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Cell Division Task and Answers

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    Mastering Concepts 8. 1 1. Explain the roles of mitotic cell division, meiosis, and fertilization in the human life cycle. Mitosis allows for growth and repair from the fertilized egg to the adult. In reproductively mature individuals, meiosis creates gametes with half the genetic material. The male gamete then fertilizes the female gamete during sexual reproduction, forming a zygote with a full set of genetic material. 2. Why are both cell division and apoptosis necessary for the development of an organism? Cell division is needed to produce the large number of cells necessary to form the individual.

    Combined with apoptosis, the structures in the organism can be carved out and constantly recycled. 8. 2 1. Why does DNA replicate? DNA replicates before a cell divides to give a complete set of genetic instructions to each daughter cell. 2. What is semiconservative replication? Semiconservative replication means that only one strand of a DNA molecule is newly formed. The other strand is original DNA from the parent cell. 3. What are the steps of DNA replication? The helicase enzyme unwinds and separates a double-stranded DNA molecule; the primase enzyme forms a short stretch of complementary RNA on each DNA emplate; starting at the end of the RNA primer, DNA polymerase adds DNA nucleotides that are complementary to the template strand, proofreading and correcting errors as it goes; RNA primers are removed; on the lagging strand, the enzyme ligase joins fragments of DNA. 4. What is the role of RNA primers in DNA replication? The RNA primer is added to the start of the DNA segment being replicated and acts to attract the DNA polymerase, which can only add nucleotides to an existing strand. 5. What happens if DNA polymerase fails to correct an error? A mutation occurs. 8. 3 1.

    What is the relationship between chromosomes and chromatin? Chromosomes are chromatin coiled tightly on itself. 2. How does DNA interact with histones? A stretch of DNA wraps around a cluster of eight histones to form a nucleosome. 3. What are the main parts of a chromosome? The main parts include two identical sister chromatids and the centromere where they are attached. 8. 4 1. What are the three main events of the cell cycle? The three main events are interphase, during which the cell grows and replicates its DNA, mitosis, during which the DNA divides, and cytokinesis, during which the cytoplasm divides. . What happens during interphase? During interphase, a cell grows and produces proteins so that its normal biochemical functions proceed. DNA replicates during interphase as a cell prepares to divide. 3. How does the mitotic spindle form, and what is its function? The mitotic spindle is composed of microtubules associated with cytoskeleton proteins. The spindle originates from centrosomes that are at opposite ends of the cell. Spindle fibers grow across to join at the midline of the cell. The function of the mitotic spindle is to form “trackways” for the movements of chromosomes as cells divide. . What happens during each phase of mitosis? The events of mitosis include: • Prophase: chromosomes condense and become visible; mitotic spindle forms • Prometaphase: nuclear envelope breaks up; spindle fibers attach to kinetochores on each chromosome • Metaphase: chromosomes line up on equator of cell • Anaphase: sister chromatids separate and move to opposite poles of the cell • Telophase: nuclear membranes reassemble around the daughter nuclei; chromosomes decondense; spindle disappears 5. Distinguish between mitosis and cytokinesis.

    Mitosis is the division of duplicated chromosomes into new daughter nuclei. Cytokinesis is the division of cytoplasm and organelles into two new daughter cells and the separation of these cells. 8. 5 1. What prevents normal cells from dividing when they are not supposed to? Interactions between signaling proteins produces checkpoints in the cell cycle to ensure that cells do not divide at improper times. 2. What happens at cell cycle checkpoints? At cell cycle checkpoints, the cell cycle is controlled, ensuring that each stage of the cycle is completed before the next stage begins.

    A cell will only proceed into mitosis if the G1, S, and G2 checkpoints have been successfully passed. If they have not, the cell may stop or suspend the cell cycle, or may enter apoptosis and die. 3. What is the difference between a benign and a malignant tumor? Benign tumors do not spread to other areas of the body and do not invade nearby tissues. A malignant tumor does invade nearby tissues and may metastasize to colonize other areas of the body. 4. How do cancer cells differ from normal cells? Cancer cells are different from normal cells in that they have lost the normal controls on the cell cycle.

