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NYSTCE Biology CST study materials
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NYSTCE Biology CST practice questions

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Question 1Biology content domain (not enumerated in source)

In a population of beetles, a mutation produces a darker cuticle that better camouflages individuals on dark soil. Over several generations on that soil, the frequency of the dark allele rises. Which evolutionary mechanism does this best illustrate?

  1. Natural selection Correct answer.
  2. Genetic drift
  3. Gene flow
  4. Assortative mating

Explanation

Natural selection is the differential survival and reproduction of individuals based on heritable variation, which is exactly what happens when darker beetles survive predators more often and leave more offspring. The allele frequency shift is driven by a consistent fitness advantage tied to the environment.

Question 2Biology content domain (not enumerated in source)

Which statement correctly describes the relationship between a codon and an amino acid during translation?

  1. A codon is a three-nucleotide sequence that specifies one amino acid Correct answer.
  2. A codon is a two-nucleotide sequence that specifies one amino acid
  3. A codon is a single nucleotide that specifies one amino acid
  4. A codon is a three-amino-acid sequence that specifies one nucleotide

Explanation

The genetic code is read in triplets, so each mRNA codon consists of three nucleotides that correspond to a specific amino acid or a stop signal. This triplet arrangement provides 64 possible codons for 20 standard amino acids, allowing redundancy in the code.

Question 3Biology content domain (not enumerated in source)

A cell is observed with chromosomes condensing and the nuclear envelope beginning to break down. Which phase of mitosis is this?

  1. Prophase Correct answer.
  2. Anaphase
  3. Telophase
  4. Metaphase

Explanation

Prophase is marked by chromatin condensing into visible chromosomes and the nuclear envelope starting to disintegrate as the spindle forms. These events occur before chromosomes align at the metaphase plate.

Question 4Biology content domain (not enumerated in source)

Which molecule serves as the primary energy currency that directly powers most cellular work, such as active transport and enzyme-driven reactions?

  1. NADH
  2. Cholesterol
  3. Glucose
  4. ATP Correct answer.

Explanation

ATP is the immediate energy currency of the cell because hydrolysis of its terminal phosphate bond releases energy that can be coupled to endergonic processes. Glucose and NADH store energy but must be processed to generate ATP before most work can proceed.

Question 5Biology content domain (not enumerated in source)

A plant cell placed in a hypertonic solution will most likely show which response?

  1. Bursting of the cell membrane
  2. No net water movement because the cell wall blocks osmosis
  3. Net uptake of water and increased turgor pressure
  4. Loss of water and shrinkage of the protoplast Correct answer.

Explanation

In a hypertonic solution, the solute concentration outside the cell exceeds that inside, so water leaves the cell by osmosis. In plant cells this causes the protoplast to shrink away from the cell wall, a condition called plasmolysis.

Question 6Biology content domain (not enumerated in source)

Which base pair is found in a DNA double helix held together by three hydrogen bonds?

  1. Adenine and thymine
  2. Guanine and cytosine Correct answer.
  3. Guanine and thymine
  4. Adenine and uracil

Explanation

Guanine pairs with cytosine through three hydrogen bonds, while adenine pairs with thymine through two. This difference contributes to the greater thermal stability of GC-rich DNA regions.

Question 7Biology content domain (not enumerated in source)

A cross between two heterozygous pea plants (Tt × Tt) for a trait showing complete dominance produces offspring. What is the expected phenotypic ratio?

  1. 3 dominant : 1 recessive Correct answer.
  2. 9 dominant : 3 recessive
  3. 1 dominant : 1 recessive
  4. 1 dominant : 2 intermediate : 1 recessive

Explanation

A monohybrid cross of two heterozygotes yields a genotypic ratio of 1 TT : 2 Tt : 1 tt. Because complete dominance masks the heterozygote, the phenotypic ratio is 3 dominant to 1 recessive.

Question 8Biology content domain (not enumerated in source)

Which structure in a eukaryotic cell is the primary site of ATP production by oxidative phosphorylation?

  1. Golgi apparatus
  2. Ribosome
  3. Mitochondrion Correct answer.
  4. Lysosome

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, where oxidative phosphorylation generates most cellular ATP. Other organelles perform unrelated roles in synthesis, modification, or degradation.

Question 9Biology content domain (not enumerated in source)

A researcher measures the rate of an enzyme-catalyzed reaction as substrate concentration increases while enzyme concentration is held constant. The rate rises and then levels off. What best explains the plateau?

