8.f^Chapter 8 Ending. The Chapter in Review^126^128^,,^7904^8281%
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The Chapter in Review
Summary
8.1
The Basics of Cellular Reproduction

Cellular reproduction occurs when growth and repair of tissues take place. Cellular reproduction is also necessary to both asexual and sexual reproduction. Following fertilization, a single cell becomes a multicellular organism by cellular reproduction.

Cellular reproduction involves two processes: growth, during which a cell grows and doubles its contents, and cell division, during which the parent cell's contents are split among two daughter cells. During cell division, one-half of the cytoplasm and a copy of the cell's DNA are passed on to each of two daughter cells.

When a cell is not dividing, the DNA is wound around histones, then arranged into a zigzag configuration to form chromatin. Just before cell division, duplicated chromatin condenses into large loops to form duplicated chromosomes consisting of two chromatids held together at a centromere.

8.2
The Cell Cycle

The cell cycle consists of interphase and the M (mitotic) stage.

Interphase has the following stages:

  • G 1 : The cell doubles its organelles and accumulates materials that will be used for DNA replication.

  • S: The cell replicates its DNA.

  • G 2 : The cell synthesizes proteins that will be needed for cell division.

The M (mitotic) stage consists of mitosis and cytokinesis. As a result of mitosis, the daughter nuclei are genetically identical to the parent nucleus and to each other. If the parent nucleus has four chromosomes, the daughter nuclei each have four chromosomes.

8.3
Mitosis and Cytokinesis

Cell division consists of mitosis and cytokinesis.

Mitosis has four phases:

  • During prophase, the chromosomes are condensing. Outside the nucleus, the spindle begins to assemble between the separating centrosomes. Prophase continues with the disappearance of the nucleolus and the breakdown of the nuclear envelope. Spindle microtubules from each pole attach to the chromosomes in the region of a centromere.

  • During metaphase, the chromosomes are aligned at the spindle equator midway between the spindle poles.

  • During anaphase, the sister chromatids separate and become daughter chromosomes. As the microtubules attached to the chromosomes disassemble, each pole receives a set of daughter chromosomes.

  • During telophase, the spindle disappears as new nuclear envelopes form around the daughter chromosomes. Each nucleus contains the same number and kinds of chromosomes as the original parent nucleus. Division of the cytoplasm begins.

Cytokinesis differs for animal and plant cells:

  • In animal cells, a furrowing process involving actin filaments divides the cytoplasm.

  • In plant cells, a cell plate forms from which the plasma membrane and cell wall develop.

8.4
The Cell Cycle Control System

Checkpoints are interacting signals that promote or inhibit the continuance of the cell cycle. Important checkpoints include these three:

  • At G1, the cell can enter G0 or undergo apoptosis if DNA is damaged beyond repair. If the cell cycle passes this checkpoint, the cell is committed to complete the cell cycle.

  • At G2, the cell checks to make sure DNA has been replicated properly.

  • At the mitotic checkpoint, the cell makes sure the chromosomes are properly aligned and ready to be partitioned to the daughter cells.

Both internal and external signals converge on checkpoints to control the cell cycle. Cyclin-kinase complexes are internal signals that promote either DNA replication or mitosis. Growth factors and hormones are external signals that promote the cell cycle in connection with growth. Inhibitory external signals are responsible for contact inhibition. Internally, when telomeres are too short, the cell cycle stops because the chromosomes become unstable.

When DNA cannot be repaired, apoptosis occurs as enzymes called caspases bring about destruction of the nucleus and the rest of the cell.

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8.5
The Cell Cycle and Cancer

Cancer results when control of the cell cycle is lost, and cells divide uncontrollably. Cancer cells are undifferentiated, have abnormal nuclei, do not undergo apoptosis, and form tumors. A tumor may stimulate angiogenesis, allowing it to obtain more nutrients and grow larger. Tumors may also undergo metastasis, which allows them to spread throughout the body. Certain behaviors, such as avoiding smoking and sunbathing and adopting a diet rich in fruits and vegetables, are protective against cancer.

Key Terms
Testing Yourself

Choose the best answer for each question.

  1. is nuclear division, whereas is division of the cytoplasm.

    1. Cytokinesis, mitosis

    2. Apoptosis, mitosis

    3. Mitosis, apoptosis

    4. Mitosis, cytokinesis

  2. The two identical halves of a duplicated chromosome are called

    1. chromosome arms.

    2. nucleosomes.

    3. chromatids.

    4. homologues.

  3. For questions 3–6, match the items to those in the key. Answers can be used more than once.

    Key:

    a. G1 stage of interphase

    b. G2 stage of interphase

    c. S stage of interphase

  4. This stage follows mitosis.

  5. DNA is replicated during this stage.

  6. Organelles are doubled in number during this stage.

  7. During this stage, the cell produces proteins that will be needed for cell division.

  8. Label the stages and phases of the cell cycle on the following diagram. Include anaphase, cytokinesis, G1 stage, G2 stage, metaphase, prophase, S stage, and telophase.

  9. For questions 8–13, match the items to those in the key. Answers can be used more than once, and each question can have more than one answer.

