The purpose of this activity is to model the formation of homologous chromosomes and the crossing over realized in meiosis I cell division. The model established through the activities conducted will allow students to visualize homologous chromosomes and the formation of crossing over among them. The model will help students to understand how homologous chromosomes occur and how crossing over is realized between homologous chromosomes whose chromatids are not sisters. The developed model is found to be an effective tool in teaching crossing over.
Constructivism is a theory of learning which claims that students construct knowledge rather than merely receive and store knowledge transmitted by the teacher (Ben-Ari, 1998, p. 1). According to constructivist educators, students need to be actively involved in the instruction process so that they understand the concepts and meaningful learning is achieved. Students need to encounter real-life events or be provided with real-life models in a teaching environment to facilitate learning in a constructivist way (Pringle 2004, p. 30). Biology concepts are abstract, and it is difficult to visualize and construct these concepts. Thus, by using a constructivist learning environment, students connect their prior knowledge and achieve meaningful learning without memorizing concepts.
Models can help students visualize abstract concepts. Models are effective for reconstructing and internalizing concepts so students can explain the concepts to others (Treagust et al., 2002). Güneᗱ (2012) found out that students who used origami for modeling nucleic acids understood these concepts and experienced fewer misconceptions related to the concepts. In Emre and Bahᗱi's (2006, p. 71) study, students indicated that crossing over realized in meiosis occurs among homologous chromosomes whose chromatids are sisters. In the same study, students also revealed misconceptions when explaining homologous chromosome, chromatids, and crossing over. Teaching students about homologous chromosomes and crossing over involves very abstract concepts. It is difficult for students to visualize how homologous chromosomes come together, how crossing over occurs between homologous chromosomes whose chromatids are not sisters, and how gene replacement occurs.
For these reasons, the purpose of the activity described here is to model the formation of homologous chromosomes and their crossing over, the abstract topics found in the eighth-grade Science and Technology lesson's “Cell division and heredity” unit. Thus, eighth-grade students actively participate in a Science and Technology lesson in accordance with a constructivist approach to learn these processes in a meaningful way. This activity is appropriate for eighth-grade students because students at this grade levels like to play with models.
The activity was carried out under the framework of the following goals:
To teach how homologous chromosomes come together in meiosis;
To elicit students' observations that homologous chromosomes come from parents;
To raise students' awareness that crossing over is realized among homologous chromosomes;
To raise students' awareness that crossing over occurs between homologous chromosomes whose chromatids are not sisters; and
To make abstract concepts concrete.
Homologous chromosomes: A pair of chromosomes of the same length, centromere position, and staining pattern, each of which possesses genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother. Also called “homologs” or a “homologous pair.” (Campbell et al., 2008, p. G-18)
Meiosis I cell division: The first division of the two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell. (Campbell et al., 2008, p. G-22)
Sister Chromatids: Either of two copies of a duplicated chromosome attached to each other by proteins at the centromere and, sometimes, along the arms. While joined, two sister chromatids make up one chromosome. Chromatids are eventually separated during mitosis or meiosis II. (Campbell et al., 2008, p. G-34)
Sets of dolls, two each (one for mother, one for father), made of the same material, such as plastic. Removable arms and legs are crucial on this type of doll. For a group of four students, one set is sufficient.
Face paint in blue and pink
Sheets of colorful paper (10 × 5cm)
Water-based glue-stick or liquid glue
Time Requirement: 80 minutes, 2 lesson hours
Students need to be careful while using scissors. There is a potential for eye splash incidents, and students must wear safety goggles while painting. These dyes are water-based, so they are not harmful to the skin, but could be harmful if they are ingested or come into contact with the eyes.
Targeted Grade Level
This activity is designed for eighth-graders because, in our country, the topics of crossing over and homologous chromosomes are found in eighth-grade science curriculum and secondary school biology curriculum. However, these topics could be found in other elementary grades curricula in other countries. Thus, the activity could be used for elementary level students if the topics of crossing over and homologous chromosomes are found in elementary school curriculum.
Note: The teacher may not be able to obtain enough dolls for the class. In this case, students will be required to bring their own dolls to class for this activity; the only requirement is that they must have removable legs and arms. Each pair of dolls have to be identical, but a pair could be different from another pair. With removable arms and legs and the teacher's help, the dolls will likely not be damaged.