  1. All active sites are saturated with substrate Correct answer.
  2. The reaction has reached chemical equilibrium
  3. Substrate molecules begin to inhibit each other
  4. The enzyme has been permanently denatured

Explanation

At high substrate concentrations, every enzyme active site is occupied, so adding more substrate cannot increase the reaction rate further. The maximum velocity reflects the enzyme's turnover capacity under those conditions.

Question 10Biology content domain (not enumerated in source)

During glycolysis, one molecule of glucose is converted to two molecules of pyruvate. What is the net yield of ATP and NADH per glucose in this pathway?

  1. 1 ATP and 1 NADH
  2. 2 ATP and 2 NADH Correct answer.
  3. 4 ATP and 2 NADH
  4. 2 ATP and 4 NADH

Explanation

Glycolysis consumes 2 ATP during the investment phase and produces 4 ATP during the payoff phase, for a net gain of 2 ATP. It also reduces 2 NAD+ to 2 NADH per glucose molecule.

Question 11Biology content domain (not enumerated in source)

A mutation inserts a single nucleotide into the coding region of a gene. Which consequence is most likely?

  1. A deletion of the entire gene
  2. A frameshift that alters every downstream codon Correct answer.
  3. A substitution of one amino acid only
  4. A silent substitution with no effect on the protein

Explanation

Inserting one nucleotide shifts the reading frame, so all codons downstream of the mutation are read differently and the resulting protein is usually nonfunctional. This is why frameshift mutations are often more severe than substitutions.

Question 12Biology content domain (not enumerated in source)

In a Hardy-Weinberg population at equilibrium, the frequency of the recessive allele is 0.3. What is the expected frequency of heterozygous individuals?

  1. 0.21
  2. 0.09
  3. 0.42 Correct answer.
  4. 0.49

Explanation

With q = 0.3, p = 0.7, and the heterozygote frequency is 2pq = 2(0.7)(0.3) = 0.42. This follows directly from the Hardy-Weinberg equation p² + 2pq + q² = 1.

Question 13Biology content domain (not enumerated in source)

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotic cells lack ribosomes
  2. Prokaryotic cells lack nucleic acids
  3. Prokaryotic cells lack a plasma membrane
  4. Prokaryotic cells lack a membrane-bound nucleus Correct answer.

Explanation

Prokaryotes keep their genetic material in a nucleoid region without a surrounding nuclear envelope, whereas eukaryotes enclose DNA within a true nucleus. Prokaryotes still possess ribosomes, a plasma membrane, and DNA.

Question 14Biology content domain (not enumerated in source)

A cell that undergoes meiosis produces four haploid gametes. Which event during meiosis is most directly responsible for generating genetic variation among these gametes?

  1. DNA replication before meiosis
  2. Attachment of spindle fibers to kinetochores
  3. Cytokinesis following telophase
  4. Crossing over between homologous chromosomes Correct answer.

Explanation

Crossing over during prophase I exchanges segments between homologous chromosomes, creating new allele combinations, and independent assortment adds further variation. DNA replication and cytokinesis are necessary but do not themselves reshuffle alleles.

Question 15Biology content domain (not enumerated in source)

In an ecosystem, which trophic level typically contains the least total energy available to the next level?

  1. Tertiary consumers Correct answer.
  2. Primary producers
  3. Primary consumers
  4. Decomposers

Explanation

Energy is lost at each trophic transfer, so higher trophic levels such as tertiary consumers retain only a small fraction of the energy captured by producers. This is why food chains rarely support many top-level consumers.

Question 16Biology content domain (not enumerated in source)

A particular enzyme functions optimally at pH 2 within the stomach. If the same enzyme is moved to a pH 7 buffer, what is the most likely result?

  1. Its activity increases because the active site becomes more flexible
  2. Its activity increases because more substrate is available
  3. Its activity decreases because its tertiary structure is disrupted Correct answer.
  4. Its activity stays the same because pH does not affect enzymes

Explanation

Each enzyme has an optimal pH at which its ionization state and shape support catalysis. A large shift away from that optimum alters charge interactions and disrupts the active site, lowering activity.

Question 17Biology content domain (not enumerated in source)

A woman who is a carrier for an X-linked recessive disorder has children with a man who does not have the disorder. What proportion of their sons is expected to be affected?

  1. One-quarter
  2. None
  3. One-half Correct answer.
  4. All

Explanation

Sons inherit their single X chromosome from their mother, so a carrier mother passes the affected allele to half of her sons. Because males have only one X, that allele is expressed, making those sons affected.

