5.09 End of Chapter Questions

Answer: D. S phase

Explanation:

• S Phase (Synthesis Phase): The second chromatid is synthesized during the S phase of the cell cycle. This is when DNA replication occurs, resulting in the duplication of each chromosome. Each chromosome originally consists of one chromatid, and after replication, it comprises two identical sister chromatids connected at the centromere.
• G₁ Phase: This is a preparatory phase for cell growth and organelle duplication, but DNA replication has not yet started.
• G₂ Phase: This phase follows DNA replication and focuses on cell growth, repair of any DNA replication errors, and preparation for mitosis. The chromatids are not formed here, as they were already synthesized in the S phase.
• Cytokinesis: This occurs after mitosis, dividing the cytoplasm and separating the two daughter cells, with no role in chromatid formation.

Thus, the second chromatid is made during the S phase of the cell cycle.

Answer: D

Explanation:

1. Speeding up growth without speeding up the cell cycle: If the cell grows faster but does not divide more frequently, the cells will become progressively larger because the time for division (mitosis) remains constant, allowing more time for size increase.
2. Speeding up the cell cycle without speeding up growth: If the cell cycle accelerates without proportional growth, the resulting cells will be smaller, as the cytoplasmic and organelle content will not keep pace with the faster division rate.

• Column D correctly reflects these consequences:
• Smaller cells for faster division without faster growth.
• Larger cells for faster growth without faster division.

Answer: D

Explanation:

• Mitosis: The chromatids remain paired as sister chromatids, totaling 8 chromatids until they are separated.
• Cell Division: After mitosis, each daughter cell has 4 chromatids (one chromatid per chromosome).
• S-phase: DNA replication occurs, doubling the number of chromatids to 8 (4 chromosomes × 2 chromatids per chromosome).

Answer: B. Differentiate into different cell types

Explanation:

Producing specific cell types, like blood cells (option C), or stimulating tissue growth (option D), are applications of differentiation rather than a definition of potency.

Cell Potency: Potency is a cell’s ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency.

• Totipotent cells can give rise to all cell types, including embryonic and extra-embryonic tissues.
• Pluripotent cells can differentiate into any cell type in the body but not extra-embryonic tissues.
• Multipotent cells are more restricted and can develop into a limited range of related cell types (e.g., hematopoietic stem cells for blood cells).

While stem cells can self-renew (option A), that process does not define “potency.”

Answer: A. Multipotent

Explanation:

• Omnipotent: This term is not used in the classification of stem cells.
• Bone Marrow Stem Cells: These are hematopoietic stem cells found in bone marrow and are classified as multipotent.
• Multipotent: These stem cells can differentiate into a limited range of related cell types. For example, hematopoietic stem cells can produce different types of blood cells (red blood cells, white blood cells, and platelets) but cannot give rise to unrelated cell types like neurons or muscle cells.
• Totipotent: Found only in the earliest stages of embryonic development, these cells can form all cell types, including extra-embryonic tissues.
• Pluripotent: These cells can differentiate into almost all cell types in the body but not extra-embryonic tissues.

Answer

The terms centrosome, centriole, and centromere are distinct in function and structure, as outlined below:

Key Differences:

• Centrosome and centrioles are involved in spindle formation, while the centromere connects chromatids to spindle fibers.
• Centrosome is a cellular organelle, centriole is a structural component of the centrosome, and centromere is part of a chromosome.

Answer

a) Stages of mitosis:

• Cell A: Anaphase
• Cell B: Interphase (not part of mitosis, but the stage preceding it)
• Cell C: Metaphase

b) Description of each stage:

Summary:

• In metaphase, chromosomes align at the cell’s center in preparation for segregation.
• In anaphase, chromatids separate and migrate to poles.
• In interphase, DNA replication occurs, and chromosomes are not visible.

Answer:

a)

• Stage of mitosis in Diagram A: Metaphase
  • At this stage, chromosomes are aligned at the metaphase plate (cell equator), and spindle fibers are attached to their centromeres.

b) Prophase Drawing Description:
In prophase, the following key features should be illustrated:

• Spindle fibers begin forming from centrosomes (or microtubule organizing centers in plants).
• Chromosomes condense and become visible.
• Nuclear envelope disintegrates (starts breaking down).