    Cancer cells therefore divide in an unregulated fashion. Cancer cells look different from normal cells and lose some of the special features of the parent cell. Cancer cells may have their own growth factors that signal when they should divide. Cancer cells lack contact inhibition; in addition, they are essentially immortal and lack apoptosis. Cancer cells can stimulate the growth of new blood vessels that supply them with nourishment and remove wastes. 5. What is the relationship between mutations and cancer? Mutations cause changes to the signaling proteins that act as checkpoints on division or that signal apoptosis.

    These changes result in cells that continually divide and do not undergo cell death. 6. How does a person acquire the mutations associated with cancer? Mutations can be inherited or caused by exposure to mutagens such as UV radiation or tobacco chemicals. Some sexually transmitted viruses also cause cancer. Lifestyle choices such as eating right and exercising might help prevent mutations associated with cancer. 7. Distinguish among the treatments for cancer. Surgical tumor removal is a direct and local treatment that removes just the cancerous mass and often nearby lymph nodes.

    Chemotherapy and radiation treatments are both “blunt” tools that target any rapidly dividing cell types, though radiation treatment is much more localized. 8. 6 1. What events happen in a cell undergoing apoptosis? A cell undergoing apoptosis first receives a signal at a membrane protein called a death receptor. The signal triggers the release of enzymes that destroy the cell from within. Eventually, immune system cells engulf the dying cell, degrading or recycling its components. 2. Describe two functions of apoptosis.

    Apoptosis carves out excess cells to form a body structure, and it eliminates damaged or aging cells. 8. 7 1. Describe the differences between embryonic, adult, and induced pluripotent stem cells. Embryonic stem cells are totipotent and give rise to any cell in the body. Adult stem cells are pluripotent, with the ability to become most (but not all) cell types.. Induced pluripotent stem cells are adult stem cells that have gene expression manipulated in such a way that they become like embryonic stem cells. 2. What are the potential medical benefits of stem cells?

    Stem cells help illustrate how animals develop and grow; they may be useful in treating diseases that involve tissue loss or damage, also they are useful in observing the development of a disease from its start. Finally, stem cells may also be useful in drug testing. 3. Why is the cloning technique called somatic cell nuclear transfer? The nucleus of a somatic body cell is used rather than the nucleus of a sex cell. 4. Summarize the steps scientists use to clone an adult mammal. Cells from the animal to be cloned are grown in culture, and then the nucleus of one of those cells is removed.

    That nucleus is then fused with a denucleated cell from an egg donor. The fused cell is allowed to divide into an embryo and is then implanted into the surrogate where it is allowed to develop to birth. 8. 8 1. How do researchers use the Sanger method and DNA microarrays to deduce a DNA sequence? The Sanger method uses 4 test tubes with the materials needed to replicate many copies of DNA. Included in each test tube are special terminator nucleotides which result in replicated strands of various lengths indicating the positions of A, C, T, and G in the unknown DNA.

    When the replicated strands are added to a gel electrophoresis plate the complimentary DNA sequence can be determined. Microarrays also determine unknown DNA sequences as the small segments bind to complimentary sequences on the microarray chip. A computer then pieces together the overlaps in sequences to determine the original sequence. 2. How do target DNA, primers, nucleotides, and Taq DNA polymerase interact in PCR? The primers attach to both ends of the target DNA to give the DNA polymerase the necessary nucleotides.

    The Taq form of the polymerase is not denatured by the heating process that occurs in PCR, and the nucleotides provide the raw material the polymerase needs to replicate the strand. 3. Why is PCR useful? PCR creates many copies of a tiny DNA sequence for analysis, especially in forensics. It can also be used to increase the amount of DNA recovered from a microorganism to aid in diagnosis or from a fossil to aid in evolutionary classification. 4. What are STRs, and how are they used in DNA profiling? Short tandem repeats are a series of just a few repeating nucleotides that occur in noncoding portions of DNA.

