Life Science Flashcards

A research team is investigating a newly discovered extremophile bacterium from a hydrothermal vent. They observe that the bacterium utilizes a unique metabolic pathway to chemosynthesize energy from sulfur compounds. This process is entirely independent of sunlight. Which of the following cellular organelles, if absent, would definitively rule out the possibility of this organism being a eukaryote?

Answer: Mitochondrion

Mitochondria are the primary sites of cellular respiration in eukaryotes, a process that generates ATP. While this extremophile uses chemosynthesis, the fundamental machinery for large-scale energy production in eukaryotes is housed in mitochondria. The absence of mitochondria is a hallmark of prokaryotic cells. Ribosomes, cell membranes, and cytoplasm are found in both prokaryotic and eukaryotic cells.

In a semi-arid ecosystem in Arizona, a prolonged drought has significantly reduced the population of a keystone species, the banner-tailed kangaroo rat, which creates burrows that many other species use for shelter. Which of the following represents the most likely cascading ecological effect of this population decline?

Answer: A decrease in the genetic diversity of predator populations like coyotes and owls.

A sharp decline in a primary prey species (kangaroo rat) would lead to increased competition and starvation among its predators (coyotes, owls). This would likely cause a population bottleneck, reducing the overall genetic diversity of the predator populations as only certain individuals survive and reproduce. While there might be some impact on producers or soil, the most direct and significant cascading effect described is on the predator populations that rely heavily on the keystone species.

A geneticist is studying a family pedigree for a rare genetic disorder. They observe that affected fathers always pass the disorder to all of their daughters, but never to their sons. Affected mothers pass the disorder to half of their sons and half of their daughters. What is the most probable mode of inheritance for this disorder?

Answer: X-linked dominant

This pattern is characteristic of X-linked dominant inheritance. Since fathers give their only X chromosome to all their daughters, an affected father will pass the trait to all of them. Fathers give their Y chromosome to their sons, so they cannot pass an X-linked trait to them. Affected mothers have a 50% chance of passing their affected X chromosome to any child, regardless of sex.

In the process of protein synthesis, a specific type of RNA molecule is responsible for reading the codons on the mRNA template and delivering the corresponding amino acid to the ribosome. A mutation occurs that alters the anticodon sequence of this RNA molecule. What is the most immediate and direct consequence of this mutation?

Answer: An incorrect amino acid will be incorporated into the polypeptide chain.

The RNA molecule described is transfer RNA (tRNA). The anticodon on tRNA is complementary to the codon on mRNA and determines which amino acid is carried. If the anticodon is mutated, it may bind to the wrong codon or bring the wrong amino acid for the correct codon, directly resulting in the incorporation of an incorrect amino acid into the growing protein.

A biologist is studying two isolated populations of the same species of desert tortoise. Population A lives in a valley with abundant, evenly distributed resources. Population B lives on a rocky hillside with scarce, patchy resources. Over many generations, which of the following evolutionary pressures would most likely lead to greater phenotypic variation in Population B compared to Population A?

Answer: Disruptive selection

Disruptive selection (or diversifying selection) is a mode of natural selection in which extreme phenotypes are favored over intermediate phenotypes. In Population B's environment with patchy resources, tortoises with specialized traits (e.g., better at climbing for high-up food, or better at digging for buried food) might have a survival advantage over generalists. This would lead to the population diverging and showing greater variation. Stabilizing selection would reduce variation by favoring the average, which is more likely in the stable environment of Population A.

A scientist is evaluating the resilience of a chaparral ecosystem following a wildfire. They measure the rate of secondary succession by monitoring the return of native plant species. They observe that in areas treated with a specific microbial soil inoculant, the rate of recovery is significantly faster than in untreated areas. What is the most plausible explanation for this observation?

Answer: The inoculant creates a physical barrier that prevents erosion and loss of topsoil.

Wildfires can sterilize soil and deplete essential nutrients like nitrogen. Many soil microbes, particularly certain bacteria, form symbiotic relationships with plants and can fix atmospheric nitrogen into a usable form (ammonia). By reintroducing these beneficial microbes, the inoculant would enrich the soil, providing the necessary nutrients for native plants to re-establish and grow more quickly, thus accelerating secondary succession.