Question 18Biology content domain (not enumerated in source)

Which process directly produces the oxygen released during photosynthesis?

  1. Synthesis of glucose from G3P
  2. Reduction of NADP+ in the stroma
  3. Fixation of carbon dioxide in the Calvin cycle
  4. Photolysis of water in the light reactions Correct answer.

Explanation

In the light reactions, the oxygen-evolving complex splits water to replace electrons lost by photosystem II, releasing O2 as a byproduct. Carbon fixation and sugar synthesis occur later and do not produce oxygen.

Question 19Biology content domain (not enumerated in source)

A population of birds is split by a new river that prevents interbreeding. After many generations, the two populations can no longer produce fertile offspring. Which term best describes this outcome?

  1. Adaptive radiation within a single habitat
  2. Convergent evolution
  3. Genetic drift within one population
  4. Reproductive isolation leading to speciation Correct answer.

Explanation

Geographic separation can lead to divergence until the populations can no longer interbreed, which is reproductive isolation and the hallmark of speciation. The inability to produce fertile offspring confirms that distinct species have formed.

Question 20Biology content domain (not enumerated in source)

A cell has a mutation that prevents the formation of the mitotic spindle. At which point will the cell cycle most likely arrest?

  1. During cytokinesis, after nuclear division
  2. During metaphase, before chromosome segregation Correct answer.
  3. During S phase, during DNA synthesis
  4. During G1, before DNA replication

Explanation

The spindle is required to align chromosomes at the metaphase plate and to separate sister chromatids, so its absence triggers the spindle assembly checkpoint. Without it, the cell cannot proceed to anaphase.

Question 21Biology content domain (not enumerated in source)

A metabolic pathway is regulated by feedback inhibition. Which description best fits this control?

  1. The substrate activates the first enzyme to accelerate the pathway
  2. The final product binds and inhibits an early enzyme in the pathway Correct answer.
  3. An intermediate product permanently denatures a late enzyme
  4. The pathway proceeds without regulation once the first enzyme is activated

Explanation

Feedback inhibition occurs when the end product of a pathway binds to and inhibits an enzyme that catalyzes an early step, preventing wasteful accumulation. This creates a self-regulating loop that matches production to need.

Question 22Biology content domain (not enumerated in source)

Two genes are located close together on the same chromosome. In a test cross, which observation would indicate that crossing over occurred between them?

  1. All offspring show the parental phenotypes
  2. The genes assort independently as if on different chromosomes
  3. Recombinant offspring appear at exactly 50 percent
  4. Recombinant offspring appear at a frequency below 50 percent Correct answer.

Explanation

Linked genes tend to be inherited together, so recombinants arise only from crossovers between them and appear at a frequency below 50 percent. The closer the genes, the lower the recombination frequency.

Question 23Biology content domain (not enumerated in source)

A keystone predator is removed from a rocky intertidal community. Which outcome is most consistent with keystone species theory?

  1. Primary productivity stops entirely
  2. A dominant competitor expands and reduces overall species diversity Correct answer.
  3. The community remains unchanged because predators do not affect diversity
  4. All other species increase equally in abundance

Explanation

A keystone predator keeps a dominant competitor in check; removing it allows that competitor to monopolize space and resources, lowering diversity. This disproportionate influence on community structure defines a keystone species.

Question 24Biology content domain (not enumerated in source)

In a signal transduction pathway, a ligand binds a G protein-coupled receptor. Which event most directly follows receptor activation?

  1. The receptor is degraded by the proteasome
  2. The G protein exchanges GDP for GTP and dissociates into active subunits Correct answer.
  3. The receptor directly phosphorylates DNA
  4. The ligand enters the nucleus and binds DNA

Explanation

Ligand binding activates the receptor, which acts as a guanine nucleotide exchange factor, causing the G protein to release GDP and bind GTP. The GTP-bound subunits then dissociate and activate downstream effectors.

Question 25Biology content domain (not enumerated in source)

A researcher finds that a trait's heritability is estimated at 0.8 in a specific population. Which interpretation is most accurate?

  1. Eighty percent of each individual's phenotype is determined by genes
  2. The trait is fixed and cannot respond to environmental change
  3. The trait will be inherited by exactly eighty percent of offspring
  4. Eighty percent of the phenotypic variation in that population is attributable to genetic variation Correct answer.

