▶️ Answer/Explanation
DNA is a double-stranded helix consisting of two polynucleotide chains.
The “backbone” on the sides consists of alternating deoxyribose sugar and phosphate groups.
The center of the molecule contains the four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
Options B and D are incorrect because uracil is found in RNA, not DNA.
Options C and D are incorrect because DNA uses deoxyribose sugar, while RNA uses ribose.
The bases pair specifically (A with T, C with G) via hydrogen bonds to hold the strands together.
▶️ Answer/Explanation
The correct answer is D.
Potatoes contain the enzyme catalase, which breaks down $\text{H}_2\text{O}_2$ into $\text{H}_2\text{O}$ and $\text{O}_2$.
In Experiment 1, the enzyme is active, resulting in vigorous bubbling (oxygen release).
In Experiment 2, heating the potato provides thermal energy that disrupts chemical bonds.
This causes the enzyme to denature, permanently changing the shape of its active site.
Because the shape is lost, the substrate can no longer bind, and the reaction stops.
▶️ Answer/Explanation
Correct Option: D
A drought reduces the availability of water and flowering plants, leading to a nectar shortage.
Hummingbirds will face intense competition and a struggle for survival due to this lack of food.
Individual organisms cannot “immediately” change their biology or diet by choice (eliminating C).
Increasing reproduction (A) is impossible without energy from food, and (B) is biologically illogical.
Natural selection occurs as only individuals with traits allowing them to use alternative sources survive.
Therefore, the population will likely decline, with survival limited to those that can adapt.
▶️ Answer/Explanation
Prokaryotic cells are characterized by the absence of membrane-bound organelles.
Mitochondria and chloroplasts are membrane-bound, so they are only found in eukaryotes.
Cell $3$ lacks both mitochondria and chloroplasts, fitting the prokaryotic profile.
Unlike eukaryotic organelles, ribosomes are found in both cell types for protein synthesis.
Many prokaryotes (like bacteria) possess a cell wall, which Cell $3$ has.
Cells $1$, $2$, and $4$ all contain mitochondria, identifying them as eukaryotic.
Therefore, Cell $3$ is the only candidate for a prokaryotic cell.
▶️ Answer/Explanation
The correct answer is A.
Three consecutive nucleotides in DNA or mRNA form a unit called a codon.
Each codon specifies a particular amino acid to be added to a polypeptide chain.
This triplet code is universal across almost all living organisms.
Multiple codons linked together eventually form a sequence that codes for a protein.
Therefore, a single triplet corresponds to only one amino acid, not a whole protein.
▶️ Answer/Explanation
• Mitosis is the process that ensures DNA is replicated and organized before the cell divides.
• Cytokinesis is the physical splitting of the cytoplasm into two daughter cells.
• If cytokinesis occurs before mitosis, the genetic material has not been properly segregated.
• This results in daughter cells receiving incomplete or random sets of chromosomes and organelles.
• Lacking essential genetic instructions, these cells cannot function and will trigger apoptosis (cell death).
• Therefore, statement D is the most accurate biological consequence of this error.
▶️ Answer/Explanation
The correct answer is B.
Every somatic cell in an organism contains the exact same DNA sequence.
The difference in cell types arises through a process called cell differentiation.
In this process, specific genes are switched “on” or “off” based on chemical signals.
Therefore, feather cells and scale cells express different sets of proteins despite having the same genome.
Options C and D are biologically incorrect as somatic cells remain diploid and share the same DNA.
▶️ Answer/Explanation
The correct answer is A.
Photosynthesis is the process where plants convert solar energy (light) into chemical energy (glucose).
Transfer $1$ correctly identifies “solar” as the starting type and “chemical” as the ending type.
Transfer $2$ describes metabolic heat loss or combustion.
Transfer $3$ represents cellular respiration, converting stored chemical energy into usable $\text{ATP}$.
Transfer $4$ is incorrect because solar energy is not converted directly into $\text{ATP}$ for cellular work without the intermediate glucose stage.
Therefore, only Transfer $1$ accurately reflects the primary energy transformation of photosynthesis.
