Meio-Socks. Other Genetic Yarns HOW-TO-DO-IT A NTHEA M. STAVROULAKIS

Transcription

1 HOW-TO-DO-IT Meio-Socks & Other Genetic Yarns Several years ago, while reviewing text choices for our general biology course, a photograph accompanying a human karyotype in Asking About Life (Tobin & Dusheck, 1998) caught my attention. Paired socks! What a simple and elegant way to illustrate homologously-paired chromosomes. This inspired me, and that textbook photograph became the foundation for teaching the concepts of maternally- and paternally-derived homologous chromosomes, sex chromosome differentiation, telomere structure, and chromosomal mutations and rearrangements. Socks are inexpensive and easily acquired. Sock modeling permits classroom discussion of many important concepts in nuclear division and cytogenetics and engages the students in the lesson. ANTHEA M. STAVROULAKIS is a Professor teaching General Biology, Botany, Genetics, and Microbiology in the Department of Biological Sciences at Kingsborough Community College, Brooklyn, NY 11235; [email protected]. A NTHEA M. STAVROULAKIS There are many different strategies and materials traditionally used to teach meiosis to biology students, including pipe cleaners and yarn. There are laboratory exercises that utilize Lego TM Building Blocks, colored wax strips, and colored playing cards (Agostino, 2001; Clark & Mathis, 2000; Haws & Bauer, 2001; Winterer, 2001), as well as articles that describe ways of incorporating current discussions into the classroom, including role playing (Ortiz et al., 2000; Taras et al., 2000; Wyn & Stegink, 2000). These are but a few ideas for teaching and engaging the students. Socks provide a hands-on method of instruction for both the students and the instructor. The sock and yarn models described here allow the students the opportunity to examine the meiotic process in a more dynamic way than by traditional methodologies of presenting this material. The methods can be used by middle and high school teachers and by college professors. MEIO-SOCKS 233

2 Materials Required Using unwanted socks and materials readily available in office supply stores, teaching meiosis becomes a more visual and hands-on experience in the classroom. The materials required are minimal, and include socks, yarn, pushpins, clips and a piece of foam board (Figure 1). A rectangular piece of foam board, that is rigid and lightweight, is easily carried into the classroom and propped on the blackboard ledge for reference. Ordinary pushpins proved too short to secure the chromosomes (socks) to the board; therefore longer styled pins were preferred. Large binder clips were easily seen and securely held the duplicated ( sister chromatids ) socks together. Classroom Presentation Examples I have used sock demonstrations in many different courses. The examples presented here were adapted to a variety of classes including the non-biology major, introductory biology, and genetics elective courses. On a desk top, a clumped pile of socks is used to illustrate the arrangement of chromosomes, such as during interphase (Figure 2). The desk top may or may not include a foam board depicting a nucleus with a circular background drawing representing the nuclear membrane. This method and prop was particularly helpful when using 23 pairs of socks. I asked groups of students to karyotype the socks, and found it was a method of simultaneously engaging their hands, minds, and mouths. Students appreciated the time needed to sort the pairs and the accuracy required for accomplishing this task during meiosis. In the advanced genetics course, I showed how the arrangements of chromosomes within the nucleus are confined to specific regions called domains (Savage, 2000). Each chromosome arm occupies a distinct location. Discussing how this compartmentalized arrangement of the nucleus is a regular feature of interphase nuclei and how it facilitates the exchange processes is helpful. A correlation between the effects of ionizing radiation on specific chromosomes and its potential consequences was facilitated by positioning the socks in various arrangements (Gasser, 2002). Meiosis, Amniocentesis & the Human Karyotype During lessons on meiotic division, the paired socks are used to show chromosome homology. In the general biology course, pairing for karyotyping is easily illustrated. By selecting a sock at random from the nuclear pile (Figure 2), students innately know which sock completes the pair. Socks that resemble each other are useful to illustrate the precise identity (e.g., exact gene for gene match) of chromosome pairs. By misaligning a matched pair, students can be shown how proper orientation is necessary for pairing, as occurs during synapsis. Matching up the ankle and heel regions helps show the correct positioning. When selecting socks to use for demonstration, select pairs that appear different from each other, both in size and coloration. If a few pairs have unique markings such as stripes or different patterns or colors, any difficulty in distinguishing the pair is facilitated (Figure Figure 1. Materials required. A few pairs of socks with unique patterns, yarn, large binder clips, pins, and foam board available from your local office supply store are all that is needed. Figure 2. Nuclear arrangement of chromosomes. A circular background, representing the nuclear membrane, containing a ball of socks helps illustrate the arrangement of chromosomes, such as during interphase. 234 THE AMERICAN BIOLOGY TEACHER, VOLUME 67, NO. 4, APRIL 2005