Explanation

Heritability describes the proportion of phenotypic variance within a population that is due to genetic variance under the conditions studied. It is a population-level statistic and does not specify how much of any individual's phenotype comes from genes.

Question 26Biology content domain (not enumerated in source)

A scientist stains a root tip cell and observes chromosomes aligned along the cell equator, each attached to spindle fibers from opposite poles. Which stage of mitosis is shown?

  1. Metaphase Correct answer.
  2. Telophase
  3. Prophase
  4. Anaphase

Explanation

Metaphase is defined by chromosomes lining up at the metaphase plate, with spindle fibers from opposite poles attaching to each sister chromatid's kinetochore. This alignment is the checkpoint that precedes sister chromatid separation.

Question 27Biology content domain (not enumerated in source)

Which molecule serves as the final electron acceptor at the end of the electron transport chain during aerobic cellular respiration?

  1. Oxygen Correct answer.
  2. NAD+
  3. Pyruvate
  4. Carbon dioxide

Explanation

Oxygen accepts electrons at the end of the electron transport chain and combines with protons to form water. This keeps electrons flowing through the chain so that ATP synthase can continue producing ATP.

Question 28Biology content domain (not enumerated in source)

A strand of DNA has the base sequence 5'-TACGGA-3'. What is the sequence of the complementary DNA strand, written 5' to 3'?

  1. 5'-TACGGA-3'
  2. 5'-TCCGTA-3' Correct answer.
  3. 5'-AUGCCU-3'
  4. 5'-ATGCCT-3'

Explanation

Complementary base pairing joins A with T and G with C, and antiparallel strands run in opposite directions. Pairing the given strand yields 3'-ATGCCT-5', which reads 5'-TCCGTA-3' when reversed.

Question 29Biology content domain (not enumerated in source)

In a pedigree, an unaffected daughter has two unaffected parents, but her brother is affected by a rare disorder. The disorder appears in every generation and affects males and females in roughly equal numbers. Which mode of inheritance is most consistent with these observations?

  1. Autosomal dominant
  2. X-linked recessive
  3. Autosomal recessive Correct answer.
  4. Mitochondrial

Explanation

Unaffected parents producing an affected child indicates that both parents carry a hidden recessive allele. Equal sex distribution and skipping generations fit autosomal recessive inheritance, since carriers show no phenotype.

Question 30Biology content domain (not enumerated in source)

A protein is synthesized and then transported through the endomembrane system for secretion from the cell. Which sequence of organelles correctly traces this pathway?

  1. Golgi apparatus → rough ER → lysosome → plasma membrane
  2. Nucleus → lysosome → Golgi apparatus → secretory vesicle
  3. Smooth ER → mitochondrion → Golgi apparatus → plasma membrane
  4. Rough ER → Golgi apparatus → secretory vesicle → plasma membrane Correct answer.

Explanation

Secretory proteins are synthesized on ribosomes of the rough ER, packaged into transport vesicles, and modified in the Golgi apparatus. They are then sorted into secretory vesicles that fuse with the plasma membrane to release their contents.

Question 31Biology content domain (not enumerated in source)

A freshwater plant cell is placed in a hypertonic solution. What is the most likely immediate effect on the cell?

  1. The cell wall ruptures from osmotic pressure
  2. Water leaves the cell and the protoplast shrinks away from the wall Correct answer.
  3. Solutes move out until concentrations equalize
  4. Water enters and the cell becomes turgid

Explanation

In a hypertonic solution, the external solute concentration exceeds that inside the cell, so water moves outward by osmosis. In a plant cell this causes plasmolysis, in which the plasma membrane pulls away from the rigid cell wall.

Question 32Biology content domain (not enumerated in source)

Which statement correctly describes the role of ATP in cells?

  1. It transfers a phosphate group to target proteins, changing their activity Correct answer.
  2. It acts as the final electron acceptor in the electron transport chain
  3. It serves as the primary structural component of cell membranes
  4. It stores genetic information for protein synthesis

Explanation

ATP hydrolysis releases energy that drives cellular work, often by transferring a phosphate group to a protein in a process called phosphorylation. This can activate or deactivate enzymes and motor proteins.

Question 33Biology content domain (not enumerated in source)

A population of insects is exposed to an insecticide over many generations. The frequency of resistant individuals rises sharply. Which mechanism best explains this change?

  1. Stabilizing selection
  2. Genetic drift
  3. Directional selection Correct answer.
  4. Gene flow

Explanation

Directional selection favors individuals at one extreme of a trait distribution, shifting the population mean over time. Insects with preexisting resistance alleles survive and reproduce more, so resistance spreads.