▶️ Answer/Explanation
Aerobic respiration is the process used by cells to release energy from glucose.
This biological reaction requires oxygen ($O_2$) to fully break down glucose molecules.
The chemical equation is: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$.
Carbon dioxide and water are produced as by-products, not as requirements.
While anaerobic respiration can occur without oxygen, it is less efficient and produces lactic acid.
Therefore, oxygen is the essential substance for the primary release of energy in most organisms.
▶️ Answer/Explanation
The carbon cycle involves the movement of carbon between the atmosphere, biosphere, and geosphere.
Option D correctly tracks the flow of carbon from an inorganic form ($\text{CO}_2$) to an organic form (glucose in plants).
This carbon is then transferred through the food chain when the plant is consumed by an animal.
Option A is incorrect because animals do not “use” $\text{CO}_2$ for respiration; they produce it as a byproduct.
Option B describes a physical arrangement of atoms, not a biological or geochemical cycle.
Option C describes the accumulation of carbon in one reservoir rather than its movement through a cycle.
▶️ Answer/Explanation
11. Answer: C
The cell membrane contributes to homeostasis by acting as a selectively permeable barrier.
This means it regulates the passage of substances, allowing essential materials like water and nutrients to enter while keeping harmful substances out.
Option A is incorrect because a completely solid membrane would prevent necessary exchanges.
Option B is incorrect because cells need to transport ions and nutrients, not just water.
Option D is incorrect because unregulated transport would disrupt the internal balance.
12. Answer: B
The correct statement is: “The cell membrane maintains homeostasis by allowing water to exit the cell.”
In a high-salinity environment (hypertonic solution), the salt concentration outside the cell is higher than inside.
Water naturally moves from an area of high water concentration (inside) to low concentration (outside) via osmosis.
By allowing water to exit, the cell balances its internal concentration with the environment, preventing damage.
13. Answer: A
To test the hypothesis, the experiment must vary the salt concentration (the independent variable) while keeping the cell type constant.
Option A correctly uses the same cell type across different concentrations and measures size frequently to determine the speed of water movement.
Option B is incorrect because it uses only one concentration, so no comparison can be made.
Options C and D introduce a confounding variable (different fish species) and fail to measure the rate of change over time.
14. Answer: C
The food web shows a direct energy flow: Algae → Zooplankton → Finger Mullet.
If the algae population decreases, the zooplankton will have less food available, causing their population to decline.
With fewer zooplankton to eat, the finger mullet population will also decrease due to a lack of food resources.
This illustrates the “bottom-up” effect where changes at the producer level impact all higher consumer levels.
15. Answer: Statements 1, 3, and 6
Statement 1: The food web diagram shows only one arrow pointing to the red drum, coming from the finger mullet, meaning it is its sole energy source.
Statement 3: Phytoplankton are producers that create glucose and oxygen through photosynthesis; these products support the entire ecosystem, including the heron.
Statement 6: Energy is highest at the producer level (phytoplankton) and decreases by approximately 90% at each trophic level as heat.
Statements 2, 4, and 5 are false because seagrass is a producer (not consumer), algae produce their own glucose, and top predators have the least available energy.
▶️ Answer/Explanation
Energy transfer in an ecosystem is always inefficient, with only about $10\%$ of energy moving to the next level.
Producers must contain the most energy to support the subsequent trophic levels.
As energy is lost as heat and through metabolic processes, each higher level receives significantly less energy.
Model B represents this “Energy Pyramid” where the base (producers) is the largest.
The narrowing shape reflects the decreasing energy available to primary, secondary, and tertiary consumers.
A stable ecosystem requires this broad base of producers to maintain higher-level consumers.
▶️ Answer/Explanation
Correct Option: D
Introducing a new species often disrupts the existing ecological balance.
The new species will require energy and nutrients to survive and reproduce.
It will occupy a niche that overlaps with native species already in the area.
This creates increased competition for limited food and nesting resources.
Native species may decline if the newcomer is a more efficient consumer.
Lack of local natural predators often allows the new population to grow rapidly.
▶️ Answer/Explanation
Global climate change is primarily driven by the accumulation of greenhouse gases like $CO_2$.