3 3). Matched pairs help the students understand the importance in the following generation, when germ cells are formed. Students are shown how idiograms are prepared during amniocentesis for genetic counseling. Sorting and pairing of like socks illustrates how chromosome pairs are identified. This exercise facilitates discussion and a better understanding of many important concepts of inherited human aberrations. It allows the instructor to demonstrate the consequences meiotic anomalies may have on a fetus and how they are detected. Patterned socks with stripes or a design help identify corresponding gene locations. Striped socks are also a nice way to introduce and describe the usefulness of chromosome banding. Alleles on un-striped socks can be delineated at various regions using paper tags or adhesive colored dots. Let s not forget children s socks; they make great G group and Y chromosomes. They can be used to represent the human G group chromosomes 21 and 22, as well as the Y chromosome. In Figure 4 there is an extra chromosome 21, which demonstrates trisomy 21, characteristic of Down syndrome. The comparable size of the Y chromosome relative to the X chromosome can also be seen. The mismatched sex chromosomes are used to discuss the hemizygous condition and its consequences for males carrying the hemophilia and redgreen color blindness alleles. Mutations & Rearrangements Have you ever accidentally washed one sock in bleach? I have used the differently-colored sock to discuss: (a) maternally vs. paternally derived homologues; though the same size, there is something different about them (b) mutated/damaged chromosomes; identical superficially, but different shades of color, perhaps denoting a weakness (c) genomic imprinting; students are asked to imagine the bleached sock representing methylated genes, which are imprinted according to their parental origin. Genes located on chromosomes inherited from each parent differ according to parental origin. We discuss how DNA methylation occurring as part of genomic imprinting affects the onset and severity of genetic disorders, such as Prader-Willi syndrome and Parkinson s disease. In these examples, students can see how the chromosomes are identical with respect to size and centromere position (e.g., the heel of the sock), yet differ (Figure 5). Don t throw out those unpaired socks either they are deletions! Thinning areas at the ankle may be used to show defective (mutated?) chromosomal areas. Any damaged or worn sock can be used to show a variety of defects. In Figures 6 and 7, holes are useful to show mutations; worn out ankle areas can be fragile sites. Many athletic socks with frayed ends are great for showing the important role telomeres have in maintaining chromosome integrity. As shown in Figure 8, frayed ends can be pulled away to show how the entire Figure 3. Karyotypes. Paired socks, with or without designs, may be used to show homologously-paired chromosomes. Figure 4. G group chromosomes and the Y chromosome. Children s socks lend themselves to a representation of human chromosomes 21 and 22, as well as the Y chromosome. A trisomy 21 is demonstrated, as well as comparison of the size of the Y chromosome relative to the X chromosome. MEIO-SOCKS 235

4 Figure 5. Chromosomal damage.when one member of the pair is bleached, mutation and genomic imprinting can be discussed. Figures 6 and 7. Chromosomal damage. Socks with holes or worn out areas can be used to demonstrate mutations, deletions, and fragile sites. chromosome could eventually unravel, thereby exposing the no-longer protected alleles beneath. But why stop there? Collect select socks of bright color differences to show chromosomal rearrangements. Several socks can be stapled together to show inversions. In Figure 9, a paracentric inversion is demonstrated, showing Mickey Mouse in an upside down position in one sock. If the inverted sock is long, the inversion loop formed during meiotic pairing is easily demonstrated. Chromosomal Replication Chromosome replication requires two identical pairs of socks. I find athletic socks work best for this; they are usually available with many pairs sold in a package. A single sock is used to represent a chromosome prior to replication, then the duplicated (sister chromatids) chromosome is held together by a large binder clip as shown in Figure 10. If three identical pairs of socks are available, the first pair would be a homologous pair, and then each homologue can be duplicated. Reductional and equational meiotic divisions are readily demonstrated. To show metacentric, submetacentric, and acro/telocentric centromere locations, simply clip the binder clip to various positions of identical socks (Figure 11). Chromosome pairs that are similar to each other in size, such as the human C group chromosomes (homologous pairs 6-12) and the similarly sized X chromosome, are best illustrated with athletic socks, which are usually of similar appearance and length. Unfortunately, sister chromatid exchange or crossing over would require several socks with alleles labeled, Figure 8. Telomeres. Frayed sock ends may be used to illustrate the role of the telomere in maintaining chromosome end integrity.the pulled ends can be used to represent their structure. Figure 9. Inversions. Using a stapler or Velcro TM strips, inversions and the resulting meiotic pairing difficulties (e.g., inversion loops) can be illustrated. 236 THE AMERICAN BIOLOGY TEACHER, VOLUME 67, NO. 4, APRIL 2005

