Answers to 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.  Without apoptosis many structures could not be modified during development and chickens, for example, would have webbed feet like ducks.

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 are the steps of DNA replication?

The helicase enzyme unwinds and separates a double-stranded DNA molecule; an enzyme breaks the hydrogen bond of the base pairs; DNA polymerase adds DNA nucleotides that are complementary to the template strand, proofreading and correcting errors as it goes; the enzyme ligase joins fragments of DNA.

3. What is a mutation, and why are mutations important?

A mutation is a change in the DNA sequence.  If a change in the DNA changes the resulting amino acid sequence, the gene may no longer function properly leading to diseases like cystic fibrosis or cancers.  Mutations may also alter the gene in a beneficial way and improve the function of the gene.

4. Why is DNA repair necessary?

DNA repair is necessary to fix any mistakes in replication or mutations so that mutations do not accumulate.

8.3
1. What are the events of binary fission?

The circular DNA replicates; the DNA attaches to the inner surface of the cell; a new cell membrane grows between the two DNA molecules.  The cell divides along the new membrane and a new cell wall is formed for the two daughter cells.

2. What are the sources of genetic variation in bacteria and archaea?

Mutation is the main source of genetic variation in bacteria and archaea.  However, they can also receive new genes from living cells or absorb DNA released from dead cells.

8.4
1. How do chromatin and histones interact?

A stretch of chromatin wraps around a cluster of eight histones to form a nucleosome.

2. Sketch and label the main parts of a duplicated chromosome?

The main parts include two identical sister chromatids and the centromere where they are attached.  Your drawing should look like the lower part of figure 8.7.

8.5
1. 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.

2. 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. The mitotic spindle attaches the centriole to the centromere to help movement the chromosomes as cells divide.

3. What happens during each stage of mitosis?

The events of mitosis include:

• Early Prophase:  chromosomes condense and become visible; mitotic spindle forms.
• Late Prophase: nuclear envelope breaks up; spindle fibers attach to the chromosome.
• Metaphase: chromosomes line up on equator of cell.
• Anaphase: centromeres split, and sister chromatids separate and move to opposite poles of the cell.
• Telophase:  nuclear membrane and nucleolus form at each of the poles; chromosomes decondense; spindle disappears.

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 and the separation of these cells.

8.6
1. What keeps 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 is the relationship between genetic 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.

3. List and describe the three most common cancer treatments.

Surgical tumor removal is a direct and local treatment that removes just the cancerous mass and often nearby tissue.  Chemotherapy drugs are designed to stop cancer cells from dividing, and radiation uses thin streams of radioactive energy to kill the cells of the tumor.  Chemotherapy and radiation treatments are both “blunt” tools that target any rapidly dividing cell types, though radiation treatment is much more localized.

8.7
1. Why are traditional chemotherapy drugs often most effective during the first few treatments?

The cancer cells mutate quickly during rapid divisions.  It is likely that after a few treatments, the cells have evolved a resistance to the medications which would lead the medications to be less effective over time.

2. Why doesn’t endostatin select for drug-resistant cancer cells?

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  without putting selective pressure on the mutations accumulating in cancer tissues. 

 

Answers to Write It Out Questions

1. How would your body look if cells divided but apoptosis never occurred?

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.

2. Use words and diagrams to describe how DNA is copied in a cell.

The helicase enzyme unwinds and separates a double-stranded DNA molecule; an enzyme breaks the hydrogen bond of the base pairs; DNA polymerase adds DNA nucleotides that are complementary to the template strand, proofreading and correcting errors as it goes; the enzyme ligase joins fragments of DNA.  Your diagram should be similar to figure 8.4.

3. If a cell contains all the genetic material it needs to synthesize protein, 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. 

4. Sketch and describe the events that occur when a bacterial cell divides.

 The circular DNA replicates; the DNA attaches to the inner surface of the cell; a new cell membrane grows between the two DNA molecules.  The cell divides along the new membrane and a new cell wall is formed for the two daughter cells.  Your sketch should be similar to figure 8.5.

5. List the ways that binary fission is similar to and different from mitosis.

In both processes there is division of a single cell which creates two identical daughter cells.  The two processes differ in that mitosis occurs in eukaryotes with linear chromosomes and includes the division of a nucleus.

6. 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.

7. Biologists once thought interphase is 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.

8. Which contains the most DNA, a cell in G1 or a cell in G2 phase?

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 occurs 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 not divide up its replicated DNA.  A cell such as this, with too much DNA, would not be normal and would undergo apoptosis.

10. 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 without proper protection those administering the radiation exposed their cells, which then produced the cancer in the physicians.

11. Why can combining a traditional chemotherapy treatment with a drug that inhibits new blood vessel growth be more effective than either treatment alone?

With radiation or chemotherapy alone, some cancer cells may survive the treatment. If you block the ability of a tumor to recruit blood vessels that carry nutrients and O2 to the tumor, it is possible to literally starve the surviving cells by not allowing vessels to form.

12. Do an Internet search for the phrase cancer risk assessment tool. Choose one 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.]
Answers to Pull It Together Questions

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. Describe the events that take place in each phase of mitosis.

The events of mitosis include:

• Early Prophase:  chromosomes condense and become visible; mitotic spindle forms.
• Late Prophase: nuclear envelope breaks up; spindle fibers attach to the chromosome.
• Metaphase: chromosomes line up on equator of cell.
• Anaphase: centromeres split, and sister chromatids separate and move to opposite poles of the cell.
• Telophase:  nuclear membrane and nucleolus form at each of the poles; chromosomes decondense; spindle disappears.

4. 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. 

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 gets mutated.  The result can be a loss of control over mitotic cell division.