Burning fossil fuels for power is the largest source of these heat-trapping emissions.
Converting to carbon-free energy (solar, wind, nuclear) directly addresses the root cause.
Option A relates to UV protection, not the greenhouse effect or global warming.
Option C helps public health but does not stop the warming of the planet.
Option D preserves ecosystems but has a negligible impact on atmospheric temperature.
Therefore, reducing carbon output is the most effective strategy for climate mitigation.
▶️ Answer/Explanation
Process X represents fertilization, where two haploid ($1n$) gametes combine to form a diploid ($2n$) zygote.
This fusion of DNA from two different parents significantly increases genetic variation.
Process Z represents mitosis, where a single diploid ($2n$) parent cell divides into two identical $2n$ daughter cells.
Mitosis is a form of asexual reproduction or growth that maintains genetic consistency rather than variety.
Therefore, cells from Process X (sexual) have more variation than those from Process Z (asexual).
This makes Statement A the most accurate biological explanation for the two diagrams.
▶️ Answer/Explanation
The diagram illustrates homologous chromosomes exchanging genetic material.
Initially, one chromosome is white and the other is solid black.
After the arrow, segments of the chromatids have “swapped” colors.
This physical exchange of DNA segments is known as crossing over.
It occurs during Prophase I of meiosis when homologous pairs form tetrads.
This process creates new combinations of alleles on a single chromosome.
Consequently, it is a primary driver of genetic variation in offspring.
▶️ Answer/Explanation
In this cross, the red allele ($R$) is not completely masked by or masking the white allele ($W$).
The heterozygous offspring ($RW$) display a pink phenotype, which is a blend of the two parental colors.
This intermediate expression is the hallmark of incomplete dominance.
In contrast, codominance would result in both red and white appearing distinctly (like spots).
Polygenic traits involve multiple genes, while this cross tracks a single gene locus.
Since a new phenotype is created by the combination of alleles, option C is correct.
▶️ Answer/Explanation
The correct answer is B.
The study shows a continuous range of neck lengths ($5.9$ to $7.9$ feet) rather than discrete categories.
Continuous variation is a hallmark of polygenic inheritance, where multiple genes influence a single trait.
In polygenic traits, each gene has a small additive effect, resulting in a bell-shaped curve distribution.
Single-gene traits (like codominance or simple dominance) typically result in distinct, “either-or” phenotypes.
Therefore, the wide spectrum of measurements most likely indicates control by multiple genes.
▶️ Answer/Explanation
In DNA fingerprinting, every band in a child’s profile must be present in at least one parent’s profile.
By comparing Baby Y to the parents, we see that its top two bands match the Woman.
The middle thick band and the next two lower bands in Baby Y match the Man’s profile.
The bottom-most band in Baby Y matches a corresponding band in the Woman’s profile.
Other babies (W, X, and Z) possess unique bands that do not appear in either the Man or the Woman.
Since all bands in Baby Y are accounted for by the potential parents, it is the biological offspring.
▶️ Answer/Explanation
The correct option is B because:
Genetically engineered corn (such as Bt corn) produces its own internal pesticide.
This biological trait allows the plant to defend itself against specific insect pests naturally.
Consequently, farmers do not need to apply as many external chemical sprays to the fields.
Reducing chemical use lowers production costs and minimizes environmental runoff.
Options A, C, and D are not direct biological outcomes of producing an internal pesticide.
▶️ Answer/Explanation
The correct option is A.
Antibiotics exert selective pressure on bacterial populations.
While the medication kills susceptible bacteria, those with genetic mutations for resistance survive.
These resistant survivors then reproduce, passing the resistance traits to the next generation.
Vaccinations (Option B) are typically preventative, not a treatment for an active infection.
Antibodies (Option C) assist the immune response but do not define the medication’s impact on reproductive success.
Medications are designed to inhibit metabolic processes, not act as an energy source (Option D).
Thus, the ongoing reproduction of resistant strains is the most accurate description of long-term reproductive success.
▶️ Answer/Explanation
Nucleotide sequences (DNA) provide the most direct and objective evidence of evolution.
