CHAPTER 11. SEXUAL REPRODUCTION AND MEIOSIS THE ESSENTIALS

[Pages:10]CHAPTER 11. SEXUAL REPRODUCTION AND MEIOSIS

THE ESSENTIALS

Students need to know: the difference between asexual and sexual reproduction. the function of meiosis and fertilization in sexual reproduction. how chromosome number is reduced from diploid to haploid in the process of meiosis. the role of homologous chromosomes in meiosis. the genetic consequences of crossing over. the three key differences between mitosis and meiosis. how crossing over, independent assortment, and random fertilization increase genetic variation.

Key Terms

gametes somatic cells zygote fertilization syngamy

diploid haploid sexual reproduction meiosis I meiosis II

synapsis chiasma (chiasmata) independent assortment asexual reproduction parthenogenesis

Strategy

Class Time: The AP "Acorn Book" recommends devoting 25% of the course to the Heredity and Evolution unit which includes Chapters 11?25. This chapter begins the unit and sets the stage for understanding genetics and patterns of inheritance. Allowing two to three days for both lecture and lab should be sufficient for this introduction.

Below is a suggested schedule based on a year-long class meeting three 45-minute periods every two days:

? Lecture 1 (40?45 minutes): Meiosis ? Lecture 2 (40?45 minutes): Meiosis (continued) ? Lab 1 (40?45 minutes): AP Lab 3. Mitosis and Meiosis (required)

Approach:

It is terribly easy for students to confuse meiosis and mitosis--replicated chromosomes, dividing cells, phases with the same names--leading them to think of this as just another dance of the chromosomes. This is compounded by the fact that meiosis is usually taught directly after mitosis. Only to be further exacerbated by the fact that most students have learned about meiosis in earlier biology courses and may

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feel overly confident that they know all about it which leaves them less than fully attentive to the defining details.

Meiosis evolved later than mitosis and hence it is built on the same machinery; the similarities are not coincidental. Yet the role of meiosis is so radically different from mitosis that its unique function cannot be over-emphasized. You are laying the groundwork for students' understanding of genetics and evolution--the rest of the unit--in this opening chapter. Simply, the function of meiosis is to make haploid reproductive cells.

A reasonable segue to this chapter from mitosis is to walk students through the mental exercise of what would happen if sexual reproduction (joining of gametes) was attempted with diploid cells. It becomes readily apparent that chromosome number would double each generation. And this would quickly create an impossible situation. Even if the genetic ramifications aren't apparent to the students, you can point out that there quickly would not be enough room in the nucleus to hold all that DNA. The stage is now set for the necessity of a reduction process--reduction to the haploid number. So, if the cell has to halve the number of chromosomes, how are they divided up to support future life? It is important at this point to review the concept that a diploid cell has two copies of every chromosome--one from each parent-- and that these are referred to as homologous chromosomes. Therefore it is necessary to sort out the chromosomes during meiosis so that one copy of each homologous pair is passed on to the gametes during the reduction division. Drawing diagrams that show the chromosomes from each parent in different colors (as conventional as blue and pink) allows you to illustrate the recombination that results from this process. To state it directly, the chromosomes are sorted so that there is a random mix of paternal (blue ones) and maternal (pink ones) chromosomes in each gamete produced. Furthermore, crossing over creates hybrid individual chromosomes (chromosomes with part blue and part pink), thereby creating completely novel combinations and vastly increasing the variation. Clarifying these processes sets the stage for understanding inheritance, variation, and evolution in later chapters.

NOTE: Once students understand the alignment of homologous pairs during meiosis I, you can point out to them the problem that would occur if an organism has an odd number of chromosomes (mules, Down's syndrome humans). One chromosome would have no homologue to pair with, rendering these individuals sterile. This will be discussed again in evolution and speciation.

The next step is to walk through the phases of meiosis. At this point the best way to deal with the problem of the mitosis-meiosis confusion is head on. Clearly point out to students the pitfalls that they are approaching. The defining point is synapsis in Prophase 1 in which homologues pair up. Point out to students that this is never seen in mitosis. For demonstrations in front of the classroom, you can use pipe insulation tubes found at the local hardware store--a patch of Velcro can hook them together like a centromere and colored tape can be used to identify individual chromosomes of an homologous pair. Popit beads, as used in the AP Lab 3, are useful to demonstrate crossing over. Have students walk step-bystep through the stages of meiosis with pop-it beads; this will quickly reveal misunderstandings to the students themselves and to their teachers.

Anchor students with some key phrases that they can use as a repeatable chorus, so that they don't get lost in the details but rather concentrate on the main themes:

"The first division of meiosis separates homologous pairs."

"The second division of meiosis separates sister chromatids."

