In general biology, the subject of cell division is one of the most difficult for students. It introduces many new concepts, describes the behavior of chromosomes at different stages of cell division. Many students cannot imagine this whole complex mechanism, which causes difficulties in studying this topic. To facilitate the perception of the material, I have developed a dynamic model of cell division, showing the behavior of chromosomes during mitosis and meiosis. The model is easy to manufacture and gives a good effect in the assimilation of complex material.
The model can be made of thick cardboard. The mobility of the parts is ensured by sliding them on a nylon line with the help of threads that perform the function of the threads of the fission spindle. Details depicting homologous chromosomes are best made from colored cardboard. Color clearly demonstrates the fact that the diploid set of chromosomes of the cell is formed from haploid sets of male and female gametes. Each detail symbolizes one strand of DNA (chromatid). Details are strung on a nylon fishing line and are mounted on a cardboard base depicting a cage. At the poles of the "cage" you need to make holes through which to stretch the threads (threads of the spindle). The strands are attached to the chromosomes in the centromere region. To avoid tearing the cardboard when pulling the strings, small pieces from the ballpoint pen shaft must be inserted into the holes depicting the cell center. The model is ready, and it can be used in lessons.
I will give examples of the use of a dynamic model of the process of cell division in the study of “Cell division. Mitosis ”,“ Meiosis ”and“ Genetics. Monohybrid and dihybrid crosses. The law of gamete purity ”,“ Linked inheritance ”,“ Hereditary variability ”.
Theme “Cell division. Mitosis ”is preceded by the theme of the cell cycle and interphase. An important interphase process is replication - DNA doubling. Using the model, it can be represented as follows (Fig. 1 and 2):
In the first position, the chromatids are located one above the other, so the chromosomes look monochromatic. When parts are shifted along the fishing line relative to each other, two-chromatid chromosomes are obtained. In this case, it is necessary to clarify that the number of DNA is increasing, and not chromosomes, i.e. 2n2c> 2n4c. It is such a cell that begins division.
The first phase of mitosis - prophase - is characterized by the spiralization of chromosomes, the divergence of the cell center to the poles of the cell and the destruction of the nuclear membrane. It is not difficult to characterize this process with the help of drawings of a textbook and other visual aids. It is more difficult for children to imagine what a metaphase plate is, a spindle of division, how the number of chromosomes is preserved when they diverge to the poles of the cell. This is clearly demonstrated by the cell division model. The mitosis model consists of three parts (Fig. 3, 5, 6). Figure 3 shows the second phase of mitosis - metaphase. It shows the arrangement of chromosomes in the equatorial plane of the cell in one layer (metaphase plate) and the attachment of fission spindle strands to the centromeres of chromosomes (two strands to each chromosome). Chromosomes are suspended on two strands from opposite poles of the cell. This structure is called the spindle of division. Shortening the threads (dragging them through holes) leads to the separation of chromosomes and the divergence of chromatids. So anaphase is carried out.
Karyokinesis ends with the formation of the nuclear membrane and despiralization of chromosomes, i.e. early telophase occurs. After the formation of the cell plate and separation of the cytoplasm (cytokinesis), division ends with the formation of daughter cells. This is the last phase of mitosis - telophase (Fig. 6).
Comparing the first and third details of the model (Figs. 1 and 5), children themselves can draw a conclusion about the biological role of mitosis as a process of cell division, as a result of which hereditary material is preserved.
When studying the topic “Meiosis”, students need to be reminded how interphase doubling of DNA occurs (Figs. 1 and 2), as 2n4c cell enters meiosis. The meiosis model consists of 10 parts (Fig. 7, 8, 10, 12, 13, 15). In the first division of meiosis, special attention is paid to prophase. Conjugation and crossingover occurring in this phase (Figs. 8 and 9) lead to the formation of combined chromosomes, and therefore to the manifestation of combinational variability in organisms. The crossing-over process will once again be considered in the subject of “Linked inheritance” (genetics) and “Hereditary variability”.
In the metaphase of the first division (I) of meiosis, homologous chromosomes line up in the equatorial plane of the cell, but, in contrast to mitosis, in two rows.
The threads of the fission spindle are attached one from each pole to the chromosomes, therefore, in the anaphase of the first division (I), two-chromid chromosomes diverge to the poles, and the number of chromosomes is halved. As a result, two n2c cells form in the telophase of the first division (I).
The second division of meiosis is very similar to mitotic division. Prophase II occurs rapidly, metaphase II (Fig. 13) and anaphase II (Fig. 14) occur. And since two n2c cells begin the second division, then 4 haploid nc cells are formed in telophase II (Fig. 15).
All parts of the kit are shown sequentially as you study the material and are hung out on the board. When studying meiosis, it is necessary to demonstrate a mitosis model so that students can compare both processes.
The model of cell division helps students better understand the law of gamete purity and the cytological basis of monohybrid and dihybrid crosses in the topic “Genetics”. For this, alphabetic symbols of genes are attached to the chromosomes of the metaphase scheme of the second division: the dominant allele BUT and recessive allele but - in case of crossbreeding or Aa and BB - with dihybrid. Moreover, the arrangement of chromosomes in metaphase II may be different (Figs. 16 and 18). If the chromosomes are located as in Figure 16, then genes will fall into one cell BUT and ATand in the second - but and at .
If chromosomes in metaphase II are located as in Figure 18, then genes will fall into one cell BUT and atand in the second - but and AT .
This arrangement of chromosomes in the meoyza is equally probable, therefore, four types of gametes can be formed with the same probability in digeterozygotes: AB, Av, aB and av.
In the same way, the law of linked inheritance can be demonstrated, only genes BUT and B (A and at) are located in one of the homologous chromosomes, and but and in (a and AT) - in another. Students see how the genes that form the linkage group fall into one gamete (Figs. 20 and 21).
Maybe someone will say that in our age of computer technology, the process of cell division can be simulated using computer and multimedia. But in conditions of insufficient supply of computers to the rural school, a dynamic model of the process of cell division helps me more easily explain complex topics, and for students to better understand them.