The diagram should represent a plant cell in telophase with the following features:

Two nuclei:

Chromosomes:

• Each nucleus should be located at opposite poles of the cell.
• A reforming nuclear envelope surrounds the chromatin within each nucleus.
• The chromosomes should appear decondensed and thread-like, returning to their chromatin state inside each nucleus.

Cell wall formation:

• A new cell wall is depicted as a plate-like structure forming at the center of the cell, spreading outward.
• This structure is the cell plate, which will develop into a permanent cell wall, separating the two daughter cells.

Spindle remnants:

• The remnants of the spindle fibers may be faintly shown outside the nuclei, as they are no longer actively pulling chromatids apart.

Cell outline:

• The plant cell’s rectangular shape with a rigid cell wall should be depicted to highlight that it is a plant cell.

Diagram for Anaphase

The diagram should include:

Sister Chromatid Separation:

• The three pairs of chromatids (sugar-cane-like, straight, and short) should now be shown separating.
• Each chromatid moves towards opposite poles of the cell, pulled by the spindle fibers.
• The chromatids appear as single, distinct “V-shaped” or “rod-like” structures during their migration.

Spindle Fibers:

• Clear spindle fibers should extend from each pole of the cell to the centromeres of the chromatids, illustrating their role in pulling the chromatids apart.

Poles and Midline:

• Ensure the poles of the cell and the central region (where chromatids were aligned during metaphase) are clear.

No Nuclear Envelope:

• At this stage, the nuclear envelope is absent, as the cell is actively dividing.

Shape of the Cell:

• The animal cell should have a rounded shape typical of animal cells during division, with no rigid cell wall as seen in plant cells.

1. Disruption of Microtubule Assembly:

• Microtubules are formed by the polymerization of tubulin protein subunits.
• Colchicine binds to tubulin, preventing the subunits from joining together.
• This inhibits the polymerization process necessary for microtubule formation.

2. Microtubule-Based Structures in Mitosis:

• The spindle fibers are made of microtubules and are essential for chromosome movement.
• Centrioles and the aster microtubules also contain microtubules and help in organizing the spindle apparatus.

3. Stage of Arrest Due to Colchicine:

• Cells treated with colchicine are typically halted at metaphase.

• Reason:
  • Microtubules are required for the spindle fibers to attach to kinetochores and align chromosomes at the metaphase plate.
  • Without functional microtubules, chromosomes cannot align properly, and the cell cannot progress to anaphase.

1. Centrosomes are replicated before M phase of the cell cycle begins.

• True: Centrosomes are duplicated during the S phase of interphase to ensure each daughter cell inherits one centrosome after division.

2. Sister chromatids contain identical DNA.

• True: Sister chromatids are formed during the S phase of the cell cycle by DNA replication, resulting in identical DNA sequences.

3. The microtubules attached to a given kinetochore extend to both poles of the spindle.

• False: Microtubules attached to a kinetochore extend to only one pole, helping to pull the chromatids apart during anaphase.

4. Microtubule polymerisation and depolymerisation is a feature of the S phase of the cell cycle.

• False: Microtubule dynamics (polymerization and depolymerization) are prominent during the M phase, particularly in mitosis for spindle formation.

5. Kinetochores are found in the centrosomes.

• False: Kinetochores are located on the centromeres of chromosomes, where they serve as attachment points for spindle microtubules.

6. Telomeres are the sites of attachment of microtubules during mitosis.

• False: Telomeres are the ends of chromosomes and are not involved in microtubule attachment; kinetochores on centromeres serve this function.

7. Sister chromatids remain paired as they line up on the spindle at metaphase.

• True: During metaphase, sister chromatids are aligned at the metaphase plate and remain paired until they are separated during anaphase.

1. Why is cancer referred to as a genetic disease?

• Cancer is termed a genetic disease because it arises due to mutations in DNA, often affecting genes that regulate cell growth and division (e.g., oncogenes or tumor suppressor genes).
• These mutations can lead to uncontrolled cell proliferation and tumor formation.

2. What is the meaning of the term “carcinogen”?

• A carcinogen is any substance, agent, or factor that can cause cancer by inducing mutations or interfering with cellular processes, such as tobacco smoke, UV radiation, or certain chemicals.

3. Cancer cases by age and trends:

a. Identify the age group with the highest cancer prevalence:

Figure: Above is a figure of another Proportional Plot (not related to this question). This graph one includes the percentages – and hopefully shows you how to read proportional graphs. You have to manually subtract the values to calculate the graph given in the question since it does not show any percentages.