Question 34Biology content domain (not enumerated in source)

Which of the following best describes the relationship between a population and a community in ecology?

  1. A population consists of all species in a community plus the abiotic factors
  2. A community consists of all populations of different species living and interacting in the same area Correct answer.
  3. A population is the set of all communities in a biome
  4. A community is a single species occupying a defined habitat

Explanation

A population is a group of individuals of one species in an area, whereas a community includes all the interacting populations of different species there. Adding the abiotic environment to a community yields an ecosystem.

Question 35Biology content domain (not enumerated in source)

A cell is treated with a drug that blocks the enzyme DNA ligase during DNA replication. Which outcome is most likely?

  1. RNA primers are never synthesized
  2. Helicase can no longer unwind the double helix
  3. The leading strand is synthesized in the wrong direction
  4. Okazaki fragments on the lagging strand remain unjoined Correct answer.

Explanation

DNA ligase seals the nicks between Okazaki fragments after primers are replaced with DNA. Without ligase activity, the lagging strand remains fragmented and the new DNA is not continuous.

Question 36Biology content domain (not enumerated in source)

A single-celled organism lacks a membrane-bound nucleus and has circular DNA and 70S ribosomes. To which group does it most likely belong?

  1. Fungi
  2. Protists
  3. Archaea
  4. Bacteria Correct answer.

Explanation

Circular DNA, 70S ribosomes, and the absence of a membrane-bound nucleus are prokaryotic features. Bacteria are the group classically described with these characteristics among the choices given.

Question 37Biology content domain (not enumerated in source)

During the light-independent reactions of photosynthesis, what is the primary role of the Calvin cycle?

  1. It fixes carbon dioxide into organic molecules using ATP and NADPH Correct answer.
  2. It generates a proton gradient across the thylakoid membrane
  3. It converts glucose into pyruvate for cellular respiration
  4. It splits water to release electrons and oxygen

Explanation

The Calvin cycle uses ATP and NADPH produced in the light reactions to reduce carbon dioxide into carbohydrate precursors. Rubisco catalyzes the initial carbon fixation step.

Question 38Biology content domain (not enumerated in source)

A trait is controlled by a gene on the X chromosome. A male with the trait has a daughter with an unaffected female who is not a carrier. What is the probability that the daughter expresses the trait?

  1. 100%
  2. 25%
  3. 0% Correct answer.
  4. 50%

Explanation

A father passes his X chromosome to all daughters, but a daughter also inherits an X from her mother. If the mother is homozygous unaffected (not a carrier), the daughter is heterozygous and shielded from expressing an X-linked recessive trait.

Question 39Biology content domain (not enumerated in source)

Which property of water is most directly responsible for its ability to moderate temperature changes in living organisms?

  1. Its low surface tension
  2. Its low molecular weight
  3. Its high specific heat capacity Correct answer.
  4. Its ability to dissolve nonpolar substances

Explanation

Water's high specific heat results from extensive hydrogen bonding, so it absorbs or releases large amounts of heat with small temperature changes. This stabilizes the internal temperature of organisms and bodies of water.

Question 40Biology content domain (not enumerated in source)

A researcher uses a respirometer to measure oxygen consumption by germinating seeds at different temperatures. Oxygen consumption increases as temperature rises from 10°C to 30°C, then declines sharply at 45°C. Which conclusion is best supported?

  1. The seeds produce more oxygen than they consume at 45°C
  2. Oxygen consumption is independent of temperature
  3. Germinating seeds perform only anaerobic respiration
  4. Metabolic rate increases with temperature until enzymes denature at high temperature Correct answer.

Explanation

Cellular respiration rates generally rise with temperature within an optimal range because molecular collisions increase. Above that range, enzymes denature and metabolic rate falls sharply, matching the observed decline.

Question 41Biology content domain (not enumerated in source)

Which process produces genetic variation by exchanging segments between homologous chromosomes during prophase I of meiosis?

  1. Fertilization
  2. Independent assortment
  3. DNA replication
  4. Crossing over Correct answer.

Explanation

Crossing over occurs when homologous chromosomes pair and exchange corresponding segments at chiasmata. This recombination creates new combinations of alleles on the resulting chromatids.

Question 42Biology content domain (not enumerated in source)

A mutation changes a codon from UAC to UAA in the middle of an mRNA coding sequence. What is the most likely effect on the protein?