The lectures can conclude with a summary of these three unique features of meiosis (reviewed nicely in the animations included in the Digital Content Manager CD):

1. synapsis (pairing of homologous chromosomes)

2. homologous recombination (crossing over)

3. reduction division (diploid to haploid)

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The pivotal role of the first division of meiosis should be re-emphasized in the next chapter on Mendelian genetics. A thorough understanding of meiosis is necessary to grasp what occurs in Mendelian genetics, enabling students to understand genetics beyond mere letters in Punnett squares. It is amazing that Mendel was able to formulate his ideas without knowledge of either mitosis or meiosis.

Concept Map

sexual reproduction

joining of gametes

fertilization

recombination

diploid organisms need to produce

haploid cells

gametes

meiosis

zygote

creates variation

meiosis I

synapsis

crossing over

independent assortment

separates homologous

pairs

reduction division

creates variation

meiosis II

separates sister

chromatids

4 haploid cells

Student Misconceptions and Common Pitfalls

? First and foremost students will confuse mitosis and meiosis. Meiosis evolved after mitosis and therefore uses the same machinery, so it is easy to confuse them. It is helpful to repeatedly use the phrase "reduction division" so students embed that concept in the same place in their brain as meiosis. At each phase of meiosis clearly state what pitfall the students are likely teetering on.

a. At the start of each there is a replication of chromosomes which seems like an odd event for a process that is trying to reduce chromosome number. Again, remind students that meiosis evolved after mitosis so it has the remnants of mitotic events; it would also seem that this allows for the production of twice as may gametes which would be an advantage.

b. Prophase of mitosis and prophase I of meiosis I is where the two processes quickly diverge. Show illustrations of homologous pairs lined up in a tetrad and emphasize that this never happens in mitosis. To push students to delineate the differences give them the following diagram of a cell and ask them to (1) draw the next stage if the cell were undergoing mitosis (metaphase) and (2) the next stage if the cell were undergoing meiosis (metaphase 1). This will quickly show you their

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level of understanding.

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c. After metaphase I, be sure to walk students through the chromosome count (ploidy) of the cell. Ask them "How many chromosomes in the original cell?"... and then "How many chromosomes in each of these cells produced?"..."Oh, look, we have half the number of chromosomes: reduction division." That will emphasize the diploid to haploid change. It will also reinforce that each double-stranded chromosomes are still counted as one chromosome.

d. Point out that after telophase I, there is not another interphase and there is not another round of replication. It is helpful to remind students that condensed chromosomes cannot replicate. Meiosis II begins with the haploid set of double-stranded chromosomes.

e. Meiosis II is essentially a mitotic division: haploid cells to haploid cells. The sister chromatids of double-stranded chromosomes separate and produce four haploid cells.

? Students have to be reminded that crossing over occurs in prophase I and not when chromosomes are lined up on the metaphase plate in metaphase I.

? The misconceptions about chromosomes that plagued students in mitosis will show up in meiosis as well--and they will add new ones too. a. Since we cannot visualize chromosomes until they are condensed in mitosis or meiosis, many students think that chromosomes are double-stranded in non-dividing diploid cells. They have rarely seen illustrations of chromosomes otherwise. They also confuse double helix with doublestranded. Address this misunderstanding early and explicitly; it establishes a faulty foundation otherwise. b. Students may also think that condensed chromosomes are the default state of chromosomes in a non-dividing cell. This is not trivial and reveals fundamental misconceptions of the nature of chromosomes. If students think that DNA can be replicated and/or genes can be transcribed while a chromosome is condensed then it is difficult for them to fully understand DNA synthesis or gene regulation in later chapters. Address these potential misunderstanding now and re-emphasize them in those later chapters. c. Students may think that single-stranded chromosomes are characteristic of haploid cells and double-stranded chromosomes are characteristic of diploid cells. Address this misunderstanding early and explicitly. d. Students may mistakenly count double-stranded chromosomes as two chromosomes rather than as two sister chromatids in one chromosome. Repeatedly make chromosome counts in diagrams and in demonstration to exorcise this error; it creeps back in often. e. Students may mistakenly pair up paternal and maternal single-stranded chromosomes as sister chromatids to make double-stranded chromosomes. Be sure to have them draw diagrams with colored pencil or walk through phases of meiosis with colored pop-it beads to reveal these errors. f. Students may think that a single centromere joins sister chromatids or homologous pairs. Be sure that students do not draw double-stranded chromosomes as an X; this perpetuates this

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misconception of chromosome structure. Always draw them as two chromosomes side-by-side.

like this:

not like this:

? Students often fail to draw the connections between Mendelian genetics and the process meiosis. Point out segregation and independent assortment in meiosis I during this chapter, so that students have an early foothold on these concepts and then can hear them reiterated and reinforced in the next chapter.

