- A student claims that since all cells in the body originate from a single zygote, all cells are technically stem cells. Which statement provides the most accurate reason why this claim is incorrect?
Answer: Most cells in the body are terminally differentiated and have lost the key stem cell properties of extensive self-renewal and potency.
The defining characteristics of a stem cell are its ability to divide repeatedly to produce more stem cells (self-renewal) and its ability to differentiate into specialized cell types (potency). While all cells descend from the zygote (which is a totipotent stem cell), most cells undergo terminal differentiation. In this process, they become specialized and lose both the ability to differentiate into other types and the capacity for extensive self-renewal. Therefore, they are no longer considered stem cells.
- A zygote is totipotent, whereas a cell from the inner cell mass of a blastocyst is pluripotent. What can a totipotent cell form that a pluripotent cell cannot?
Answer: The extra-embryonic tissues, such as the placenta and amnion.
Totipotency is the highest level of potency. A totipotent cell (like a zygote) can differentiate into all cell types of the embryo proper (which pluripotent cells can also do) AND the extra-embryonic tissues required for development, such as the placenta. Pluripotent cells can form all embryonic tissues but not the extra-embryonic tissues.
- Cancers are characterized by uncontrolled cell proliferation and a lack of proper function. This pathology can be described as a failure in which two fundamental, coupled processes?
Answer: Cell cycle regulation and cell differentiation
Cancer involves a breakdown in the normal control of the cell cycle, leading to uncontrolled proliferation (uncontrolled mitosis). Additionally, cancerous cells often fail to differentiate properly or they de-differentiate, losing their specialized function and structure. This combination of uncontrolled growth and lack of specialization is a hallmark of malignancy.
- Adult stem cells, such as hematopoietic stem cells in bone marrow, reside in specific microenvironments called niches. What is the most likely function of these niches?
Answer: To regulate the balance between stem cell self-renewal and differentiation via signalling molecules.
Stem cell niches are complex environments that provide signals (e.g., growth factors, cell-to-cell contacts) that carefully control stem cell behavior. These signals maintain the stem cell population by regulating when they divide to make more stem cells (self-renewal) and when they divide to produce cells that will differentiate to replace tissues.
- The specialized function of a mature erythrocyte (red blood cell) is oxygen transport. Which structural change during its differentiation is most directly linked to maximizing this function?
Answer: The development of a biconcave shape to increase surface area-to-volume ratio.
While the expulsion of the nucleus is key to maximizing hemoglobin content, the biconcave shape is a critical specialization that increases the surface area-to-volume ratio. This facilitates faster diffusion of oxygen across the plasma membrane to and from hemoglobin molecules inside the cell, directly enhancing its primary function of gas exchange.
- Researchers are developing a stem cell therapy to treat patients with Type 1 diabetes by replacing damaged pancreatic β-cells. For this therapy to be successful, the stem cells used must be able to perform which action?
Answer: Differentiate into insulin-producing cells when placed in the pancreatic environment.
The goal of this therapy is to replace the specific function of the lost β-cells, which is to produce insulin. Therefore, the therapeutic stem cells must have the potential (potency) and be successfully induced to differentiate into functional, insulin-secreting β-cells. The other options describe incorrect or irrelevant biological processes.
- The human body consists of trillions of cells organized into approximately 200 distinct cell types. What does this observation imply about the process of gene regulation during development?
Answer: A limited number of regulatory pathways are combined in different ways to produce diverse cell types.
The complexity of an organism is not just about the number of genes, but how they are regulated. A relatively small number of key regulatory genes and signaling pathways are used combinatorially during development to generate a much larger number of cell types. Different combinations of signals and transcription factors lead to different outcomes, creating cellular diversity from the same genome.
- A muscle cell and a neuron from the same person differ greatly in structure and function. What is the fundamental genetic basis for these differences?
Answer: Both cells contain the same genome, but they express different sets of genes.
Cell specialization (differentiation) does not involve changing the underlying genome. All somatic cells in an individual contain the same set of genes. The differences arise because different cell types activate or silence specific genes, leading to the production of different proteins and thus different structures and functions. This is known as differential gene expression.
- In a healthy adult mammal, the identity of a terminally differentiated cell, like a hepatocyte (liver cell), is very stable. What cellular mechanism is primarily responsible for maintaining this specialized state through cell division? [HL]
Answer: The inheritance of epigenetic markers, such as DNA methylation patterns, during mitosis.
Cellular identity is maintained through a concept known as 'cell memory'. This is achieved largely through epigenetic mechanisms. Patterns of DNA methylation and histone modifications that were established during differentiation are faithfully copied and passed on to daughter cells during mitosis. This ensures that a dividing liver cell produces more liver cells, as the gene expression patterns for 'liverness' are maintained.
- In the process of cell differentiation, what is the direct role of specific transcription factors? [HL]
Answer: They bind to specific DNA sequences to control the rate of transcription of target genes.
Transcription factors are key regulatory proteins that control gene expression. In differentiation, a unique combination of transcription factors is expressed in a cell. These proteins bind to promoter or enhancer regions of specific genes, either activating or repressing their transcription into mRNA. This selective gene expression is the underlying mechanism of cell specialization.
- Which property is necessarily lost when a pluripotent embryonic stem cell differentiates into a mature cardiac muscle cell?