  1. The reading frame shifts downstream of the mutation
  2. Translation terminates early, producing a shortened polypeptide Correct answer.
  3. The mRNA is degraded before translation begins
  4. A single amino acid is substituted with no other change

Explanation

UAA is a stop codon, so the ribosome releases the polypeptide prematurely. The result is a truncated protein that often lacks normal function.

Question 43Biology content domain (not enumerated in source)

In an energy pyramid, approximately what proportion of energy at one trophic level is typically transferred to the next level?

  1. About 1%
  2. About 90%
  3. About 10% Correct answer.
  4. About 50%

Explanation

Most energy is lost at each trophic level through metabolism, heat, and excretion, leaving roughly 10% available to the next level. This loss limits the number of trophic levels an ecosystem can support.

Question 44Biology content domain (not enumerated in source)

A cell biologist observes that a protein is present in the cytoplasm but is also found inside the nucleus. Which property of the protein best explains its nuclear localization?

  1. It contains a nuclear localization signal recognized by importins Correct answer.
  2. It is transported across the nuclear envelope by the Golgi apparatus
  3. It is small enough to diffuse freely through the nuclear envelope
  4. It is synthesized inside the nucleus by nuclear ribosomes

Explanation

Proteins destined for the nucleus carry a nuclear localization signal that is recognized by importin proteins. The importin-cargo complex passes through nuclear pores into the nucleus.

Question 45Biology content domain (not enumerated in source)

Two populations of a lizard species live on opposite sides of a mountain range and have not interbred for thousands of years. They can still interbreed in the laboratory and produce fertile offspring. What term best describes their relationship?

  1. They are reproductively isolated species
  2. They have undergone sympatric speciation
  3. They are members of different genera
  4. They belong to the same species Correct answer.

Explanation

The biological species concept defines species as groups that can interbreed and produce fertile offspring. Because these populations remain reproductively compatible, they are still the same species despite geographic separation.

Question 46Biology content domain (not enumerated in source)

Which statement best explains why the enzyme-catalyzed reaction rate plateaus at high substrate concentration?

  1. All active sites are occupied, so the enzyme is saturated Correct answer.
  2. The activation energy of the reaction decreases to zero
  3. The substrate begins to inhibit the enzyme at high concentration
  4. The enzyme is consumed faster than it can be replaced

Explanation

At high substrate concentration, essentially every enzyme active site is bound by substrate, so the reaction reaches its maximum velocity. Adding more substrate cannot increase the rate until more enzyme is available.

Question 47Biology content domain (not enumerated in source)

A bacterial cell acquires a gene for antibiotic resistance from a plasmid transferred by a neighboring bacterium through direct contact. Which process is described?

  1. Binary fission
  2. Conjugation Correct answer.
  3. Transformation
  4. Transduction

Explanation

Conjugation involves direct cell-to-cell contact through a pilus, allowing transfer of plasmid DNA between bacteria. This is a common route for spreading antibiotic resistance genes.

Question 48Biology content domain (not enumerated in source)

A cell cycle checkpoint detects damaged DNA during the G1 phase. Which outcome is most consistent with normal checkpoint function?

  1. The cell doubles its DNA content without repair
  2. The cell undergoes apoptosis only after completing S phase
  3. The cell immediately enters mitosis with the damage
  4. The cell cycle pauses to allow DNA repair before proceeding Correct answer.

Explanation

G1 checkpoints monitor DNA integrity and halt progression so that repair can occur before replication. If damage is irreparable, the cell may instead trigger apoptosis.

Question 49Biology content domain (not enumerated in source)

Which structural feature of a phospholipid bilayer directly allows small nonpolar molecules such as oxygen to cross membranes readily?

  1. The hydrophobic interior formed by fatty acid tails Correct answer.
  2. The presence of integral proteins spanning the bilayer
  3. The rigid cholesterol layer covering the surface
  4. The hydrophilic phosphate heads facing inward

Explanation

The interior of the bilayer is hydrophobic because the fatty acid tails face inward. Small nonpolar molecules dissolve in this region and diffuse across without a transport protein.

Question 50Biology content domain (not enumerated in source)

A DNA sample from a newly discovered organism contains 30% adenine. What percentage of its bases is guanine?

  1. 20% Correct answer.
  2. 40%
  3. 60%
  4. 30%

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 30%, totaling 60%. The remaining 40% is shared equally between guanine and cytosine, making guanine 20%.