- The production of transgenic bacteria that can synthesize human insulin relies on creating a recombinant plasmid. A critical step in this technology is to cut both the human insulin gene and the bacterial plasmid with the same restriction enzyme. What is the direct consequence of this step?
Answer: It generates compatible 'sticky ends' on both DNA fragments, allowing them to anneal.
Most restriction enzymes make staggered cuts in the DNA, creating short, single-stranded overhangs known as 'sticky ends'. By using the same enzyme to cut both the plasmid vector and the gene insert, their sticky ends will be complementary. This allows the insert to anneal (base-pair) with the plasmid, holding it in place so that DNA ligase can form permanent phosphodiester bonds to create the recombinant molecule.
- In the Hershey-Chase experiment, bacteriophages were grown in two separate media: one with radioactive sulfur (³⁵S) and another with radioactive phosphorus (³²P). What was the critical technological rationale for selecting these specific isotopes to trace the genetic material?
Answer: ³²P is incorporated into the phosphate backbone of DNA, while ³⁵S is found in the amino acids methionine and cysteine within proteins.
The design of the experiment relied on the unique elemental composition of DNA and proteins. DNA contains phosphorus in its sugar-phosphate backbone but lacks sulfur. Proteins, conversely, lack phosphorus but contain sulfur in some amino acids (methionine and cysteine). This allowed Hershey and Chase to selectively label and trace the DNA and protein coat of the virus separately.
- Ecologists use sealed glass mesocosms to investigate the resilience of small ecosystems. Despite allowing for the control of variables, this technology has inherent limitations. Which of the following represents a significant limitation of using a mesocosm to model a large, natural lake ecosystem?
Answer: The small volume prevents the establishment of realistic nutrient concentrations and thermal stratification.
Large ecosystems like lakes have complex physical properties, such as thermal stratification (layers of different temperatures) and vast nutrient sinks and sources, that are impossible to replicate at the small scale of a mesocosm. The edge effects (influence of the container walls) and the lack of volume and depth prevent the model from accurately representing these large-scale physical and chemical processes.
- During the preparation of a human karyogram, cells are treated with a substance like colchicine that disrupts mitotic spindle formation. What is the direct technological purpose of this specific step?
Answer: To arrest the cells in metaphase, when chromosomes are maximally condensed and clearly visible.
For a karyogram, chromosomes must be clearly visible and distinguishable. They reach their most condensed and compact state during metaphase of mitosis. By disrupting the mitotic spindle, colchicine prevents the cell from progressing past metaphase into anaphase. This causes cells to accumulate at the metaphase stage, providing a large number of cells with chromosomes suitable for imaging and analysis.
- mRNA vaccines represent a different technology from traditional vaccines using inactivated viruses. What is the fundamental difference in how these two technologies initiate an immune response?
Answer: Traditional vaccines introduce a foreign antigen, while mRNA vaccines cause the host's cells to produce the foreign antigen.
The core technological difference lies in the source of the antigen. A traditional inactivated virus vaccine contains the actual viral proteins (antigens) which are introduced into the body directly. An mRNA vaccine contains the genetic instructions (mRNA) for a viral antigen. The host's own cells take up this mRNA and use their ribosomes to synthesize the foreign antigen. This self-produced antigen is then what stimulates the immune response.
- A cell biologist can easily visualize a mitochondrion using a standard light microscope but cannot resolve an individual ribosome within the same cell. What is the fundamental technological limitation of the light microscope that explains this difference?
Answer: The wavelength of visible light is longer than the diameter of a ribosome, preventing its resolution.
The ability of a microscope to distinguish two close objects as separate is called resolution. The resolution is fundamentally limited by the wavelength of the illumination source. Ribosomes (~25 nm) are smaller than the shortest wavelength of visible light (~400 nm). Consequently, light waves diffract around the ribosome rather than resolving its structure. Electron microscopes achieve higher resolution by using electrons, which have a much shorter wavelength.
- When designing a viral vector for use in human gene therapy, a critical technological modification is the removal of the virus's own replication and pathogenesis genes. What is the primary safety reason for this modification?
Answer: To prevent the virus from causing disease in the patient or from reproducing uncontrollably.
The main goal is to use the virus as a delivery vehicle (a vector) for the therapeutic gene, harnessing its natural ability to enter cells. However, the original virus is pathogenic and can replicate, which would cause disease in the patient. By removing the genes responsible for these functions, the vector is 'gutted' and rendered replication-incompetent and non-pathogenic, making it a safe tool for gene delivery.
- A genetic engineer is designing a CRISPR-Cas9 system to correct a specific point mutation in a target gene. The remarkable specificity of this gene-editing technology primarily relies on which of its components?
Answer: The nucleotide sequence of the guide RNA (gRNA), which is complementary to the target DNA sequence.
The CRISPR-Cas9 system's specificity comes from the guide RNA (gRNA). A portion of the gRNA is engineered to have a sequence that is complementary to the target DNA. This gRNA molecule guides the Cas9 enzyme to the precise location in the genome where the cut is to be made. The Cas9 protein itself does not have sequence-specificity for the target.
- A researcher is using cell culture technology to grow human skin cells in a laboratory. The cells are provided with a complex medium containing glucose, amino acids, vitamins, and salts in a buffered, isotonic solution. To stimulate the cells to progress through the cell cycle and undergo mitosis, which additional substance is typically required?
Answer: Growth factors from serum.
Most mammalian cells require external signals in the form of growth factors to progress through the cell cycle. These are typically supplied by adding serum (like fetal bovine serum) to the culture medium. Growth factors bind to cell surface receptors and trigger signaling pathways that allow cells to pass cell cycle checkpoints and enter mitosis.
- Paper chromatography is a technology used to separate pigments from a plant extract. The separation into distinct bands of colour occurs because the different pigment molecules possess different...
Answer: degrees of solubility in the solvent and adhesion to the stationary paper.
Paper chromatography is a form of partition chromatography. Separation depends on the differential partitioning of components between a stationary phase (the paper, which is polar) and a mobile phase (the solvent, which is typically non-polar). Pigments that are more soluble in the non-polar solvent and have weaker adhesion to the polar paper will travel further up the paper, resulting in separation.
- Hybridoma technology for producing monoclonal antibodies involves fusing antibody-producing plasma cells with myeloma (cancer) cells. What is the primary technological advantage conferred by the myeloma cell in the resulting hybridoma?
Answer: It confers the property of unlimited cell division, allowing for sustained antibody production in culture.
Plasma cells are specialized to produce a single type of antibody but have a limited lifespan. Myeloma cells are cancerous and are effectively immortal, meaning they can divide indefinitely in culture. Fusing the two creates a hybridoma cell that has both desired properties: it produces the specific antibody from the plasma cell and can be cultured long-term due to the immortality conferred by the myeloma cell.