Answer: The ability to differentiate into other cell types like neurons or liver cells.
The essence of differentiation is becoming specialized. In this process, a pluripotent cell, which has the potential to become any cell type, commits to a single lineage and loses the ability to form other types of cells. A mature cardiac cell is terminally differentiated and cannot become a neuron. Basic cellular functions like protein synthesis (A), respiration (B), and in some cases, mitosis (D), are retained.
- The therapeutic use of stem cells to treat Stargardt's disease involves differentiating them into retinal pigment epithelium (RPE) cells. What is the primary function these new cells must perform after transplantation into the eye?
Answer: To support and nourish the photoreceptor cells, which are dying in this disease.
Stargardt's disease is caused by the dysfunction and death of RPE cells. The function of RPE cells is not to detect light themselves, but to provide critical metabolic support to the light-detecting photoreceptor cells. The therapeutic goal is to replace the failing RPE cells with healthy ones derived from stem cells to halt the degeneration of photoreceptors.
- A major risk in therapies using pluripotent stem cells is the formation of teratomas, which are tumors containing a mixture of different tissue types. This risk is directly related to which characteristic of the cells?
Answer: Their potential to differentiate into all three germ layers in an uncontrolled manner.
A teratoma is a direct, albeit undesirable, demonstration of pluripotency. If pluripotent stem cells are injected without being fully differentiated into the desired cell type, their inherent ability to form cells of the ectoderm, mesoderm, and endoderm can lead to the growth of a tumor containing tissues like hair, teeth, muscle, and nerves. This risk is lower with more restricted multipotent cells.
- Which statement best distinguishes pluripotent from multipotent stem cells regarding their differentiation capacity?
Answer: Pluripotent cells can form any cell type from the three primary germ layers, while multipotent cells are limited to a specific lineage.
The defining difference lies in their potency. Pluripotent stem cells (like embryonic stem cells) can differentiate into any cell type derived from the ectoderm, mesoderm, or endoderm. Multipotent stem cells (like hematopoietic stem cells) are more limited and can only differentiate into a range of related cell types within a specific lineage (e.g., blood cells).
- In gene regulation, how does the process of methylation typically affect gene expression during cell specialization? [HL]
Answer: Methylation of DNA, particularly in promoter regions, is often associated with gene silencing.
DNA methylation is a key epigenetic mark. When methyl groups are added to CpG islands in the promoter regions of genes, it typically leads to the condensation of chromatin, making the DNA less accessible to transcription factors and RNA polymerase. This results in the long-term silencing or repression of that gene, which is a crucial mechanism for establishing and maintaining cell identity.
- A scientist is attempting to coax pluripotent stem cells to differentiate into neurons in a petri dish. What would be the most critical type of substance to add to the culture medium to direct this specific differentiation pathway?
Answer: A specific cocktail of growth factors and signaling molecules known to induce neural development.
Differentiation is controlled by external signals that activate internal gene expression programs. Scientists have identified specific growth factors, hormones, and other signaling molecules that mimic the developmental cues present in an embryo. By adding a specific combination of these molecules to the culture medium, they can direct the pluripotent cells to differentiate down a desired lineage, such as the neural pathway.
- What is the key cellular process that is largely reversed when scientists create induced pluripotent stem cells (iPSCs) from somatic cells like fibroblasts?
Answer: Cellular differentiation
The creation of iPSCs involves taking a specialized, differentiated cell (like a skin fibroblast) and 'reprogramming' it by introducing specific genes. This process forces the cell to de-differentiate, losing its specialized characteristics and regaining the pluripotent state of an embryonic stem cell. It is essentially a reversal of the natural differentiation process.
- Which of the following represents a key difference in the therapeutic use of hematopoietic stem cells (for leukemia) versus embryonic stem cells (for Stargardt's disease)?
Answer: Hematopoietic stem cell therapy has been established for decades, while therapies using embryonic stem cells are more recent and experimental.
Bone marrow transplants (using hematopoietic stem cells) have been a standard medical procedure for treating leukemia and other blood disorders for many years. In contrast, therapies using embryonic stem cells, such as for Stargardt's disease, are still in clinical trial phases and are considered experimental regenerative medicine. This reflects the different stages of development and approval for these two types of stem cell therapies.
- The use of a patient's own induced pluripotent stem cells (iPSCs) for therapy is being explored. What is a primary advantage of this approach over using embryonic stem cells (ESCs) from a donor?
Answer: iPSCs are immunologically compatible with the patient, reducing the risk of tissue rejection.
Because iPSCs are created by reprogramming a patient's own somatic cells (like skin cells), they are genetically identical to the patient. This means that if they are differentiated into therapeutic cells and transplanted back into the patient, the immune system will recognize them as 'self' and will not mount an immune response, avoiding rejection.
- Which piece of evidence most directly refutes the misconception that somatic cells lose genes as they differentiate?
Answer: The ability to create a clone of an entire organism from a single differentiated adult cell.
Cloning, such as in the case of Dolly the sheep, involves taking the nucleus from a differentiated somatic cell (e.g., a mammary cell) and transferring it to an enucleated egg cell. The fact that this nucleus can direct the development of a complete, new organism proves that it must contain the entire genome—all the genes required to make every cell type in that organism. Therefore, genes were not lost during differentiation.
