1. Cell replication is a normal part of maintaining a healthy body.
2. Errors in cell division can lead to cancer, infertility, abortion, or the production of children with serious genetic disorders.
3. Understanding the details of cell division help us recognize the causes of these problems and help us develop better strategies to address the challenges they produce.

1. Explain why cell division is an essential part of life.
2. Describe the roles of cell division in living organisms.
3. Compare the cellular processes and cellular products of asexual and sexual reproduction.
4. Describe the basic structure of a chromosome. Explain how DNA is packaged into an elaborate, multilevel system of coiling and folding.
5. Explain how and when chromosomes are duplicated.
6. Describe the key events of each phase of the cell cycle.
7. Describe the key events of each phase of mitosis.
8. Compare the processes of cytokinesis in animal and plant cells.
9. Describe how the cell cycle control system normally functions and explain the consequences of errors in this system.
10. Explain how cancer cells are different from healthy cells of the body.
11. Distinguish between benign and malignant tumors. Explain what is meant by the “slash, burn, and poison” approach to treating cancer.
12. Explain how you can reduce your risks of developing cancer.
13. Distinguish between the following pairs of terms: sex chromosomes versus autosomes, somatic cells versus gametes, and diploid versus haploid cells .
14. Explain why the generation of haploid gametes is necessary in sexually reproducing organisms.
15. Compare the processes and products of meiosis I and meiosis II.
16. Compare the overall processes and products of meiosis and mitosis.
17. Explain how independent assortment of chromosomes during meiosis, random fertilization, and crossing over contribute to genetic diversity in offspring.
18. Describe the consequences of nondisjunction in autosomes and sex chromosomes.

19. Explain why asexual and sexual reproduction are adaptive.
anaphase asexual reproduction autosome benign tumor cancer cell cycle cell cycle control system cell division cell plate centromere centrosome chemotherapy chiasma chromatin chromosome cleavage furrow crossing over cytokinesis diploid
Down syndrome fertilization gamete genetic recombination haploid histone homologous chromosome interphase karyotype life cycle malignant tumor meiosis metaphase metastasis mitosis mitotic phase mitotic spindle nucleosome nondisjunction prophase radiation therapy sex chromosome sexual reproduction sister chromatid

somatic cell telophase trisomy 21 tumor zygote
a = not or without (asexual: type of reproduction not involving fertilization) ana = again (anaphase: mitotic stage when sister chromatids separate) auto = self (autosomes: the chromosomes that do not determine gender) centro = the center; mere = a part (centromere: the centralized region joining two sister chromatids) chemo = chemical (chemotherapy: type of cancer therapy using drugs that disrupt cell division) chiasm = cross-mark (chiasma: the sites where crossing over have occurred) chroma = colored (chromosome: DNA-containing structure) cyto = cell; kinet = move (cytokinesis: division of the cytoplasm) di = two (diploid: cells that contain two homologous sets of chromosomes) fertil = fruitful (fertilization: process of fusion of sperm and egg cell) gamet = a wife or husband (gamete: egg or sperm) haplo = single (haploid: cells that contain only one chromosome of each homologous pair) homo = like (homologous: like chromosomes that form a pair) inter = between (interphase: time when a cell metabolizes and performs its various functions) karyo = nucleus (karyotype: a display of the chromosomes of a cell) mal = bad or evil (malignant: type of tumor that migrates away from its site of origin) mei = less (meiosis: the division of a diploid nucleus into four haploid daughter nuclei) meta = between (metaphase: mitotic stage when the chromosomes are lined up in the cell’s middle) mito = a thread (mitosis: the division of a diploid cell into two diploid cells) non = not; dis = separate (nondisjunction: the result when paired chromosomes fail to separate) pro = before (prophase: mitotic stage when the nuclear membrane first breaks up) soma = body (somatic: body cells with 46 chromosomes in humans) telo = end (telophase: final mitotic stage when the nuclear envelope reforms) tri = three (trisomy 21: a condition in which a person has three number 21 chromosomes)

