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ILTS Science: Biology (239) study materials
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ILTS Science: Biology (239) practice questions

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Question 1Biology content domain (official subdomain breakdown not published in source)

A student examines a cell and observes a rigid cell wall, a large central vacuole, and numerous chloroplasts. Which conclusion is best supported by these observations?

  1. The cell is an animal cell
  2. The cell is a virus particle
  3. The cell is a plant cell Correct answer.
  4. The cell is a prokaryote

Explanation

A rigid cell wall, a large central vacuole, and chloroplasts are characteristic features of plant cells. Animal cells lack cell walls and chloroplasts, and prokaryotes lack membrane-bound chloroplasts and large vacuoles.

Question 2Biology content domain (official subdomain breakdown not published in source)

During aerobic cellular respiration, the greatest number of ATP molecules is produced by which process?

  1. Lactic acid fermentation
  2. Substrate-level phosphorylation in the Krebs cycle
  3. Glycolysis in the cytoplasm
  4. Oxidative phosphorylation along the inner mitochondrial membrane Correct answer.

Explanation

Oxidative phosphorylation, driven by the electron transport chain and chemiosmosis, yields the majority of ATP in aerobic respiration. Glycolysis and the Krebs cycle contribute only small amounts through substrate-level phosphorylation.

Question 3Biology content domain (official subdomain breakdown not published in source)

A DNA strand has the base sequence 5'-ATGCCT-3'. What is the sequence of the complementary strand?

  1. 5'-TACGGA-3' Correct answer.
  2. 5'-TACCGA-3'
  3. 5'-ATGCCT-3'
  4. 5'-AGGCTA-3'

Explanation

Complementary base pairing places adenine opposite thymine and guanine opposite cytosine. The complement of ATGCCT is TACGGA, and antiparallel orientation is reflected by writing the strand in the 5' to 3' direction.

Question 4Biology content domain (official subdomain breakdown not published in source)

Which process directly produces gametes with half the chromosome number of the parent cell?

  1. Budding
  2. Meiosis Correct answer.
  3. Mitosis
  4. Binary fission

Explanation

Meiosis involves two successive divisions that reduce the chromosome number by half, producing haploid gametes. Mitosis and binary fission maintain the diploid or parental chromosome number, and budding is a form of asexual reproduction.

Question 5Biology content domain (official subdomain breakdown not published in source)

In a population of mice, individuals with darker fur are more difficult for predators to see against dark soil. Over many generations, the frequency of dark-fur alleles increases. This best illustrates which mechanism?

  1. Gene flow
  2. Genetic drift
  3. Natural selection Correct answer.
  4. Mutation pressure

Explanation

Natural selection occurs when heritable traits that improve survival and reproduction become more common in a population. Here the dark-fur phenotype confers a survival advantage, increasing allele frequency over generations.

Question 6Biology content domain (official subdomain breakdown not published in source)

Which molecule serves as the primary energy currency that cells use to drive endergonic reactions?

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

Explanation

ATP is the universal energy currency because hydrolysis of its terminal phosphate bond releases energy that can be coupled to endergonic reactions. Glucose and NADH store energy but must be processed to generate ATP.

Question 7Biology content domain (official subdomain breakdown not published in source)

A human somatic cell in G1 phase contains 46 chromosomes. After completing the S phase and entering G2, how many DNA molecules does the cell contain?

  1. 138
  2. 46
  3. 92 Correct answer.
  4. 23

Explanation

During S phase each chromosome is replicated into two sister chromatids, doubling the number of DNA molecules while the chromosome count stays at 46. Thus the G2 cell contains 92 DNA molecules.

Question 8Biology content domain (official subdomain breakdown not published in source)

In a cross between two heterozygous tall pea plants (Tt x Tt), what is the expected phenotypic ratio of tall to short offspring?

  1. 1:3
  2. 3:1 Correct answer.
  3. 1:1
  4. 9:3:3:1

Explanation

A monohybrid cross of two heterozygotes yields a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio when the tall allele is completely dominant. Three of four offspring show the dominant tall phenotype.

Question 9Biology content domain (official subdomain breakdown not published in source)

A woman who is a carrier for an X-linked recessive disorder has children with an unaffected man. What proportion of their sons is expected to be affected?

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

Explanation

Sons inherit their X chromosome from the mother and a Y from the father. Half of the mother's X chromosomes carry the recessive allele, so each son has a 50 percent chance of being affected.

Question 10Biology content domain (official subdomain breakdown not published in source)

Which statement best describes the role of the promoter in gene expression?

  1. It is the sequence that signals the end of translation
  2. It encodes the amino acid sequence of the protein
  3. It is a DNA sequence where RNA polymerase and transcription factors bind Correct answer.
  4. It is the site where the ribosome assembles the polypeptide

Explanation

A promoter is a regulatory DNA region upstream of a gene where RNA polymerase and transcription factors assemble to initiate transcription. It does not code for amino acids and is not the site of translation.