Molecular data allows scientists to quantify the exact number of genetic differences between species.
While structures can be subject to convergent evolution, DNA sequences track lineage more accurately.
The more similar the nucleotide sequences, the more recently the species shared a common ancestor.
This molecular “clock” is more precise than comparing physical traits or embryos.
Therefore, biochemical evidence is considered the “gold standard” for determining evolutionary relationships.
▶️ Answer/Explanation
1. The sharks have been able to adapt to changes in their environment.
Natural selection drives adaptation, allowing a population to survive shifting environmental pressures.
2. Individuals differ from one another as a result of genetic diversity.
Variation is the raw material for selection; without differences, “survival of the fittest” cannot occur.
3. The average number of births has been greater than the average number of deaths in the population.
A successful population must produce more offspring than are lost to maintain growth and spread favorable traits.
Genetic diversity ensures that some individuals possess traits that offer a competitive advantage.
These advantageous traits are passed to the next generation through successful reproduction (births > deaths).
Over time, this process results in a population that is better adapted to its specific ecological niche.
▶️ Answer/Explanation
The introduction of wolves creates a selective pressure on the rabbit population.
Rabbits with the mutation for extra-powerful hind legs have a survival advantage.
These individuals are more likely to escape predation and live long enough to reproduce.
They pass the beneficial mutation (alleles) to their offspring.
Over generations, the frequency of the mutation increases within the population.
This process is a classic example of natural selection driving evolution.
Options A, C, and D are incorrect as they ignore the adaptive advantage of the mutation.
▶️ Answer/Explanation
The correct answer is D.
Enzymes function most efficiently at their optimal temperature, which is $25^\circ\text{C}$ in this case.
Increasing the temperature from $15^\circ\text{C}$ toward $25^\circ\text{C}$ provides more kinetic energy, increasing molecular collisions.
As the temperature reaches the optimum, the rate of reaction hits its maximum point.
Option B is incorrect because temperatures above the optimum can cause the enzyme to denature.
Options A and C would either have no effect or would decrease the reaction rate.
▶️ Answer/Explanation
The correct option is (B).
Species \(1\) has \(5\) needles per cluster. Following the key: Step \(1b\) (\(2-5\) needles) \(\rightarrow\) Go to \(2\) \(\rightarrow\) Step \(2a\) (\(5\) needles) identifies it as Pinus strobus.
Species \(2\) has clusters with more than \(5\) needles. Following the key: Step \(1a\) (more than \(5\) needles) identifies it as Larix laricina.
Species \(3\) has long needles in pairs (\(2\)). Following the key: Step \(1b\) \(\rightarrow\) Step \(2b\) (\(2\) needles) \(\rightarrow\) Go to \(3\) \(\rightarrow\) Step \(3a\) (long needles) identifies it as Pinus resinosa.
Species \(4\) has short needles in pairs (\(2\)). Following the key: Step \(1b\) \(\rightarrow\) Step \(2b\) \(\rightarrow\) Step \(3b\) (short needles) identifies it as Pinus banksiana.
▶️ Answer/Explanation
The nuclear envelope acts as a barrier, while nuclear pores serve as gatekeepers for transport.
Transcription occurs inside the nucleus, where mRNA is synthesized from the DNA template.
However, translation (protein synthesis) takes place on ribosomes located in the cytoplasm.
Without pores, mRNA molecules are trapped inside the nucleus and cannot reach the ribosomes.
Consequently, the cell would be unable to produce the proteins necessary for survival.
Note: DNA normally remains inside the nucleus and does not need to leave to facilitate protein synthesis.
▶️ Answer/Explanation
The correct answer is D.
Ribosomes are the sites of translation, where the genetic code is turned into a protein.
$\text{mRNA}$ binds to the ribosome to provide the template for the sequence.
$\text{tRNA}$ molecules carry specific amino acids to the ribosome.
The anticodons on $\text{tRNA}$ attach to the codons on $\text{mRNA}$ through complementary base pairing.
This alignment allows the ribosome to catalyze the formation of peptide bonds between amino acids.