Asexual and Sexual Life Cycles
The Cell Cycle
Mitosis and Cytokinesis Animation
Mitosis and Cytokinesis Video
Causes of Cancer
Human Life Cycle
Meiosis Animation
Origins of Genetic Variation
Mitosis
Meiosis
Cytokinesis in Plant Cells
Genetic Variation: Independent Assortment
Genetic Variation: Fusion of Gametes
Mitosis and Meiosis
Mitosis
Meiosis
Comparing Mitosis and Meiosis
How Much Time Do Cells Spend in Each Phase of Mitosis?
How Can the Frequency of Crossing Over Be Estimated?
Discovery Channel Video: Cells
Discovery Channel Video: Fighting Cancer
Hydra Budding
Animal Mitosis (time-lapse)
Sea Urchin Embryonic Development (time-lapse)

Current Issues in Biology, volume 2 (ISBN 0-8053-7108-7)
Tumor-Busting Viruses
Current Issues in Biology, volume 5 (ISBN 0-321-54187-1)
Cancer Clues from Pet Dogs
Current Issues in Biology, volume 6 (ISBN 0-321-59849-0)
Your Cells Are My Cells
“Miserable Bastard” (from the album Zygote), John Popper
“Reproduction,” Grease 2 Soundtrack
“I Want Your Sex,” George Michael
“Two Kinds of Seagulls,” Tom Chapin
“Let’s Talk About Sex,” Salt-N-Pepa
“Love Will Tear Us Apart,” Joy Division
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction.
Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.

3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G
1
and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Campbell Essential
Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain

attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G
1
, the chromosomes have not replicated. But by G
2
, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile
(unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items.
Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?

2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes
(chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate.
After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 2 23 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 2 23 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22,181.66 every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years
(actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.
13. Suggested answer: Each chromosome lines up singly in mitosis; chromosome replication and the separation of sister chromatids occur independently for each horse or donkey chromosome. Therefore, mitotic divisions, starting with the zygote, are not impaired. In meiosis, however, homologous chromosomes must pair in prophase I. This pairing of homologous chromosomes cannot occur properly because horse and donkey chromosomes do not match in number or content. Further, when the homologous pairs line up in metaphase I of meiosis, there will be one extra chromosome, which will be unpaired.

14. Suggested answer: (a) metaphase, (b) anaphase, (c) telophase, (d) prophase.
15. Some issues and questions to consider: Could it be that less money is spent on prevention because effective prevention is so much cheaper? Or because prevention has been tried, and it does not work well? Are lifestyle changes the kind of measures that could benefit from a shift in resources? Is prevention an individual matter of avoiding exposure or a social matter of preventing exposure? How might the answer to this question shape prevention policy? If more money were devoted to prevention, how could it be used to encourage you or others to make lifestyle changes? Would prevention work better for younger or older people? Might older people, already exposed to cancer-causing agents, actually be harmed by a shift of resources to prevention?
16. Some issues and questions to consider: Is this an issue that needs to be legally regulated? If one person agrees to such an arrangement, should that person’s right to sell gametes be halted by laws against this process? Do you think the financial motivation interferes with a person’s normal judgment on an issue such as this? Sperm donation has virtually no risks to the donor, but egg donation can be very risky for the donor. Should there be separate rules for male and female gamete donation? Should eggs be considered more valuable based on the considerable risk to the donor?
1. Down syndrome is one of the few genetic conditions involving an alteration of chromosome number that allows fetal development to be completed. Nondisjunction of autosomes other than chromosome 21 can also occur but is rarely seen in full-term infants because of spontaneous abortions during fetal development. Why would a child with trisomy 21 be more likely to come to term than a child with, say, trisomy 3?
Suggested answer: Chromosome 21 is the smallest autosome, so it may be that having additional copies of the smaller number of genes on 21 is not lethal (though it does have detrimental effects). The larger the chromosome, the more genes it is likely to have; and the more additional copies of genes there are, the more likely it is that there will be resulting problems. Trisomy of large chromosomes, like chromosome 3, is more likely to be lethal to the developing fetus.
2. Suppose you read in the newspaper that a genetic engineering laboratory has developed a procedure for fusing two gametes from the same person (two eggs or two sperm) to form a zygote. The article mentions that an early step in the procedure prevents crossing over from occurring during the formation of the gametes in the donor’s body. The researchers are in the process of determining the genetic makeup of one of their new zygotes. Which of the following predictions do you think they would make? Justify your choice, and explain why you rejected each of the other choices. a. The zygote would have 46 chromosomes, all of which came from the gamete donor
(its one parent), so the zygote would be genetically identical to the gamete donor. b. The zygote could be genetically identical to the gamete donor, but it is much more likely that it would have an unpredictable mixture of chromosomes from the gamete donor’s parents.

c. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor’s parents. d. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor’s grandparents.
Suggested answers:
Answer a. No. For this to happen, all of the chromosomes of the two gametes that fused would have to represent a complete set of the donor’s maternal chromosomes (the ones that originally came from the donor’s mother) and a complete set of the donor’s paternal chromosomes (from the donor’s father). It is much more likely that the zygote would be missing one or more maternal chromosomes and would have an excess of paternal chromosomes, or vice versa.
Answer b. Correct. Consider what would have to happen to produce a zygote genetically identical to the gamete donor: The zygote would have to have a complete set of the donor’s maternal chromosomes and a complete set of the donor’s paternal chromosomes. The first gamete in this union could contain any mixture of maternal and paternal chromosomes, but once that first gamete was “chosen,” the second one would have to have one particular combination of chromosomes—the combination that supplies whatever the first gamete did not supply. So, for example, if the first three chromosomes of the first gamete were maternal, maternal, and paternal, the first three of the second gamete would have to be paternal, paternal, and maternal. The chance that all 23 chromosome pairs would be complementary in this way is only one in 2 23 (or one in 8,388,608). Because of independent assortment, it is much more likely that the zygote would have an unpredictable combination of chromosomes from the donor’s father and mother.
Answer c. No. First, the zygote could be genetically identical to the gamete donor (see b). Second, the zygote could not be identical to either of the gamete donor’s parents because the donor only has half the genetic material of each of his or her parents. For example, even if the zygote were formed by two gametes containing only paternal chromosomes, the combined set of chromosomes could not be identical to that of the donor’s father because it would still be missing half of the father’s chromosomes.
Answer d. No; see answer c.
3. Typical neurons are not able to perform mitosis after maturity. A patient has just been diagnosed with brain cancer and is undergoing treatment. Based on what you know about cell division and cancer, why does this seem unusual?
Suggested answer: Since cancer is unregulated cell division, it would seem unusual that brain cells would become cancerous since they are not supposed to be dividing at all.
Most likely, the cancer has metastasized from another location or originated in glial
(supporting) cells in the brain.
4. The recent availability of genetic tests for susceptibility to some types of cancer has led some patients to take extreme measures, such as having a mastectomy when there is a risk, though not a certainty, of developing breast cancer. If you were a doctor, how would you advise a patient found to be at high risk for breast cancer?
Some issues and questions to consider: Does the doctor have the responsibility of educating the patient about all of the preventive and diagnostic measures? Should the patient’s wishes always be followed, regardless of the outcome? How accurate are the testing measures for susceptibility?

5. As women age, the cells in their ovaries that ultimately produce eggs also age. The older a woman is when she becomes pregnant, the older that egg cell is and the more likely it is that chromosomal damage has occurred. If this is the case and the embryo has a chromosomal abnormality, the mother’s system may spontaneously abort; however, the aborting mechanism also becomes defective with age. This is one reason why older women are more likely to have children born with birth defects than younger women.
At menopause, the woman stops releasing eggs, which ends her fertility. Some postmenopausal women are now using reproductive technology to go to extreme measures to have their own biological child. How do you feel about the use of this technology?
Some issues and questions to consider: How risky is pregnancy at an older age? If the body has naturally stopped releasing eggs, should this be overridden by technology?
How likely is it that the child will have a birth defect? How expensive is the technology?