    Each individual varies in the number of repeats they possess, so by combining analysis of several STR sites between individuals, a DNA profile can be produced. 5. Why does mitochondrial DNA provide different information from nuclear DNA? Mitochondrial DNA is much shorter than nuclear DNA and exists in many copies between all the mitochondria in a cell. It is inherited only from the female parent and does not contain a recombination of male and female nuclear DNA. For this reason it cannot be used to distinguish between siblings. 8. 9 1. Why doesn’t endostatin select for drug-resistant cancer cells, as other chemotherapy drugs do?

    Since cancer cells divide so rapidly, mutations are common, and a drug that selects for a particular drug-resistant cell wouldn’t work on a cell with a different resistance mutation. By inhibiting the formation of new blood vessels, however, endostatin can treat a cancer no matter what its mutations may be. 2. Suppose you learn of a study in which ginger slowed tumor growth in mice for 30 days. What questions would you ask before deciding whether to recommend that a cancer-stricken relative eat more ginger? You would want to know what variables were controlled for; perhaps there was another combined variable that had the inhibition effect.

    You might want to know about any potential side effects. You might also want to know if any human trials had been done and if the results in the human trials were comparable to those in mice. Write It Out 1. If a cell contains all the genetic material it needs to synthesize protein (see chapter 7), why must the DNA also replicate? The DNA must replicate to maintain genetic information when the cell divides, so that each daughter cell receives the same genetic instructions. 2. State the functions of each of the following participants in DNA replication: primer, DNA polymerase, ligase, and helicase.

    The functions are: • primer: a short complementary piece of RNA at the start of each DNA segment to be replicated. The primer attracts DNA polymerase. • DNA polymerase: enzyme that adds new DNA nucleotides complementary to the bases on the exposed strand. • ligase: enzyme that catalyzes the formation of covalent bonds between DNA segments. • helicase: enzyme that unwinds replicating DNA so that other enzymes can guide the assembly of new DNA strands. 3. A person with deficient DNA repair may have an increased cancer risk, and his or her chromosomes cannot heal breaks. The person is, nevertheless, alive.

    How long would an individual lacking DNA polymerase be likely to survive? DNA polymerase is essential for DNA replication. A cell lacking this enzyme would not be able to complete cellular division, so the individual could not grow larger than one cell in size. 4. Explain the relationships among chromatin, chromosome, chromatid, and centromere. Chromatin is a collective term for DNA and its associated proteins that make up chromosomes. A chromosome is a discrete package of DNA and its proteins arranged in a linear manner. A chromatid refers to one of two replicated chromosomes that are attached to each other (sister chromatids).

    The centromere is a small section of DNA that connects sister chromatids. 5. Obtain a rubber band and twist it as many times as you can. What happens to the overall shape of the rubber band? How is this similar to what happens to chromosomes as a cell prepares to divide? How is it different? The more you twist the rubber band the more it wraps on itself and becomes spherical. Similarly chromatin condenses in on itself to become a chromatid, a tightly compacted structure. Chromatin however wraps around scaffold proteins to help direct its condensation, and there are two sister hromatid copies of the DNA. 6. Biologists once thought interphase was a time of cellular rest, but it is not. What happens during interphase? During interphase, the cell is not resting. Rather, the cell grows and produces molecules needed to build new organelles. In addition, during this stage, some organelles duplicate, the DNA replicates, and the cell prepares to divide. 7. Why is G1 a crucial time in the life of a cell? During G1, the cell grows, carries out basic functions, and produces molecules important in later stages of cell division.

    In addition, G1 is a crucial time in the life of a cell because at this point the cell is checked for its integrity. If the cell composition is correct, then the cell advances to the next stage. If the cell composition is not correct, it is either repaired or destroyed. 8. Does a cell in G1 contain more, less, or the same amount of DNA as a cell in G2? Explain your answer. A cell in G1 contains less DNA than a cell in G2 since to get to G2 a cell first goes through the S phase where DNA replicates. 9. Describe what will happen to a cell if interphase happens, but mitosis does not.

