Mastering Concepts

8.1

1. Explain the roles of mitotic cell division, meiosis, and fertilization in the human life cycle.

Mitotic cell division allows for tissue repair/replacement and for an organism’s development from a fertilized egg (zygote) to an adult. Meiosis creates genetically variable gametes that each contain half the genetic material of a nonreproductive cell. The male gamete 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 produces the new cells that make up an organism. Apoptosis (programmed cell death) carves out structures such as fingers and toes; it also protects an organism by eliminating damaged cells that might otherwise become cancerous.

8.2

1. Why does DNA replicate?

DNA replicates before a cell divides so that each daughter cell receives a complete set of genetic information.

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; binding proteins keep the strands separate. The primase enzyme forms a short stretch of complementary RNA on each DNA template. 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. Another enzyme removes the RNA primers, and DNA polymerase fills in the gaps left by the primers. The ligase enzyme seals the resulting nicks in the newly synthesized DNA.

4. Could DNA replication occur if primase were not present in a cell? Explain your answer.

Primase’s job is to form an RNA primer on each DNA template molecule; this step is essential because DNA polymerase can only attach nucleotides to an existing nucleic acid strand. Without primase, these primers would not exist, and DNA replication could not begin.

5. What happens if DNA polymerase fails to correct an error?

A mutation occurs.

8.3

1. Which cell types divide by binary fission?

Prokaryotic cells (bacteria and archaea) divide by binary fission.

2. What are the events of binary fission?

The cell’s DNA replicates, and the two chromosomes attach to the cell membrane. The cell grows between the two attachment points, pulling the two chromosomes apart. The cell wall and cell membrane then pinch inward, dividing the cell into two identical daughter cells.

8.4

1. How are chromosomes and chromatin related?

Each chromosome is composed of chromatin.

2. What might happen to a cell if DNA packing fails?

A failure of DNA packing would mean that a cell that could not organize its DNA into discrete, portable chromosomes. It would therefore not be able to complete its preparations for the division of its genetic material into two daughter cells.

8.5

1. What are the three main events of the cell cycle?

The three main events are interphase, mitosis, and cytokinesis.

2. What happens during interphase?

During interphase, the cell grows and produces proteins needed to carry out its functions. In addition, DNA replicates as the cell prepares to divide.

3. Suppose a centromere does not split during anaphase. Describe the chromosomes in the daughter cells.

If a centromere failed to split during anaphase, one daughter cell would have an extra chromosome, and the other daughter cell would be missing one chromosome.

4. 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.

8.6

1. What prevents normal cells from dividing when they are not supposed to?

Signaling proteins ensure that cells do not divide at improper times.

2. What happens at cell cycle checkpoints?

Cell cycle checkpoints ensure that key steps in interphase and mitosis have been successfully completed before the next step begins. If a cell does not successfully pass a checkpoint, the cell cycle may stop, or the cell may enter apoptosis and die.

3. What is the difference between a benign and a malignant tumor?

Benign tumors are surrounded by a capsule; they do not spread to other areas of the body or invade nearby tissues. A malignant tumor invades nearby tissues and may metastasize, spreading to 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 control of the cell cycle. The division of cancer cells is therefore unregulated. In addition, cancer cells look different from normal cells; they have high levels of telomerase enzymes, making the cells essentially immortal; they may produce their own growth factors or divide in the absence of growth factors; they lack contact inhibition; and they can stimulate the growth of new blood vessels that supply them with nourishment and remove wastes.

5. Distinguish among the treatments for cancer.

Surgery can remove the cancerous mass. Chemotherapy and radiation treatments use drugs or radiation to kill actively dividing cancer cells.

6. What is the relationship between mutations and cancer?

DNA encodes each of the many proteins that regulate the cell cycle and apoptosis. These genes include proto-oncogenes and tumor suppressor genes. Mutations in these genes can cause the signaling proteins to be abnormal, cause malfunctions in the cell’s response to the signaling proteins, or cause apoptosis to fail. Any of these changes can result in cells that divide out of control.

7. How does a person acquire the mutations associated with cancer?

Mutations can be inherited from the parents or they may be caused by exposure to mutagens such as UV radiation or tobacco chemicals. Some sexually transmitted viruses also cause cancer.

8.7

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.

