BIOLOGY 3RD EDITION BROOKER

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BROOKER BIOLOGY, 3e

CHAPTER 15: THE EUKARYOTIC CELL CYCLE, MITOSIS, AND MEIOSIS

WHERE DOES IT ALL FIT IN?

Chapter 15 begins a new conceptual theme by addressing the cell cycle and replication. It provides students the principles of asexual reproduction in prokaryotes and eukaryotes. It is important to briefly review the basic cell structure information in Chapter 4 before proceeding with Chapter 15. The information in Chapter 15 is crucial for students to understand the principles of sexual reproduction and embryology covered later in the book.

Synopsis

Eukaryotic cell division is more complicated than that of prokaryotic cells because the eukaryotic genome is larger and more complex. Eukaryotic chromosomes are linear structures composed of chromatin, mostly DNA and protein with a small amount of RNA. Eukaryotic DNA is a long double-stranded fiber. Every 200 nucleotides it coils around a core of eight histone polypeptides forming a nucleosome. The string of nucleosomes is further wrapped into supercoils. Heterochromatin is highly condensed chromatin while euchromatin is relatively uncondensed. Some portions of the DNA are permanently heterochromatic to prevent DNA expression; the remainder is uncondensed at the proper time to facilitate transcription.

The number of chromosomes in eukaryotic organisms varies widely from species to species. Human cells possess a diploid complement of 23 homologous pairs of chromosomes each with a characteristic appearance. Prior to cell division each homologue replicates producing two

Copyright © 2014 McGraw-Hill Education. All rights reserved. No reproduction or identical sister chromatids joined by a common centromere. The process of growth and division in a typical eukaryotic cell is called the cell cycle and is composed of five phases. The G1 phase is the cell’s primary growth phase while the genome is replicated during the S phase. During the G2 phase, various organelles are replicated, the chromosomes start to condense, and microtubules are synthesized. All of these are preparatory for mitosis or M phase. Actual cell division occurs in the final C phase, cytokinesis.

Education.

Cell cycle control is based on a check-point feedback system. When certain conditions at a checkpoint are met, the cell proceeds to the next stage of activity or division. Cyclin-dependent kinases (Cdk’s) and cyclins are intimately associated with these control processes. Unicellular organisms make independent decisions on whether or not to divide. Multicellular organisms must limit independent cell proliferation to maintain the integrity of the whole. Eukaryotes utilize various growth factors to do this. Disruption of these control mechanisms is characteristic of cancer.

Mitosis is a continuous process that is divided into four stages for ease of examination: prophase, metaphase, anaphase, and telophase. Much of the preparation for mitosis occurs during interphase, a collective stage that includes G1, S, and G2. Preparations include chromosome replication, centriole replication (in animals only), and tubulin synthesis. Chromatin condensation begins near the end of interphase and continues through prophase when individual chromosomes become visible. At the same time, the nuclear envelope breaks down and the centrioles of animal cells move apart. One set of microtubules assembles between the nucleolar organizing regions while another set grows outward from each centromere toward the poles. Metaphase begins when the pairs of sister chromatids align across the center of the cell at the metaphase plate. The end of this phase is signalled by the division of the centromeres. During anaphase, each chromatid moves toward the pole to which it is attached. Separation occurs when the central spindle fibers slide past one another, moving the poles farther apart. The chromatids also move toward the poles as the microtubules to which they are attached shorten. The nucleus begins to reform around the uncoiling chromosomes during telophase. The spindle apparatus breaks down and the nucleolus reappears as rRNA genes are again expressed.

There are significant differences in cytokinesis in animals and plants. Animal cells are pinched in two by a belt of constricting microfilaments at the cleavage furrow. Rigid plant cells are not easily deformed and divide from the inside outward. This expanding partition is called the cell plate. The final addition of cellulose to either side of the membrane results in two separate cells.

Meiosis and syngamy constitute a cycle of sexual reproduction. Fertilization would double the chromosome number of each subsequent generation except that the gametes possess only a haploid complement of DNA. Thus the resultant zygote inherits genetic material from both its father and its mother, in the case of humans, twenty-three chromosomes from each. Sexual reproduction produces offspring that are genetically different from either parent while asexual reproduction produces progeny that are genetically identical to the parent cell. The specific events of sexual reproduction varies from kingdom to kingdom. For example, in most unicellular eukaryotes, the individual cells function directly as gametes. In plants, specific haploid cells are produced by meiosis, these cells then divide by mitosis to form a multicellular haploid phase which further produces eggs and/or sperm. In animals special gamete-producing cells differentiate from the other somatic cells early on in development. Only these cells are able to undergo meiosis to create haploid eggs or sperm.