Setting the Stage
To get students' attention, wonder and engage them in the introductory part of the course, the students are shown photographs of parents and children, and asked what characteristics children inherit from their fathers and mothers (Figure 1). The purpose of showing Figure 1 is to motivate students and understand the connection between homologous chromosomes and their similarity with their relatives. Questions for students with Figure 1 are: Are these people related or not? Why do you think they are relatives? Which characteristics of the boy resembles his father? Do you resemble your parents? Which characteristics of yours resemble to your parents? And finally: Why do you look like your parents? The students name some of the characteristics people receive from their parents.
Following this, the teacher asks students why children are not identical to their parents, to increase students' sense of wonder. By asking these questions, the teacher will help students to connect what they already know with the concept of homologous chromosomes and crossing over. Thus, students connection their daily life experiences with these concepts. By understanding these concepts, students comprehend the underlying information related to the resemblance between children and their parents. Thus, the teacher helps students to construct crossing over and daily life, using the correct model for elucidating this process.
Creating the Model
The teacher states that one doll represents the mother, and the other, the father. Then, the teacher instructs the students to dress the dolls, leaving only legs and arms uncovered. The teacher tells them to paint the arms and legs of one doll pink (Figures 2 and 3) and the other doll blue, for the mother and father, respectively. Students are warned to be careful while painting the dolls.
Students are asked what genetic concept the Figure 3 resemblance. Students stated that pink legs and arms represent maternal chromosomes, while blue ones represent paternal chromosomes. The teacher should help students to understand that chromosomes form tetrads (four chromatids), which come from the mother and father. Students should understand that tetrads are composed of a pair of homologous chromosomes. Students will be able to grasp the fact that a pair of homologous chromosomes bear the characteristics of the chromosomes of both mother and father. The teacher should guide students to understand that the chromatids that form the chromosome of the mother are sisters, as are the chromatids that form the chromosome of the father. Painting the mother and the father dolls in different colors helps the students to better understand that the chromatids from the mother and father are not sisters. Thus, students understand that which chromatids are sisters to each other and which are not.
The next stage involves explaining how crossing over occurs between chromosomes that are not sisters. Students are asked, What does “exchange of genetic material” mean? Brainstorming begins. Students are instructed to change the positions of the arms and legs of the mothers and fathers. Some students attempted, for example, to replace an arm with a leg, or a left arm with a right arm, but were unable to manage this. Thus, the students can conclude that there would not be any genetic exchange in these cases. As seen in Figure 4, students will understand that exchanges between sister chromatids can only be made between corresponding parts. Thus, students understood that gene exchanges are enabled with the help of crossing over, which contributes to diversity among living beings. Therefore, the teacher helps students to construct the link between homologous chromosomes and the resemblance between themselves and their parents.
Each student is given two sheets of paper in different colors and asked to draw homologous chromosomes. Then they are given scissors and glue, and instructed to show the crossing over between chromosomes (Figure 5). This activity could be extended via demonstration with Legos. Two chromosomes in the shape of an H could be built from the two different colors. Then exchange the Legosacross the cross-bar of the H. In line with the crossing over process, some parts can be taken from homologous chromosomes whose chromatids are not sisters, which leads to a change in the gene structure. With each crossing over process, the gene structure of the homologous chromosomes changes; thus, complete resemblance among the members of species is impossible due to the changes in gene structure. Thus, each person's identity must be different. Students' work is shown in Figure 6.
By using this activity, almost all of the students understood the meiosis I and crossing over and were able to summarize the process. In the activity, students learned that in the beginning of meiosis I, homologous chromosomes from the mother and father come together. Then, at the end, the homologous chromosomes separate. This model facilitates the teaching of homologous chromosomes more easily and helps to prevent misconceptions. The complicated and abstract structures were rendered simple, understandable, and concrete. In books, crossing over must be explained through drawings, which makes it more difficult for students to understand this abstract topic. The activity performed in the lesson through models makes it easier for students to understand difficult topics and observe the process. The models do not totally reflect reality, but they do facilitate understanding and pave the way for meaningful learning and visualization of the concepts. All of the students liked this activity and enjoyed the class. Students have chance to play with models and learn about crossing over.