5 Figure 10. Chromosome replication. Athletic socks are useful for showing chromosome duplication. The binder clip is used to show the common centromere adjoining sister chromatids. Figure 11. Centromere positions. Metacentric, acrocentric, and telocentric centromeric locations are illustrated with binder clips. then switching position. This could be done using Velcro TM to hold the sock segments together prior to switching, but requires additional pre-instruction preparation and may be a bit cumbersome to include in most lessons along with other aspects of meiosis. The possible uses of socks in the classroom for illustrating meiosis and related topics seem unlimited. Even the colored yarns students used for creating models of crossing over and non-disjunction during meiosis in the general biology course are useful. In the genetics elective I ve used yarn to represent exons and introns, and to demonstrate splicing loops formed during RNA processing (Figure 12). As DNA segments, they are used to show selection and joining of VD and J segments during immunogenetics lessons. First, the students are provided an explanation of immunoglobulin (antibody) structure. They are shown the arrangement of the V (variable), D (diversity), J (joining), and C (constant) regions; then are shown how DNA segments containing differing V, D, and J information can be spliced together during immunoglobulin formation. Showing this helps the student visualize how this mechanism allows a great variety of antibodies to be generated. Conclusion I have found the aforementioned models a helpful instructional strategy, that engages the students in the lesson, and enriches and improves their understanding Figure 12. RNA processing. Intron segment (white yarn) splicing is illustrated. of both nuclear division and (human) cytogenetics. With modification, these same socks can be used to introduce students to cell division processes and to teach advanced students about more in-depth genetic phenomena. Anecdotal responses by students and feedback from colleagues support the benefit of using alternative materials, such as socks, to illustrate various genetic phenomena (especially the concepts of processes pertaining to meiotic cell division) at different class levels. As a teacher, I am always interested in novel pedagogical methods for presenting complex concepts and material in genetics that interest and engage the students, while helping them understand and recall the lesson. These are but a few ideas for teaching and engaging the students. Socks provide a hands-on method of instruction for both the students and the instructor. Students come to appreciate the importance of chromosome homology and the precision of the meiotic process. In addition to student statements of really MEIO-SOCKS 237

6 understanding (this) better, I believe their comprehension of this material is better than before I incorporated these approaches. In conclusion, this is a simple, inexpensive way to initiate discussions of inheritance, that is adaptable to a variety of courses, and provides a visual demonstration of our hereditary material. These approaches are a stimulating alternative to a passive learning process where the instructor (using textbook illustrations) simply describes the origin and behavior of homologous chromosomes during meiosis. These activities supplement traditional strategies for teaching meiosis and related genetic phenomena. Acknowledgments I thank my students and colleagues whose enthusiasm and feedback helped further develop these ideas and approaches and to Mr. Evans Calymios, Ph.D., for his assistance on the final formatting of these photographs. References Agostino, J. (2001). Modeling the classic Meselson & Stahl experiment. The American Biology Teacher, 63(5), Clark, D. & Mathis, P. (2000). Modeling mitosis & meiosis. A problem solving activity. The American Biology Teacher, 62(3), Gasser, S.M. (2002). Visualizing chromatin dynamics in interphase nuclei. Science, 296, Haws, L. & Bauer, S. (2001). A genetics game. The American Biology Teacher, 63(7), Ortiz, M.T., Taras, L. & Stavroulakis, A. (2000). The Hardy- Weinberg Equilibrium: some helpful suggestions. The American Biology Teacher, 62(1), Savage, J. (2000). Proximity matters. Science, 290, Taras, L., Ortiz, M.T. & Stavroulakis, A. (2000). Human cloning Let s discuss it. The American Biology Teacher, 61(5), Templin, M.A. & Fetters, M.K. (2002). A working model of protein synthesis using Lego TM building blocks. The American Biology Teacher, 64(9), Tobin, A. & Dusheck, J. (1998) Asking About Life. New York: Saunders College Publishing. Winterer, J. (2001). A lab exercise explaining Hardy-Weinberg Equilibrium and evolution effectively. The American Biology Teacher, 63(9), Wyn, M. & Stegink, S. (2000). Role-playing mitosis. The American Biology Teacher, 62(5), THE AMERICAN BIOLOGY TEACHER, VOLUME 67, NO. 4, APRIL 2005