Option A describes transcription, which occurs in the nucleus, not on a ribosome.
▶️ Answer/Explanation
Photosynthesis captures solar energy to convert \(\text{CO}_2\) and \(\text{H}_2\text{O}\) into energy-rich carbohydrates.
This process effectively stores energy within the chemical bonds of glucose.
Cellular respiration then breaks down these organic carbohydrates in the presence of oxygen.
This breakdown releases the stored energy to synthesize \(\text{ATP}\), the cell’s primary energy currency.
Therefore, photosynthesis is an energy-storing process, while respiration is an energy-releasing process.
Options B, C, and D are incorrect as they misidentify the energy source or the direction of energy flow.
▶️ Answer/Explanation
Phytoplankton are producers that convert solar energy into chemical energy via photosynthesis.
In an ecological pyramid, producers must have the largest biomass and population to support higher levels.
As energy moves up the food chain (from larvae to frogs to trout), approximately $90\%$ of energy is lost as heat.
Because only $10\%$ of energy is transferred to each succeeding level, fewer organisms can be supported at the top.
Therefore, a massive base of phytoplankton is required to sustain a very small number of apex predators like trout.
▶️ Answer/Explanation
Correct Option: B
A longer snow season provides a white background for a greater portion of the year.
White foxes possess camouflage in snow, making them less visible to predators and more effective hunters.
This selective advantage leads to higher survival and reproduction rates for white foxes.
Conversely, red foxes are easily spotted against white snow, increasing their risk of predation.
As a result, the white fox population is likely to increase while the red fox population decreases.
This is a classic example of natural selection driven by environmental changes.
▶️ Answer/Explanation
36. (C)
Gametes (sperm and egg) are haploid cells produced by meiosis. When they fuse during the process of fertilization, they combine their genetic material to form a zygote (embryo). The resulting embryo is diploid, meaning it contains two full sets of chromosomes (one from each parent).
37. (C)
Height is a polygenic trait determined by multiple alleles (represented by \(T\) and \(t\)). Because parents are heterozygous for various genes, independent assortment during meiosis creates diverse combinations. The sister received a higher percentage of recessive “short” alleles (\(t\)) compared to Judd, resulting in a shorter phenotype.
38. (B)
Figure 2 shows a continuous bell-shaped distribution of height. Traits that show a continuous range of phenotypes (rather than distinct categories like “tall” or “short”) are typically polygenic, meaning they are controlled by the interaction of multiple genes.
39. (A)
In a normal distribution histogram (Figure 2), the height of the bars represents the frequency of the trait. The ranges 50–55 inches and 75–80 inches are located at the extreme “tails” of the graph where the bars are very short or non-existent. This indicates that both ranges represent rare phenotypes.
40. (B)
The data shows a correlation between socioeconomic status/health and average height. Neighborhood 1 (positive health/high status) has the tallest average, while Neighborhood 2 (negative health/low status) has the shortest. This suggests that environmental factors like nutrition and healthcare availability significantly influence the phenotypic expression of height.
▶️ Answer/Explanation
The correct answer is C.
Natural selection occurs when a specific trait provides a survival advantage in a given environment.
Individuals possessing the mutated receptor protein were resistant to Yersinia pestis.
These survivors had a higher fitness, meaning they lived long enough to reproduce.
As they passed the beneficial mutation to their offspring, the frequency of the allele increased.
This process is known as being “selected for” by environmental pressure.
Vaccines (Option A) were not available in the $1300\text{s}$ and do not cause genetic mutations.
Variation (Option B) must exist before selection; the outbreak itself did not create the mutation.
▶️ Answer/Explanation
Correct Option: C
The change in dairy cows is a result of selective breeding, also known as artificial selection.
Humans identify parent cows with desirable traits, such as high milk volume or fat content.
These specific individuals are then bred together to ensure those genes are passed to the next generation.
Unlike natural selection, which is driven by environmental fitness, this process is controlled by human preference.
Over many generations, this directed breeding leads to significant physiological changes in the population.
Options A and B focus on diversity levels rather than the mechanism of change, making C the most accurate explanation.