    The cell will grow larger and produce more organelles in G1; it will copy its DNA in S and prepare for division in G2, but will then not divide up its replicated DNA. A cell such as this with too much DNA would not be normal and would then undergo apoptosis. 10. If a drug prevents microtubules from forming, at which stage of mitosis would a cell become “stuck”? Without spindles the cell will fail the metaphase checkpoint and become stuck. 11. Cytochalasin B is a drug that blocks cytokinesis by disrupting the actin and myosin microfilaments in the contractile ring. What effect would this drug have on cell division?

    Explain your answer. The DNA would divide and be formed into two nuclei in telophase, but without cytokinesis the two nuclei would remain in the same cell. 12. How do biochemicals from outside the cell control the cell cycle? What signals inside the cell affect cell division? Growth factors are signals to divide that come from outside of the cell. These growth factors bind to receptors on the receiving cell membrane and stimulate the cell to divide. From inside of the cell, there are specific checkpoints such as after G1 phase. The checkpoints ensure that the cell’s DNA is not damaged.

    If the cell’s DNA is damaged, the cell undergoes apoptosis. 13. A microorganism called Agrobacterium tumefaciens infects plant cells. It causes a plant disease called crown gall by inserting a piece of DNA that causes the plant cell to divide rapidly and produce molecules that the bacteria eat. In what ways is the gall similar to a malignant tumor? In what ways is it different? Like a tumor, the cells in the gall are dividing rapidly and creating an enlarged tumor. Unlike a malignant tumor, however, the cells do not metastasize, and they are under control, just not control by the plant’s genes. 14.

    List the ways that cancer cells differ from normal cells. A cancer cell is rounder; its cell membrane is more fluid and may lose some of its specialized features; cancer cells are immortal and keep on dividing; cancer cells produce their own signals to divide; cancer cells lack “contact inhibition” and tend to over-divide in a culture. 15. Use the Internet to find information about Henrietta Lacks and her contribution to medical research. At the same time, consider the issues surrounding informed consent, bioethics, and the ownership of biological materials. How have the ethical standards of medical research changed since Lacks’ time?

    Which ethical issues remain unsettled to this day? [Answer will vary. ] 16. A cell from a newborn human divides 19 times in culture and is then frozen for 10 years. After thawing, how many times is the cell likely to divide? What accounts for this limit? Human cells usually divide 50 times before they cease dividing. If the human cell has divided 19 times, it should undergo approximately another 31 divisions. 17. How might the observation that more advanced cancer cells have higher telomerase activity be developed into a test that could help physicians treat cancer patients?

    The test needs to be designed to measure the telomerase activity in the cells of normal patients to establish a norm. Telomerase activity should be then monitored and studied in patients suffering from various stages of cancer. 18. In the early 1900s, scientists began to experiment with radiation as a cancer treatment. Many physicians who administered the treatment subsequently died of cancer. Why? Radiation is a mutagen, and so without proper protection those administering the radiation exposed their cells, which then produced oncogenes and mutated tumor suppressor genes. 19.

    A protein called p53 regulates the cell cycle in multicellular organisms. Explain the observation that many cancer cells have mutations in the p53 gene. In a multicellular organism, what would be the selective advantage of p53 triggering apoptosis in a cell with severe DNA damage? The p53 protein detects abnormalities during cell division. In cancer cells, the p53 gene is mutated or inactivated, so the p53 protein does not function. Cancer cells go undetected and continue dividing. 20. Scientists sometimes compare the genes that influence cancer development to the controls of a car.

    In this comparison, oncogenes are like an accelerator stuck in the “full throttle” position, and mutated tumor suppressor genes are like brakes that don’t work. How do the roles of proto-oncogenes and tumor suppressor genes relate to this analogy? Proto-oncogenes would be like the automatic gears that a car shifts through as it accelerates. It has to reach a certain RPM before it shifts into the next automatic gear. Tumor suppressor genes are the brake that stops the car if it is going to crash. 21. Do an Internet search for the term cancer risk assessment tool.

