Lewis/Life - Teaching Techniques

Chapter 10 - Teaching Techniques

A. General Tips
Students generally really like this chapter because it has to do with the reproductive organs but they are often afraid of meiosis because they just didn't get it in high school.
With this entire unit (chapters 10-12) I urge you to stress reproductive commonality.
You might wish to copy and distribute to your students the Overview of Chapter Objectives flowchart found at the beginning of this Instructor's Manual Chapter.
B. Possible Approaches to the Chapter
You might wish to sketch a diagram of the different types of reproduction. Work from this diagram.
Throughout this chapter, I urge you to stress commonality. Lewis does a good job of bringing in numerous examples from different parts of the living world. Nevertheless, the representative organism is the human. This is good. But, there is sometimes a tendency for us to slight the rest of the living world and our students don't quite see that the "human principles" are applicable across the board.
Notice the number of gamet- words, oo- words, and sperm- words. What degree of technicality do you require of your students? Be certain they know what is expected of them in this area because it can be confusing to those who have never had it before.
Students often have trouble with meiosis because in many schools it is taught with mitosis and the students do not see meiosis as a totally separate event.
There are several ways you can successfully approach meiosis.
1. Sketch the process on the board from the duplicated DNA at mitotic interphase through the production of the four haploid cells. Do this WITHOUT labels and WITHOUT crossing over. Use colored chalk and state that you are giving a generic overview. After you are finished with the process, add the prophase, metaphase, anaphase, etc. labels. When the process makes sense, you can add crossing over.
2. As a variation on the above, start with the duplicated DNA but put on a hypothetical gene, making certain a different allele is on each chromatid. Name the gene (allele) -- such as the allele for Type A blood and the allele for Type B blood. Follow the genes through the meiotic process so that the students can see that (in this case) two of the gametes have the Type A and the other two gametes have the Type B allele.
3. Although this idea may sound somewhat "childish", it really does work -- even for college students. If there is room in your lecture hall, have the students walk through meiosis.

Start by appointing chromosomes, which are all milling around in one cell. Then duplicate the chromosomes; in other words, appoint partner chromatids. Literally have the students line up at the equator, migrate to poles, separate into new cells, reconvene as two new cells, rest briefly, line up again equatorially, migrate, and separate into four cells. Some students may laugh, but they will not forget the process. If you do this, make certain you know in advance how and where which "chromatid" will be going.

Regardless of how you approach meiosis, I do recommend teaching the conceptual overview before mentioning crossing over. For students with poor science backgrounds, some of these ideas are a bit overwhelming.

If your students are not doing a lab exercise on meiosis, suggest that they work through the steps at home with toothpicks. This could be an especially valuable assignment for students going into education. Suggest that they design a lab exercise for junior or senior high students using toothpicks to demonstrate meiosis. Toothpicks can be marked with felt pens.

C. Hints Related to Specific Chapter Topics

II. Sometimes students are confused by the alternation of generations concept. You will be coming back to this idea later in the course. Don't attempt to teach the whole process now. Instead, at this point, use it to demonstrate how even in diversity, there is a fundamental commonality in all types of sexual reproduction.
II.A. As a sidelight, you might note that conjugation in the bacterial world has some frightening genetic engineering implications. Think of what would happen if a gene coding for a deadly toxin were deliberately spliced into a bacterial cell that is a part of the normal flora. Also, it is a given that conjugation can be "trans-species." Think of the implications! E. coli, as shown in Figure 10.3, exhibits a classic "trans-species" conjugation as it can "mate" with various species of Salmonella, Shigella, Proteus, and Enterobacter.
II.B. Stress the importance of genetic diversity. You might ask why genetic diversity is important.
III.B. Lewis mentions that stimulating the clitoris leads to orgasm. Some of the present-day outcries against female genital mutilation center on cultures that routinely perform either clitoridectomies or clitoridotomies on young girls. Interestingly, orgasm is often still possible, even without the clitoris.
IV. Note on page 198 that Lewis says, "Meiosis is preceded by an interphase period where DNA replicates...." In the last chapter, if you drew a "to meiosis" arrow coming off the cell cycle at the S-G2 junction, return to that drawing now. Explain that certain cells, having duplicated their DNA in the normal fashion right there in the S phase, are now leaving the regular cell cycle to traverse the process of meiosis.
It is important to note that although there are some variations, meiosis is virtually universal; it is the same for people, trees, mushrooms, and unicellular algae, and everything in between. The biggest differences are usually just in the time frames. These time differences might include haploid versus diploid dominance (found in certain plants), the dikaryotic stage of certain fungi, or zygotic meiosis (such as is found in Chlamydomonas, {Figure 10.4}). Regardless, the cycle is still the same.
In some organisms, after the four haploid cells are produced, there may be one or more "quasi-mitotic" divisions so that the end result is eight (or more) haploid cells. The initial haploid number, however, is constant at four.
** See the notes above under Possible Approaches to the Chapter.
Table 10.2 summarizes the stages of meiosis. Note the term Interphase between meiosis I and meiosis II. This interphase is sometimes called interkinesis.
Some students will see Figures 10.9 and 10.10 one way; some will see it another. Be certain these figures make sense to you first.

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