▶️ Answer/Explanation
At Generation $40$, the antibiotic is introduced, causing a sharp decline in the population.
By Generation $70$, the population begins to increase again despite the presence of the antibiotic.
This recovery occurs because certain individuals possessed genetic variations or mutations conferring resistance.
These resistant bacteria survived the initial treatment while the non-resistant ones died off.
The survivors kemudian passed these resistant traits to their offspring through reproduction.
Over subsequent generations, the population of antibiotic-resistant bacteria grew, leading to the observed increase.
This process is a classic example of natural selection acting on a bacterial population.
▶️ Answer/Explanation
Correct Option: D
Comparing DNA sequences provides the most objective and precise data for classification.
Physical traits (A) can be misleading due to convergent evolution or environmental factors.
Cellular structures (B) and feeding habits (C) often overlap significantly across different genera.
Modern taxonomy relies on molecular phylogenetics to identify unique genetic markers.
A distinct genetic signature confirms reproductive isolation and evolutionary divergence.
Therefore, DNA analysis is the best and most definitive method for identifying a new species.
▶️ Answer/Explanation
The correct option is D.
Mutations are the primary source of genetic variation within a population.
They create new alleles, which changes the composition of the gene pool.
Natural selection requires this variation to “choose” traits that improve fitness.
Without mutations, populations would lack the diversity needed to adapt to environmental changes.
Somatic mutations (Option C) are not inherited; only germline mutations contribute to evolution.
Mutations are random and do not occur in response to observation or specific disasters.
▶️ Answer/Explanation
The offspring are genetically identical because they are clones, meaning their DNA sequences are the same.
Variation in physical traits (phenotypes) like height often results from environmental factors.
Different zoos provide different diets, climates, and care, which trigger different gene expressions.
This interaction between genes and the environment is known as phenotypic plasticity.
Mutations (Options A and C) are rare and unlikely to occur identically or insignificantly across all five.
Environmental influence on gene expression (epigenetics) perfectly explains how identical DNA produces different results.
▶️ Answer/Explanation
The correct answer is C.
In a phylogenetic tree, the most recent common ancestor is represented by the node (branching point) closest to the tips.
The node connecting the chicken and the mouse is further to the right than the nodes for other pairs listed.
Specifically, the chimpanzee and mouse share the most recent ancestor overall.
Among the given choices, the chicken and mouse share a more recent ancestor than frogs/chickens or drosophila/lancelets.
This indicates a more recent evolutionary divergence and closer genetic relationship.
Therefore, option C is the most accurate choice based on the branching structure provided.
▶️ Answer/Explanation
The correct answer is C.
Photosynthesis captures solar energy to convert $CO_2$ and $H_2O$ into glucose ($C_6H_{12}O_6$).
This glucose serves as the primary chemical energy source for the plant and the food web.
In cellular respiration, cells break down this glucose in the presence of oxygen.
This process releases stored chemical energy to produce ATP, the energy currency of the cell.
Thus, the products of photosynthesis act as the essential reactants for respiration.
This creates a biological cycle where energy is captured, stored, and then utilized.
▶️ Answer/Explanation
The correct answer is A.
Kudzu is an invasive species that lacks natural predators in the U.S.
It grows aggressively, often outcompeting and smothering native plants for sunlight.
By displacing native vegetation, it significantly reduces local biodiversity.
A “weed” is typically defined as a plant growing aggressively where it is not wanted.
While it controlled erosion, its ecological damage eventually outweighed its benefits.
This rapid spread and negative impact led to its reclassification by the USDA.
▶️ Answer/Explanation
In a phylogenetic tree, the vertical or diagonal axis typically represents time.
The base of the tree represents the common ancestor, which is the furthest point in the past.
Organisms that branch off closer to the root (the bottom) appeared earlier in evolutionary history.
Looking at the diagram, the branch for organism $T$ is the first to diverge from the main lineage.
Because it splits off at the lowest point relative to the others, its fossil would be the oldest.
Organisms like $W$, $Z$, $X$, and $Y$ appear at higher nodes, indicating they evolved more recently.