    Choose a tool that helps you assess your risk for a type of cancer that interests you. Which of the risk factors mentioned in the tool are under your control? Which are not? Can you think of risk factors that are not mentioned in the assessment tool? [Answers will vary. ] 22. A researcher removes a tumor from a mouse and breaks it up into individual cells. He injects each cell into a different mouse. Although all of the mice in the experiment are genetically identical and were raised in the same environment, they develop cancers that spread at different rates. Some mice die quickly, some linger, and others recover.

    What do these results indicate about the cells that made up the original tumor? As the tumor cells divide, random mutations accumulate in the DNA. The cells in the original tumor are therefore not identical to one another. 23. Breast cancer, the most lethal form of cancer for women, is associated with exposure to chemicals called PAHs (from car exhaust, tobacco smoke, and industrial air pollution), alcohol, pesticides on food and in drinking water, styrene from food containers, PCBs, dioxin, ionizing radiation, hormone supplements, and solvents in household chemicals.

    Design an experiment that would test the hypothesis that any one of these chemicals causes breast cancer. The experiment would need to have a clear control group exposed to none of the above and experimental groups with exposure to only one potential risk factor in each group. 24. List the three most common categories of cancer treatments. Why do many cancer treatments have unpleasant side effects? The three most common treatments are surgical removal, chemotherapy and radiation.

    Because chemotherapy and radiation treatments affect any rapidly dividing cells in the body (chemo) or area (radiation), they cause widespread and unpleasant affects like nausea (effects in digestive tract lining), hair loss (effects at hair follicles), and fatigue (loss of bone marrow cells). 25. Why can combining a traditional cancer treatment with an angiogenesis inhibitor be more effective than either treatment alone? Angiogenesis inhibitors are drugs that block the ability of a tumor to recruit blood vessels that carry nutrients and O2 to the tumor.

    With radiation or chemotherapy alone, some cancer cells may survive the treatment. Adding the angiogenesis inhibitor literally starves the surviving cells by not allowing vessels to form. 26. Use the Internet to research any drug used in chemotherapy to treat cancer. Describe how the drug interferes with the cell cycle and stops cancer cells from dividing. [Answers will vary. ] 27. What is the role of apoptosis in the development of a human from fertilized egg to adult? What role does apoptosis play in an adult’s body?

    As a human develops, apoptosis helps to eliminate excess cells to carve out functional structures such as fingers, toes, nostrils, and ears. Apoptosis also plays a role in the fine tuning of the immune system by killing the white blood cells that react with the body’s own molecules. In an adult, apoptosis also helps in eliminating cells that could cause harm to the organism. Apoptosis plays an important role in the protection of UV radiation to the skin causing damage to the DNA. If the damage from UV radiation is severe, apoptosis triggers a mechanism in which the damaged skin at the surface peels away. 8. List the steps of somatic cell nuclear transfer. Culture somatic cells from donor, remove the nucleus from one of the cultured cells, denucleate a surrogate egg cell, fuse the donor nucleus with the surrogate egg, culture the fertilized egg to become an embryo, implant the embryo in a surrogate mother. 29. Describe the potential practical applications of human reproductive cloning and therapeutic cloning, and list some of the ethical dilemmas associated with each. Reproductive cloning application: allow infertile couples to produce children.