• The age group with the highest prevalence of cancer is typically the elderly, often 60 years and older. This is exactly what we see in the graph, the 50-69 year old age group has the largest ratio (prevalence) in the proportional plot.

b. Why do you think this age group experiences the most cancer cases?

Why the 50 to 69 year olds?

• Accumulation of mutations: DNA damage accumulates over time, increasing the likelihood of mutations in older individuals.
• Weakened immune system: Aging reduces the immune system’s ability to detect and eliminate abnormal cells.
• Longer exposure to carcinogens: Older people have had more prolonged exposure to carcinogenic factors like smoking, UV light, or environmental toxins.

Why not the <15 year olds?

1. Limited Cumulative Genetic Mutations

• Cancer often arises from the accumulation of genetic mutations over time. Since young people have had less time for such mutations to accumulate, their risk of developing cancer is lower.

2. Fewer Environmental Exposures

• Cancer can be triggered by long-term exposure to environmental carcinogens such as tobacco, UV radiation, and pollutants. Young children have had less time to be exposed to these risk factors compared to adults.

3. Rapid Cell Turnover and Repair Mechanisms

• Children have more efficient DNA repair mechanisms and cell regeneration processes. This allows their bodies to fix mutations before they lead to cancer.

4. Different Biological Characteristics

• Some cancers are associated with aging, such as those linked to hormonal changes, prolonged inflammation, or the long-term effects of chronic diseases. These factors are generally not present in children.

5. Types of Cancers in Children

• While cancer is rare in children, when it occurs, it is often due to inherited genetic mutations or developmental issues. Examples include:
  • Leukemia: The most common cancer in children.
  • Brain and spinal cord tumors.
  • Lymphomas.
  • Bone cancers like osteosarcoma.
• These cancers are less related to lifestyle or environmental factors and more to developmental biology and genetics.

6. Protective Lifestyle

• Children typically have not adopted risky behaviors such as smoking, drinking alcohol, or following unhealthy diets, which can increase cancer risk later in life.

Why not the +70 year olds?

1. Survivor Bias

• Many individuals with cancer do not survive to older ages. As a result, the population above 70 years may consist disproportionately of individuals who have not developed cancer or who have survived cancer and are in remission.

2. Competing Risks

• Older adults face a higher risk of dying from other age-related conditions, such as cardiovascular disease, dementia, or infections. These competing risks reduce the likelihood of living long enough to be diagnosed with or die from cancer.

3. Diagnostic Underreporting in Older Adults

• In very elderly populations, cancer may be underdiagnosed or underreported due to:
  • Fewer screenings or diagnostic tests being conducted.
  • A focus on palliative care rather than aggressive diagnostic measures in the presence of other comorbidities.
  • Misattribution of symptoms to aging or other chronic conditions.

4. Reduced Cancer Screening

• Cancer screening rates decline in older age groups. Many screening programs, such as those for breast and colon cancer, have upper age limits (often 70–75 years). This leads to lower detection rates of cancers in those over 70.

5. Cancer Incidence Peaks in Middle Age

• Many cancers, such as breast, prostate, and colon cancer, have a peak incidence in the 50–69 age range. Beyond this, incidence may plateau or decline because:
  • Susceptible individuals may have already been diagnosed.
  • The biology of certain cancers may result in fewer new cases in older populations.

6. Age-Related Changes in Cancer Biology

• Older adults may experience a shift in the types of cancers they are prone to. For instance:
  • Slower-growing cancers may remain undiagnosed.
  • The immune system’s role in identifying and clearing cancer cells may change with age.

7. Population Composition

The proportion of individuals without cancer might be higher among those over 70 because individuals with aggressive cancers might have passed away before reaching this age bracket.

c. Discuss the overall trends in cancer prevalence between 1990 and 2016:

• I am too lazy to do this question right now, I have a life, but if you don’t, this is what it should look like. You should talk about the general trends for each year. For example:
• Between 1990 and 2000, there was a sharp/drastic/small/large/big/etc. increase/decrease in the number of cancer prevalence for the 50 to 69 year olds.
  • Do the same for the other year groups.
• Between 2000 and 2010, there was a sharp/drastic/small/large/big/etc. increase/decrease in the number of cancer prevalence for the 50 to 69 year olds.
  • Do the same for the other year groups.
• Repeat the steps above for ALL the years and ALL the year groups, including the sneaky few years after 2015…