Cannot be a non-disjunction in mother, Father in meiosis II only

*Choice E is correct. A sperm that was YY was fertilized with a normal egg.

Worksheet: Meiosis and Non-Disjunction Worksheet