    Key:

    a. prophase

    b. metaphase

    c. anaphase

    d. telophase

  10. The nucleolus disappears, and the nuclear membrane breaks down.

  11. The spindle disappears, and the nuclear envelope forms.

  12. Sister chromatids separate.

  13. Spindle poles move apart as spindle fibers slide past each other.

  14. Chromosomes are aligned on the spindle equator.

  15. Each chromosome is composed of two sister chromatids.

  16. Plants cannot use a cleavage furrow to undergo cytokinesis because they

    1. lack a plasma membrane.

    2. have a cell wall.

    3. have too many chromosomes.

    4. are too small.

  17. Which cell cycle checkpoint allows damaged DNA to be repaired before it is passed on to daughter cells?

    1. G1

    2. G2

    3. S

    4. mitotic

  18. Which of the following is not an event of apoptosis?

    1. loss of contact with neighboring cells

    2. blistering of the plasma membrane

    3. increase in the number of mitochondria

    4. fragmentation of the nucleus

  19. Rats have 42 chromosomes. How many total chromatids does a rat cell have during G1 stage? Explain your answer.

    1. 21

    2. 42

    3. 63

    4. 84

    5. 126

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  21. Rats have 42 chromosomes. How many total chromatids does a rat cell have during G2 stage? Explain your answer.

    1. 21

    2. 42

    3. 63

    4. 84

    5. 126

  22. Which of the following is not a feature of cancer cells?

    1. exhibit contact inhibition

    2. have enlarged nuclei

    3. stimulate the formation of new blood vessels

    4. are capable of traveling through blood and lymph

  23. The spindle begins to assemble during

    1. prophase.

    2. metaphase.

    3. anaphase.

    4. interphase.

  24. What is the checkpoint for the completion of DNA synthesis?

    1. G1

    2. S

    3. G2

    4. M

  25. Programmed cell death is called

    1. mitosis.

    2. meiosis.

    3. cytokinesis.

    4. apoptosis.

  26. In human beings, mitosis is necessary for

    1. growth and repair of tissues.

    2. formation of the gametes.

    3. maintaining the chromosome number in all body cells.

    4. the death of unnecessary cells.

    5. Both a and c are correct.

  27. In which phase of mitosis is evidence of cytokinesis present?

    1. prophase

    2. metaphase

    3. anaphase

    4. telophase

    5. Both c and d are correct.

  28. Which of these is not a behavior that could help prevent cancer?

    1. maintaining a healthy weight

    2. eating more dark green leafy vegetables, carrots, and various fruits

    3. not smoking

    4. maintaining estrogen levels through hormone replacement therapy

    5. consuming alcohol only in moderation

  29. Cytokinesis in plant cells differs from cytokinesis in animal cells because

    1. plant cells form a cleavage furrow that constricts the daughter cells.

    2. in plant cells, many small vesicles form a cell plate that eventually becomes a new cell wall.

    3. animal cells seldom undergo cytokinesis.

    4. Both b and c are correct.

Thinking Scientifically
  1. The survivors of the atomic bombs that were dropped on Hiroshima and Nagasaki have been the subjects of long-term studies of the effects of ionizing radiation on cancer incidence. The frequencies of different types of cancer in these individuals varied across the decades. In the 1950s, high levels of leukemia and cancers of the lung and thyroid gland were observed. The 1960s and 1970s brought high levels of breast and salivary gland cancers. In the 1980s, rates of colon cancer were especially high. Why do you suppose the rates of different types of cancer varied across time?

  2. During prophase of mitosis, the nuclear membrane breaks down. This is undoubtedly a complex, energy-consuming process that has to be carried out again, in reverse, at the end of mitosis. Why do you suppose the nuclear membrane must break down at the beginning of mitosis? What mechanism can you envision for the dismantling of the nuclear membrane?

  3. Why is it advantageous that checkpoints evolved during the cell cycle?

Bioethical Issue
Paying for Cancer Treatment

As you have read, new drugs and treatment options are now available for cancers once thought incurable. However, many of these new cures are very expensive. Taxol treatment, for example, can cost in excess of $4,000 per dose. Most of these costs are passed on to the public, largely through higher health insurance rates and higher taxes. Since many of the risk factors for cancer, such as smoking and obesity, are controllable, this has led to an obvious question—should some cancer patients be held responsible for the cost of their treatment?

Many individuals feel that people who develop cancer because of poor lifestyle choices should be held fully responsible for paying for treatment. By doing so, they argue, people have a direct financial incentive to avoid risky behaviors, and are no longer a financial burden on society. But while an individual's poor lifestyle can often be implicated, cancer sometimes strikes otherwise healthy people without any known risk factors. And it can sometimes be difficult to determine whether a person's poor behavior is to blame, even when certain risk factors are present. In such cases, is it really fair to incriminate the patient for their disease when there may be other causes?

Many people also disagree on who should bear the high costs of developing new drugs and treatments. Many, including insurance companies, feel that cancer patients who benefit from new or novel cancer treatments should bear at least some of those costs. Others, however, argue that we as a society owe it to our fellow citizens to fund the research that can often save many innocent lives. There are no easy answers for any of these questions, but as cancer continues to exact a high toll in both human life and financial resources, future generations may soon face some difficult choices.

Essentials of Biology Website

The companion website for Essentials of Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.

http://www.mhhe.com/maderessentials2