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Gamete-producing cells differentiate from somatic cells early in development. While they themselves are diploid, their products are haploid as a result of meiosis. Although meiosis and mitosis share many features, including microtubule formation, meiosis is unique for three reasons: synapsis, homologous recombination, and reduction division. During synapsis homologous chromosomes physically pair along their length. In homologous recombination genetic exchange, called crossing over, occurs between the homologues. Reduction division is the two separate rounds of nuclear division that occur in the remainder of the process. In the first division, homologous chromosomes pair, exchange material, and separate. No genetic replication occurs before the second division when the non-identical sister chromatids separate into individual gametes. Each division is composed of prophase, metaphase, anaphase, and telophase, additionally labeled I or II.

Some of the most important events of meiosis occur during prophase I. The ends of the sister chromatids attach to specific sites on the nuclear envelope. The attachment sites for the two homologues are near one another ensuring that each chromosome associates closely with its homologue. Each gene corresponds with its partner forming the synaptonemal complex. Certain genes are exchanged between homologues, an event called crossing over. The homologues are released from the membrane but remain tightly connected to one another. The homologues line up along the central plate of the cell during metaphase I. Only one face of each centromere is accessible to microtubule attachment, thus each homologue attaches to only one polar spindle fiber. The microtubules shorten at anaphase I and pull the homologues apart to opposite ends of the cell. Each pole ends up with a complete set of haploid chromosomes. Telophase I finishes division I, cytokinesis may or may not occur.

Meiosis II is essentially a mitotic process. During metaphase II, the still connected sister chromatids line up along their new metaphase plate with spindle fibers from each pole attached to each centromere. During anaphase II, the centromeres split and the sister chromatids are drawn to opposite poles. The result is four cells containing a haploid complement of genetic material.

Learning Outcomes

15.1 The Eukaryotic Cell Cycle

1. Describe the features of chromosomes and how sets of chromosomes are examined microscopically.

2. Outline the phases of the eukaryotic cell cycle.

3. Explain how cyclins and cdks work together to advance a cell through the eukaryotic cell cycle.

15.2 Mitotic Cell Division

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1. Describe how the replication of eukaryotic chromosomes produces sister chromatids.

2. Explain the structure and function of the mitotic spindle.

3. Outline the key events that occur during the phases of mitosis.

15.3 Meiosis and Sexual Reproduction

1. Describe the processes of synapsis and crossing over.

2. Outline the key events that occur during the phases of meiosis.

3. Compare and contrast mitosis and meiosis, focusing on key steps that account for the different outcomes of these two processes

4. Distinguish between the life cycles of diploid-dominant species, haploid-dominant species, and species that exhibit an alternation of generations.

15.4 Variation in Chromosome Structure and Number

1. Describe how chromosomes can vary in size, centromere location, and number.

2. Identify the four ways that the structure of a chromosome can be changed via mutation.

3. Compare and contrast changes in the number of sets of chromosomes and changes in the number of individual chromosomes.

4. Give examples of how changes in chromosome number affect the characteristics of animals and plants.

Concept Map

Concept mapping is a structured graphical presentation of the concepts covered in a particular topic. The following concept map represents the links between the information covered in this chapter. It is important to tell students to develop their own concept maps after covering the particular information covered in class.

Common Student Misconceptions

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 15 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

• Students believe that binary fission is the same as mitosis

• Students do not distinguish between the cell cycle and mitosis

• Students believe asexual reproduction is restricted to microorganisms only.

• Students conceptualize all DNA as being X-shaped

• Students do not distinguish between the terms chromatin and chromosomes

• Students believe that spindles work like rubber bands during replication

• Students are not aware that endosymbionts are attached to spindles

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• Students are not fully aware that mitochondria and chloroplasts self-replicate

• Students believe that asexual reproduction always produces identical offspring cells

• Students believe asexual reproduction results in weakness and sexual reproduction always produces stronger individuals

• Students think haploid cells have half the traits needed to make an organism

• Students have the idea that cancer is merely a condition of uncontrolled cell division

• Students believe that all tumors are cancerous

Instructional Strategy Presentation Assistance

It is more efficient to move by packing your belongings in boxes and bags than to move each item individually. Similarly, condensing the chromatin into discrete chromosomes makes it easier to separate them during mitosis.

Remember the order of mitotic stages via PMAT (or IPMAT if interphase is included). Any student named Matthew deserves apologies on this mnemonic!

Stress that the purpose of mitosis is to produce many identical copies of a cell.

Most students merely memorize when the nucleolus disappears and reappears. If they associate its presence with its function synthesizing rRNA, it is obvious when the transient organelle will be present and when it will be absent.

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 15.

