Laura Coronado Bio 10 Chapter 8

Laura Coronado Bio 10 Chapter 8
Laura Coronado Bio Chapter 8

Biology and Society: Rain Forest Rescue
Endangered species of plants that normally reproduce sexually can be propagated by asexual reproduction. Cell division is at the heart of organisms reproduction, whether by sexual or asexual means. Laura Coronado Bio Chapter 8

WHAT CELL REPRODUCTION ACCOMPLISHES
May result in the birth of new organisms More commonly involves the production of new cells When a cell undergoes reproduction, or cell division, two “daughter” cells are produced that are genetically identical to each other and to the “parent” cell. Laura Coronado Bio Chapter 8

Cell Division Cell division plays important roles in the lives of organisms. Replaces damaged or lost cells Permits growth Allows for reproduction Laura Coronado Bio Chapter 8

FUNCTIONS OF CELL DIVISION Cell Replacement Growth via Cell Division Figure 8.1a Three functions of cell division. (Part 1) Colorized TEM LM Human kidney cell Early human embryo Laura Coronado Bio Chapter 8 Figure 8.1a

Asexual Reproduction Single-celled organisms reproduce by simple cell division There is no fertilization of an egg by a sperm The parent and its offspring have identical genes. Mitosis is the type of cell division responsible for: Asexual reproduction Growth and maintenance of multicellular organisms Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces. Growing a new plant from a clipping Laura Coronado Bio Chapter 8

FUNCTIONS OF CELL DIVISION Asexual Reproduction Figure 8.1b Three functions of cell division. (Part 2) LM Amoeba Sea stars African Violet Laura Coronado Bio Chapter 8 Figure 8.1b

Sexual Reproduction Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis. Thus, sexually reproducing organisms use: Meiosis for reproduction Mitosis for growth and maintenance Laura Coronado Bio Chapter 8

Figure 8.3 A plant cell just before division (colored by stains). LM Chromosomes Laura Coronado Bio Chapter 8 Figure 8.3

Chromosomes Chromosomes: Are made of chromatin, a combination of DNA and protein molecules Are not visible in a cell until cell division occurs Before a parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the organism’s DNA. During cell division, each daughter cell receives one set of chromosomes. Laura Coronado Bio Chapter 8

Number of chromosomes in body cells Species Indian muntjac deer 6 Koala 16 Opossum 22 Giraffe 30 Mouse 40 Human 46 Figure 8.2 The number of chromosomes in the cells of selected mammals. Duck-billed platypus 54 Buffalo 60 Dog 78 Red viscacha rat 102 Laura Coronado Bio Chapter 8 Figure 8.2

Eukaryotic Cell Genetic Information
Most genes are located on chromosomes in the cell nucleus A few genes are found in DNA in mitochondria and chloroplasts Each chromosome contains one very long DNA molecule, typically with thousands of genes. The number of chromosomes depends on the species. The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding. Histones are proteins used to package DNA. Nucleosomes consist of DNA wound around histone molecules. Laura Coronado Bio Chapter 8

DNA double helix Histones “Beads on a string” TEM Nucleosome Figure 8.4 DNA packing in a eukaryotic chromosome. Tight helical fiber Looped domains Duplicated chromosomes (sister chromatids) TEM Centromere Figure 8.4 Laura Coronado Bio Chapter 8

Chromosomes Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids. Sister chromatids are joined together at a narrow “waist” called the centromere. When the cell divides, the sister chromatids separate from each other. Once separated, each chromatid is: Considered a full-fledged chromosome Identical to the original chromosome Laura Coronado Bio Chapter 8

Chromosome (one long piece of DNA) Centromere Sister chromatids Figure UN 8.2 Summary: Duplicated chromosome Duplicated chromosome Laura Coronado Bio Chapter 8 Figure 8.UN2

Chromosome duplication Sister chromatids Figure 8.5 Duplication and distribution of a single chromosome. Chromosome distribution to daughter cells Laura Coronado Bio Chapter 8 Figure 8.5

The Cell Cycle A cell cycle is the orderly sequence of events that extend from the time a cell is first formed from a dividing parent cell to its own division into two cells. The cell cycle consists of two distinct phases: Interphase The mitotic phase Laura Coronado Bio Chapter 8

Interphase Most of a cell cycle is spent in interphase. During interphase, a cell: Performs its normal functions Doubles everything in its cytoplasm Grows in size Laura Coronado Bio Chapter 8

S phase (DNA synthesis; chromosome duplication)
Interphase: metabolism and growth (90% of time) G1 G2 Mitotic (M) phase: cell division (10% of time) Figure 8.6 The eukaryotic cell cycle. Cytokinesis (division of cytoplasm) Mitosis (division of nucleus) Laura Coronado Bio Chapter 8 Figure 8.6