Activity

Dance of the Meiosis Chromosomes Have students walk through meiosis with colored pop-it beads and magnetic centromeres (available through the major science supplies catalogues) to demonstrate their understanding of the events at each stage. Set up four chromosomes in a clear plastic cup as the nucleus. ? Be sure that you have two chromosomes of one color (red) as paternal chromosomes and two chromosomes of another color (yellow) as maternal chromosomes. ? Be sure that the two paternal chromosomes are each of different lengths so that you can clearly see if the student is moving them and matching them up correctly. The same goes for the maternal chromosomes.

? Give students a container of loose beads and magnetic centromeres as well as a large piece of colored construction paper as their cell. Have students walk through the phases of meiosis, demonstrating replication, crossing over, segregation, and independent assortment. At each step review the ploidy number of each cell (diploid vs. haploid) and the number of cells produced. Contrast this with mitosis. The colored pop-it beads offer a nice visual to show the great variation created by both independent assortment and crossing over.

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Web & Media Resources

? Movie: Why Sex? The fifth installment in the PBS series is entitled "Why Sex?" and discusses evolutionary hypotheses of why sexual reproduction would have evolved, including the "Red Queen" hypothesis as mentioned in the evolution chapters. This video can also fit in well with the evolution unit. The associated website is also full of resources.

? Meiosis Tutorial The Biology Project from University of Arizona is a great resource for biology tutorials and demonstrations. This site offers a series of lessons on meiosis which include text, graphics, animations, and quiz questions.

? John Kyrk Animations This site from John Kyrk offers great Flash animations illustrating biological processes.

? Mitosis vs. Meiosis This site offers a comparison of mitosis vs. meiosis from the NOVA movie, Life's Greatest Miracle, and includes a Flash movie illustrating the differences between the two processes.

On the Lighter Side

1. Alternative chromosomes Colored sports socks with knots as centromeres, pipe cleaners, or pipe insulation tubes are all quick and easy alternatives to pop it beads if you don't have the budget.

2. Meiosis, the Play It is always useful to have students demonstrate a biological process as a play to force them to work through the details and be creative. Assign the task one day and have them put on the play the next, so they can get props. The students themselves should be the chromosomes or parts of chromosomes. Do not let them just repeat the pop-it bead exercise with larger props.

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Multiple Choice Questions

1. In meiosis, all of the following are true EXCEPT a. the cell divides twice without a pause for growth b. the number of chromosomes is reduced in half c. DNA replicates twice d. sister chromatids separate in the second anaphase e. crossing over occurs in the first prophase

2. Meiosis produces a. two haploid daughter cells. b. two diploid daughter cells. c. four haploid daughter cells. d. four diploid daughter cells. e. four daughter cells with unequal number of chromosomes.

3. The segregation of genes (which produces genetic diversity) happens at what stage of meiosis? a. prophase I b. anaphase I c. cytokinesis I d. metaphase II e. anaphase II

4. The joining of male and female gametes is called a. synapsis. b. cytokinesis. c. parthenogenesis. d. fertilization. e. binary fission.

5. All of the following occur during synapsis EXCEPT a. DNA replication b. pairing of homologous chromosomes c. crossing over d. formation of chiasmata e. formation of the synaptonemal complex

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6. The life cycle of ferns alternates generations as diploid and haploid cells. After meiosis, haploid cells grow into a multicellular form. Then single cells are produced for the purpose of mating. These mating cells are called a. germ-line cells. b. spores. c. gametes. d. sporophytes. e. somatic cells.

7. How is the function of spindle microtubules different in metaphase I of meiosis as compared to mitosis? a. In meiosis, the spindle microtubules are attached to the ends of chromosomes, rather than to the centromeres. b. The spindle microtubules do not attach to chromosomes in meiosis. c. In meiosis, the spindle microtubules only attach to one of each pair of homologous chromosomes. d. In meiosis, the spindle microtubules attach to only one side of the centromere for each pair of sister chromatids. e. In meiosis, the spindle microtubules are not attached to centrioles.

8. The second cell division of meiosis is similar to mitosis EXCEPT a. The spindle microtubules bind to kinetochores and pull apart sister chromatids to opposite poles of the dividing cell. b. The nuclear membrane breaks down during prophase and re-forms during telophase. c. Centrioles guide the formation of spindle microtubules. d. After cytokinesis, the daughter cells have half the number of chromosomes as somatic cells. e. Chromosomes consisting of sister chromatids align alone the metaphase plate.

9. Germ-line cells are a. haploid. b. present only in females. c. capable of producing all of the different specialized cell types in the body. d. specialized for mitosis. e. specialized for meiosis.

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