Application

• Have students explain how drugs that alter cytoskeleton function would affect mitosis in animal cells.

• Have student explain why food poisoning is likely to occur if foods such as meats are sitting at room temperature for 30 to 60 minutes.

• Ask students why bacterial infections spread more quickly on the skin than yeast infections.

Analysis

• Ask students to explain what would happen to offspring cells if the centromeres did not separate easily during anaphase.

• Ask students to explain why diabetes, a condition in which glucose is not taken up readily by cells, slows down mitosis.

• Have students explain how amino acid deficiencies can affect the progression of the G1 phase of the cell cycle.

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Synthesis

• Ask students to think about the properties of a drug that would selectively harm cancer cells without causing death or injury to normal body cells undergoing cell division.

• Have students develop a rationale for the use of a chemical that causes telomeres, the tips of chromosomes, to shorten rapidly during mitosis.

• Ask students come up with a strategy that would inhibit binary fission without affecting the mitosis of microorganisms.

Evaluation

• Ask students to evaluate the effectiveness of an anticancer drug that inhibits the formation and growth of blood vessels.

• Ask students to support or debate the claim that nicotine, which affects cytoskeleton function, reduces the body’s ability to repair damaged body parts.

• Have the evaluate why using stem cell treatments that replace dead cells are more likely an effective treatment for repairing brain damage than for treating wounds to the skin.

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Visual Resources

1. Bring in a ball of yarn to simulate DNA as chromosomes and some unraveled yarn to represent DNA in chromatin form. Question the likelihood of knitting a scarf with the yarn in a ball. This is like trying to transcribe DNA as chromosomes. Also question the ease of separating two bunches of identically colored yarn when unraveled as compared to the same yarn when rolled into two separate balls.

2. In a small classroom, use clay or plastic foam and colored straws to represent chromosomes. In a large classroom with an overhead projector, cut rod-shaped chromosomes out of colored acetate. Make a second set to show chromatid replication during the S phase and hold the two chromatids together with overlapped post-it-note centromere circles. Cut similar-shaped, but different-colored chromosomes to show homologues.

3. Use colored beads and two sets of spaghetti to simulate chromosomes and spindle microtubules in a cell bounded by yarn. The pieces of spaghetti anchored at the poles push the yarn boundary apart as they slide past one another. Shorten the spaghetti attached to each chromosome to move the chromosomes to the poles. (One might want to use string instead of spaghetti, but the latter is more accurate.

4. The DNA content of bacteria can be illustrated using an audiocassette. The cassette represents a single bacterium. Pulling out all of the tape (without tearing it away from the cassette) represents the amount of uncoiled DNA in a single bacterium.

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Name That Phase Introduction

Laboratory sessions on animal and plant cell mitosis are often confusing adventures for students. In addition, it is difficult for instructors to troubleshoot every student’s microscope issues in large laboratory section. This demonstration assists students with recognizing the stages of mitosis before a laboratory session. It can also be used as a quick review strategy for tests the ask students to recognize or describe the stages of mitosis.

Materials

• Computer with internet access

• Downloaded PDF images found at the Jdenuno website: http://www.jdenuno.com/PDFfiles/Mitosis.pdf#search=%22mitosis%20images%22

• LCD projector

• Laser pointer

Procedure and Inquiry

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1. Show the low power image of the onion root tip slide. Ask students to name the structure and tell if the tissues making up the structure are growing or mature. Have them explain their answers.

2. Then show the close-ups of the dividing cells and use the laser pointer to select various cells at different stages of mitosis.

3. Ask the students to identify the stages and explain what features of the cells gave them a clue to their answers.

4. Show the low power image of the whitefish blastula slide. Ask students to name the structure and tell if the tissues making up the structure are growing or mature. Have them explain their answers.

5. Then show the close-ups of the dividing cells and use the laser pointer to select various cells at different stages of mitosis.

6. Ask the students to identify the stages and explain what features of the cells gave them a clue to their answers.

B. Modeling Cell Division Introduction

This fun activity asks students to be model of cell division using various craft and hobby materials. It reinforces retention of the cell features and cell events involved in binary fission and mitosis. Materials

• Small paper plates

• Scissors

• Assorted dried noodles & spaghetti

• Assorted color pipe cleaners

• Glue

• Colored markers or crayons

• Cellophane tape

• Wrapping twine

• Assorted buttons

Procedure & Inquiry

1.Have students break up into teams of two.

2.Assign them to a particular stage of mitosis or cell cycle

3.Tell them they must make a accurate model of the that stage of binary fission, mitosis or cell cycle

4.Have the students show the model to the class and explain each feature including the justification for using a particular craft or hobby material to represent a cell structure.

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