Mitosis The mitotic (M) phase includes two overlapping processes: Mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei Cytokinesis, in which the cytoplasm is divided in two Laura Coronado Bio Chapter 8

Mitosis and Cytokinesis
Mitosis consists of four distinct phases: (A) Prophase (B) Metaphase (C) Anaphase (D) Telophase Cytokinesis typically: Occurs during telophase Divides the cytoplasm Is different in plant and animal cells Laura Coronado Bio Chapter 8

INTERPHASE PROPHASE Centrosomes (with centriole pairs) Early mitotic spindle Fragments of nuclear envelope Centrosome Chromatin Centromere Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Spindle microtubules Figure 8.7a Cell reproduction: A dance of the chromosomes. (Part 1) LM Laura Coronado Bio Chapter 8 Figure 8.7.a

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Nuclear envelope forming Cleavage furrow Spindle Daughter chromosomes Figure 8.7b Cell reproduction: A dance of the chromosomes. (Part 2) Laura Coronado Bio Chapter 8 Figure 8.7b

Mitosis and Cytokinesis
During mitosis the mitotic spindle, a football-shaped structure of microtubules, guides the separation of two sets of daughter chromosomes. Spindle microtubules grow from two centrosomes, clouds of cytoplasmic material that in animal cells contain centrioles. Laura Coronado Bio Chapter 8

SEM Cleavage furrow Cleavage furrow Contracting ring of microfilaments Figure 8.8a Cytokinesis in animal cells. Daughter cells Laura Coronado Bio Chapter 8 Figure 8.8a

Wall of parent cell Cell plate forming Daughter nucleus LM Vesicles containing cell wall material Cell wall Cell plate New cell wall Figure 8.8b Cytokinesis in plant cells. Daughter cells Laura Coronado Bio Chapter 8 Figure 8.8b

Figure 8.10 The varied products of sexual reproduction. Laura Coronado Bio Chapter 8 Figure 8.10

Homologous Chromosomes
Different individuals of a single species have the same number and types of chromosomes. A human somatic cell: Is a typical body cell Has 46 chromosomes A karyotype is an image that reveals an orderly arrangement of chromosomes. Homologous chromosomes are matching pairs of chromosomes that can possess different versions of the same genes. Laura Coronado Bio Chapter 8

Pair of homologous chromosomes LM Centromere Figure 8.11 Pairs of homologous chromosomes in a male karyotype. Sister chromatids One duplicated chromosome Laura Coronado Bio Chapter 8 Figure 8.11

Human Chromosomes Humans have: Two different sex chromosomes, X and Y Twenty-two pairs of matching chromosomes, called autosomes Humans are diploid organisms in which: Their somatic cells contain two sets of chromosomes Their gametes are haploid, having only one set of chromosomes Laura Coronado Bio Chapter 8

Gametes and the Life Cycle of a Sexual Organism
The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next. Laura Coronado Bio Chapter 8

Haploid gametes (n  23) Egg cell n n Sperm cell MEIOSIS FERTILIZATION Multicellular diploid adults (2n  46) Diploid zygote (2n  46) Figure 8.12 The human life cycle 2n MITOSIS and development Key Haploid (n) Diploid (2n) Laura Coronado Bio Chapter 8 Figure 8.12

Meiosis Humans are diploid organisms in which: Their somatic cells contain two sets of chromosomes Their gametes are haploid, having only one set of chromosomes In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote. Sexual life cycles involve an alternation of diploid and haploid stages. Meiosis produces haploid gametes, which keeps the chromosome number from doubling every generation. Laura Coronado Bio Chapter 8

Homologous chromosomes separate. Chromosomes duplicate. Sister chromatids separate. Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Figure 8.13 How meiosis halves chromosome number. (Step 3) INTERPHASE BEFORE MEIOSIS MEIOSIS I MEIOSIS II Laura Coronado Bio Chapter 8 Figure

The Process of Meiosis In meiosis: Haploid daughter cells are produced in diploid organisms Interphase is followed by two consecutive divisions, meiosis I and meiosis II Crossing over occurs Laura Coronado Bio Chapter 8

MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Sites of crossing over Microtubules attached to chromosome Sister chromatids remain attached Spindle Sister chromatids Nuclear envelope Centromere Pair of homologous chromosomes Figure 8.14a The stages of meiosis. (Part 1) Chromatin Homologous chromosomes pair up and exchange segments. Pairs of homologous chromosomes line up. Pairs of homologous chromosomes split up. Chromosomes duplicate. Laura Coronado Bio Chapter 8 Figure 8.14a

MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Haploid daughter cells forming Figure 8.14b The stages of meiosis. (Part 2) Two haploid cells form; chromosomes are still doubled. During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes. Laura Coronado Bio Chapter 8 Figure 8.14b

Figure 8.14bc The stages of meiosis. (Part 2) Metaphase II in a lily cell. LM Laura Coronado Bio Chapter 8 Figure 8.14bc

Review: Comparing Mitosis and Meiosis
In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase. The number of cell divisions varies: Mitosis uses one division and produces two diploid cells Meiosis uses two divisions and produces four haploid cells All the events unique to meiosis occur during meiosis I, while meiosis II is the same as mitosis since it separates sister chromatids. Laura Coronado Bio Chapter 8

(before chromosome duplication)
MITOSIS MEIOSIS Prophase Prophase I MEIOSIS I Chromosome duplication Chromosome duplication Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) 2n  4 Homologous chromosomes come together in pairs. Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes align at the middle of the cell. Homologous pairs align at the middle of the cell. Anaphase Telophase Anaphase I Telophase I Chromosome with two sister chromatids Sister chromatids separate during anaphase. Figure 8.15 Comparing mitosis and meiosis Homologous chromosomes separate during anaphase I; sister chromatids remain together. Haploid n  2 2n 2n Daughter cells of meiosis I Daughter cells of mitosis MEIOSIS II Sister chromatids separate during anaphase II. n n n n Daughter cells of meiosis II Figure 8.15 Laura Coronado Bio Chapter 8

Independent Assortment of Chromosomes
When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance. Every chromosome pair orients independently of the others during meiosis. For any species the total number of chromosome combinations that can appear in the gametes due to independent assortment is: 2n where n is the haploid number. For a human: n = 23 223 = 8,388,608 different chromosome combinations possible in a gamete Laura Coronado Bio Chapter 8

POSSIBILITY 1 POSSIBILITY 2 Metaphase of meiosis I Metaphase of meiosis II Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I. (Step 3) Gametes Combination a Combination b Combination c Combination d Laura Coronado Bio Chapter 8 Figure

Random Fertilization A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote. If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion, different possible chromosome combinations. Laura Coronado Bio Chapter 8

Figure 8.17 The process of fertilization: a close up view. Laura Coronado Bio Chapter 8 Figure 8.17

Crossing Over In crossing over: Homologous chromosomes exchange genetic information Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs Laura Coronado Bio Chapter 8

Prophase I of meiosis Duplicated pair of homologous chromosomes Homologous chromatids exchange corresponding segments. Chiasma, site of crossing over Metaphase I Spindle microtubule Sister chromatids remain joined at their centromeres. Metaphase II Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5) Gametes Recombinant chromosomes combine genetic information from different parents. Recombinant chromosomes Laura Coronado Bio Chapter 8 Figure

The Process of Science: Do All Animals Have Sex?
Observation: No scientists have ever found male bdelloid rotifers, a microscopic freshwater invertebrate. Hypothesis: Bdelloid rotifers have thrived for millions of years using only asexual reproduction. Prediction: Bdelloid rotifers would display much more variation in their homologous pairs of genes than most organisms. Laura Coronado Bio Chapter 8

The Process of Science: Do All Animals Have Sex?
Experiment: Researchers compared sequences of a particular gene in bdelloid and non-bdelloid rotifers. Results: Non-bdelloid sexually reproducing rotifers had nearly identical homologous genes Bdelloid asexually reproducing rotifers had homologous genes that differed by 3.5–54%. Conclusion: Bdelloid rotifers have evolved for millions of years without any sexual reproduction. 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 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 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, 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 upon 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. Laura Coronado Bio Chapter 8

LM Figure 8.19 A bdelloid rotifer. Laura Coronado Bio Chapter 8 Figure 8.19

How Accidents during Meiosis Can Alter Chromosome Number
In nondisjunction, the members of a chromosome pair fail to separate during anaphase, producing gametes with an incorrect number of chromosomes. Nondisjunction can occur during meiosis I or II. If nondisjunction occurs, and a normal sperm fertilizes an egg with an extra chromosome, the result is a zygote with a total of 2n + 1 chromosomes. If the organism survives, it will have an abnormal number of genes. Laura Coronado Bio Chapter 8

NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II Meiosis I Nondisjunction: Pair of homologous chromosomes fails to separate. Meiosis II Nondisjunction: Pair of sister chromatids fails to separate. Gametes Figure 8.20 Two types of nondisjunction. (Step 3) Number of chromosomes n  1 n  1 n – 1 n – 1 n  1 n – 1 n n Abnormal gametes Abnormal gametes Normal gametes Laura Coronado Bio Chapter 8 Figure