    Ethical dilemmas: the cloned organisms often have abnormalities or die early; in humans it requires harvesting a woman’s eggs. Some people question whether we even have the right to clone an organism. Therapeutic cloning applications: produce customized organs that a person’s immune system would accept. Ethical dilemmas: requires the destruction of a human embryo and the harvesting of a woman’s eggs. 30. Use the clinicaltrials. gov website to search for current research on stem cell therapies. Choose a study that interests you. Which disease are the researchers studying, and what is the source of the stem cells used in the treatment? Answers will vary. ] 31. Explain why medical researchers are concerned that stem cell therapy may increase a patient’s risk of cancer. Because stem cells have a greater ability to divide and their placement in the body cannot yet be precisely controlled, there is the risk that their division can become uncontrolled. 32. How does the Sanger method help researchers determine the sequence of a piece of DNA? The Sanger method helps researchers determine the sequence of a fragment of DNA by generating a series of chemically labeled DNA fragments that are complementary to the DNA being sequenced.

    These fragments differ in length from one another so that once a collection of such pieces is generated, a technique called electrophoresis can be used to separate the fragments by size. The original DNA sequence can be revealed by reading the end bases of these fragments in order by size. 33. Explain how the ingredients of a PCR reaction tube replicate DNA. In a PCR reaction tube, heat separates the two strands of target DNA. Short laboratory-made primers attach to the separated target strands by complementary base pairing.

    Taq DNA polymerase (a heat-tolerant DNA polymerase produced by Thermus aquaticus, a bacterium that inhabits hot springs) adds nucleotides to the primers and builds sequences complementary to the target sequence. The newly synthesized strands then act as templates in the next round of replication, which can be initiated immediately by raising the temperature to separate the strands once more. 34. To diagnose encephalitis (brain inflammation) caused by West Nile virus infection, a researcher needs a million copies of a viral gene. She decides to use PCR on a sample of cerebrospinal fluid, which bathes the person’s infected brain.

    If one cycle of PCR takes 2 minutes, how long will it take the researcher to obtain her million-fold amplification if she starts with a single copy of the viral gene? If she starts with a single copy and multiplies it by 2 every 2 minutes, then it would take her about 40 minutes (20 rounds) to reach her goal of a million copies. 35. Search the Internet to find an example of a criminal trial in which DNA profiling was used as evidence to convict or exonerate a suspect. Suppose you are an investigator in the case. Explain the logic and science behind DNA profiling. How would you select people to collect DNA evidence from?

    Every person exhibits a different number of repeats for various STRs. When enough STRs are investigated simultaneously the statistical likelihood of any two individuals having identical patterns is so small that there can be confidence in the results. However, this confidence is only as good as the number of STRs chosen and the controls used in the processing. People should be chosen in a random manner with inclusion of the suspect and victim in the profile as well as any law enforcement individuals who could potentially have accidentally contaminated the evidence. Pull It Together 1.

    Add DNA polymerase, nucleotides, and complementary base pairing to this concept map. “DNA replication” leads to “Nucleotides” with “involves the addition of new”, which connects to “DNA polymerase” with “are added by”, which connects to “Complementary base pairing” with “pairs the nucleotides based on” 2. What sort of molecule is DNA polymerase? DNA polymerase is a protein with enzymatic properties. It adds DNA nucleotides to an initial RNA primer. 3. Sketch and describe the events that take place in each phase of mitosis Your drawings should look like figure 8. 10 and contain similar descriptions. . How do mitotic cell division and meiosis fit into the human life cycle? Meiosis is used to create new gametes. Mitosis is used for tissue growth and healing, which takes place through cell division. 5. Which types of cells undergo mitotic cell division? All cells that have two copies of the genome can undergo mitosis, and are called somatic cells. Some cells, like gametes, have only one copy of the genome and cannot undergo mitosis. However, note that plants have a stage of their life cycle in which mitosis occurs in cells with only one copy of the genome (see chapter 18). 6.

    What is the relationship between mitotic cell division and apoptosis? Apoptosis is programmed cell death. When a cell attempts mitotic cell division, if it is unable to complete the process it will either repair its errors or enter apoptosis. 7. What can cause a cell to lose control over mitotic cell division? One way that a cell can lose control over mitotic cell division is if a gene that codes for a cell cycle checkpoint protein, called cyclin, gets mutated. Those mutations might change cyclin gene expression or cyclin protein function. The result can be a loss of control over mitotic cell division.

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