8.8

1. Why doesn’t endostatin select for drug-resistant cancer cells, as other chemotherapy drugs do?

Since cancer cells divide so rapidly, they frequently undergo mutations. A drug might kill most of the cells in a tumor, but a few might have mutations that confer resistance to the drug. These surviving cells would then divide, growing a drug-resistant tumor. Endostatin, however, inhibits the formation of new blood vessels. It can therefore shrink any tumor, no matter what mutations have occurred in the tumor’s cells.

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 study the experimental design to determine the sample size, whether the doses were realistic, and whether the controls were adequate. You might want to know what kind of cancer the mice had and about any potential side effects of long-term use of ginger. 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. Explain how cell division and cell death work together to form a functional multicellular organism.

Cell division produces new cells and is therefore necessary for growth. Cell death removes certain cells, as needed for the development of specialized structures.

2. Why does DNA replication precede cell division?

Before a cell divides, its DNA must replicate so that there are two sets of DNA, one for each daughter cell.

3. Write and explain an analogy for each of these DNA replication enzymes: helicase, binding proteins, ligase.

Helicase is like scissors because it separates the two DNA strands. Binding proteins are like hair clips because they hold the separated DNA strands apart. Ligases are like staplers because they fasten two segments of DNA together.

4. Tightly packed DNA cannot be used for protein synthesis. Why has evolution favored the histones and other proteins that help DNA fold into visible chromosomes?

Although DNA is not available for protein synthesis, coiled tightly around histones and other proteins has another benefit: it makes the DNA highly compact and portable. DNA can move to the cell’s center and then into the new daughter cells much more easily and safely if it is tightly packed than if it remains loose in the cell.

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, with coils forming supercoils. The more you twist, the more compact the rubber band. Similarly, chromatin forms coils and supercoils to become a tightly compacted visible chromosome. Unlike a rubber band, however, chromatin wraps around scaffold proteins to help direct its compaction.

6. If a cell somehow skipped G1 of interphase during multiple cell cycles, how would the daughter cells change?

The daughter cells would get smaller and smaller as the cytoplasm divided multiple times without synthesizing new proteins.

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 cell division.

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. To get to G2, the cell first goes through S phase, during which 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 phase 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 probably undergo apoptosis.

10. List the ways that binary fission is similar to and different from mitotic cell division.

Both binary fission and mitotic cell division are asexual, meaning they produce identical daughter cells. Both are preceded by DNA replication. However, binary fission and mitotic cell division take place in different cell types (prokaryotic cells undergo binary fission, whereas eukaryotic cells divide mitotically). Binary fission does not require that chromosomes condense; mitosis does. Binary fission does not require a spindle; in mitotic cell division, a spindle moves the chromosomes within the cell.

11. List the ways that cancer cells differ from normal cells.

A cancer cell’s division is uncontrolled unlike that of a normal cell. Cancer cells are also shaped differently from normal cells, contain more telomerase than normal cells, produce their own signals to divide, lack “contact inhibition,” and secrete signals that stimulate angiogenesis.

12. 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 should 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 and types of cancer. Cells with high telomerase could be targeted for removal, even if they had not yet developed into tumors.

13. 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. Without proper protection from radiation, those administering the treatment also exposed their own cells. DNA in their cells mutated, producing oncogenes and mutated tumor suppressor genes.

14. 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?

A proto-oncogene normally stimulates cell division; when a proto-oncogene gene is mutated into an oncogene, cells divide out of control, much as a car with a stuck accelerator can drive much too fast for safety. A tumor suppressor gene normally acts like a car’s brake, blocking cancer development. Mutations in these genes stop the “brakes” from working, and cells divide uncontrollably.

15. 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, they cause widespread and unpleasant effects like nausea (from killing cells in the digestive tract lining), hair loss (from killing cells in hair follicles), and fatigue (from killing bone marrow cells).

Pull It Together

1. Add DNA polymerase, nucleotides, and complementary base pairing to this concept map.

“DNA replication” connects with the phrase “requires the addition of new” to “Nucleotides,” which connects with the phrase “are added by” to “DNA polymerase.” “DNA polymerase” connects with the phrase “adds nucleotides based on” to “Complementary base pairing.”

2. Add cell growth and tissue repair to the concept map.

“Cell growth” connects with the phrase “occurs during” to “Interphase.” “Mitotic cell division” connects with the phrase “produces cells required for” to “Tissue repair.”

3. What is the relationship between mitotic cell division and apoptosis?

Mitotic cell division creates new cells; apoptosis (programmed cell death) removes unneeded or damaged cells.