Abnormal egg cell with extra chromosome n  1 Figure 8.21 Fertilization after nondisjunction in the mother. Normal sperm cell Abnormal zygote with extra chromosome 2n  1 n (normal) Laura Coronado Bio Chapter 8 Figure 8.21

Down Syndrome Down Syndrome: Is also called trisomy 21 Is a condition in which an individual has an extra chromosome 21 Affects about one out of every 700 children The incidence of Down Syndrome increases with the age of the mother. 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 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 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, 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 upon 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. Laura Coronado Bio Chapter 8

LM Figure 8.22 Trisomy 21 and Down syndrome. Chromosome 21 Laura Coronado Bio Chapter 8 Figure 8.22

Infants with Down syndrome
90 80 70 60 Infants with Down syndrome (per 1,000 births) 50 40 30 20 Figure 8.23 Maternal age and Down syndrome. 10 20 25 30 35 40 45 50 Age of mother Laura Coronado Bio Chapter 8 Figure 8.23

Abnormal Numbers of Sex Chromosomes
Nondisjunction can also affect the sex chromosomes. Laura Coronado Bio Chapter 8

Table 8.1 Abnormalities of Sex Chromosome Number in Humans Laura Coronado Bio Chapter 8 Table 8.1

Evolution Connection: The Advantages of Sex
Asexual reproduction conveys an evolutionary advantage when plants are: Sparsely distributed Superbly suited to a stable environment Sexual reproduction may convey an evolutionary advantage by: Speeding adaptation to a changing environment Allowing a population to more easily rid itself of harmful genes Laura Coronado Bio Chapter 8

Figure 8.24 Sexual and asexual reproduction. Laura Coronado Bio Chapter 8 Figure 8.24

Cancer Cells: Growing Out of Control
Normal plant and animal cells have a cell cycle control system that consists of specialized proteins, which send “stop” and “go-ahead” signals at certain key points during the cell cycle. 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 G1and 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 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 G1, the chromosomes have not replicated. But by G2, 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. Laura Coronado Bio Chapter 8

What Is Cancer? Cancer is a disease of the cell cycle. Cancer cells do not respond normally to the cell cycle control system. Cancer cells can form tumors, abnormally growing masses of body cells. The spread of cancer cells beyond their original site of origin is metastasis. Malignant tumors can: Spread to other parts of the body Interrupt normal body functions A person with a malignant tumor is said to have cancer. Laura Coronado Bio Chapter 8

Lymph vessels Tumor Blood vessel Glandular tissue A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Metastasis: Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 8.9 Growth and metastasis of a malignant tumor of the breast. Laura Coronado Bio Chapter 8 Figure 8.9

Cancer Treatment Cancer treatment can involve: Radiation therapy, which damages DNA and disrupts cell division Chemotherapy, which uses drugs that disrupt cell division Laura Coronado Bio Chapter 8

Cancer Prevention and Survival
Certain behaviors can decrease the risk of cancer: Not smoking Exercising adequately Avoiding exposure to the sun Eating a high-fiber, low-fat diet Performing self-exams Regularly visiting a doctor to identify tumors early Laura Coronado Bio Chapter 8

Distribution via mitosis Duplication of all chromosomes Figure UN 8.1 Summary: Cell division Genetically identical daughter cells Laura Coronado Bio Chapter 8 Figure 8.UN1

DNA synthesis; chromosome duplication chromosome duplication
S phase DNA synthesis; chromosome duplication Interphase Cell growth and chromosome duplication G1 G2 Mitotic (M) phase Figure UN 8.3 Summary: The cell cycle Genetically identical “daughter” cells Cytokinesis (division of cytoplasm) Mitosis (division of nucleus) Laura Coronado Bio Chapter 8 Figure 8.UN3

Human Life Cycle Haploid gametes (n  23) Key n Haploid (n) Egg cell Diploid (2n) n Sperm cell MEIOSIS FERTILIZATION Figure UN 8.4 Summary: Human life cycle Diploid zygote (2n  46) Male and female diploid adults (2n  46) 2n MITOSIS Laura Coronado Bio Chapter 8 and development Figure 8.UN4

MITOSIS MEIOSIS Parent cell (2n) Parent cell (2n) MEIOSIS I Chromosome duplication Pairing of homologous chromosome Chromosome duplication Crossing over Figure UN 8.5 Summary: Comparing Mitosis and Meiosis Daughter cells 2n 2n MEIOSIS II n n n n Daughter cells Laura Coronado Bio Chapter 8 Figure 8.UN5

LM (a) (b) (c) Figure UN 8.6 Question 14: Slide of onion root tip (d) Laura Coronado Bio Chapter 8 Figure 8.UN6

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