Question 11Biology content domain (official subdomain breakdown not published in source)

A mutation changes a single nucleotide but the resulting polypeptide has the same amino acid sequence as the original. This type of mutation is best described as which of the following?

  1. Missense mutation
  2. Silent mutation Correct answer.
  3. Nonsense mutation
  4. Frameshift mutation

Explanation

A silent mutation changes a nucleotide without altering the encoded amino acid, often because of the redundancy of the genetic code. Missense and nonsense mutations change the amino acid sequence, and frameshifts alter reading frame.

Question 12Biology content domain (official subdomain breakdown not published in source)

Which level of protein structure is stabilized primarily by interactions between R groups such as hydrogen bonds, ionic bonds, and hydrophobic interactions?

  1. Quaternary structure
  2. Tertiary structure Correct answer.
  3. Primary structure
  4. Secondary structure

Explanation

Tertiary structure is the three-dimensional folding of a single polypeptide, stabilized largely by interactions among R groups. Secondary structure refers to local alpha helices and beta sheets, and quaternary structure involves multiple polypeptides.

Question 13Biology content domain (official subdomain breakdown not published in source)

In a food chain, only about 10 percent of the energy at one trophic level is transferred to the next. Which statement best explains the fate of the remaining energy?

  1. It is lost as heat and used in metabolism and excretion by the organisms Correct answer.
  2. It is transferred directly to decomposers without passing through consumers
  3. It is stored indefinitely in the bodies of primary producers
  4. It is converted to matter that cannot be used by any organism

Explanation

Most energy at each trophic level is used for metabolism, lost as heat during respiration, or excreted, leaving only a small fraction available to the next level. This loss explains why food chains rarely exceed four or five levels.

Question 14Biology content domain (official subdomain breakdown not published in source)

A population of bacteria is exposed to an antibiotic, and a few cells survive because they carry a resistance gene. After several generations, most of the population is resistant. Which conclusion is most consistent with evolutionary theory?

  1. The bacteria learned to resist the antibiotic during exposure
  2. The antibiotic caused resistant mutations in individual cells
  3. Resistant bacteria were already present and were selected by the antibiotic Correct answer.
  4. Antibiotic exposure increased the mutation rate in all cells

Explanation

Selection acts on existing heritable variation: resistant cells survive and reproduce, increasing their frequency. The antibiotic does not create resistance mutations in response to exposure, nor do bacteria acquire resistance through learning.

Question 15Biology content domain (official subdomain breakdown not published in source)

Which condition is required for a population to remain in Hardy-Weinberg equilibrium?

  1. Natural selection favoring one phenotype
  2. Random mating with no mutation, migration, or selection Correct answer.
  3. High rates of gene flow from neighboring populations
  4. Small population size with frequent genetic drift

Explanation

Hardy-Weinberg equilibrium requires a large population, random mating, no mutation, no migration, and no selection. Under these conditions allele and genotype frequencies remain constant across generations.

Question 16Biology content domain (official subdomain breakdown not published in source)

A cell placed in a hypertonic solution will most likely do which of the following?

  1. Actively pump solutes inward to maintain volume
  2. Remain unchanged because water cannot cross the membrane
  3. Swell and possibly burst as water enters
  4. Lose water and shrink as water exits Correct answer.

Explanation

In a hypertonic solution the solute concentration outside the cell is higher, so water moves out by osmosis and the cell shrinks. Animal cells may shrivel, while plant cells undergo plasmolysis.

Question 17Biology content domain (official subdomain breakdown not published in source)

Which structure is directly responsible for synthesizing proteins destined for secretion from the cell?

  1. Peroxisome
  2. Smooth endoplasmic reticulum
  3. Lysosome
  4. Rough endoplasmic reticulum Correct answer.

Explanation

Ribosomes attached to the rough endoplasmic reticulum synthesize proteins that enter the secretory pathway. Smooth ER lacks ribosomes and functions mainly in lipid synthesis and detoxification.

Question 18Biology content domain (official subdomain breakdown not published in source)

A gardener notices that a plant wilts during the hottest part of the day even though the soil is moist. Which mechanism best explains this observation?

  1. Chlorophyll breaks down at high temperatures
  2. Transpiration exceeds water uptake, reducing turgor pressure Correct answer.
  3. Stomata remain closed, preventing carbon dioxide entry
  4. Roots stop absorbing water when soil is moist

Explanation

On hot days transpiration through stomata can outpace water uptake by roots, lowering turgor pressure and causing temporary wilting. With adequate soil moisture, the plant typically recovers when temperatures fall.

Question 19Biology content domain (official subdomain breakdown not published in source)

Which statement accurately distinguishes mitosis from meiosis?

  1. Mitosis halves the chromosome number, whereas meiosis maintains it
  2. Mitosis produces two genetically identical diploid cells, whereas meiosis produces four genetically varied haploid cells Correct answer.
  3. Mitosis produces four haploid cells, whereas meiosis produces two diploid cells
  4. Mitosis involves crossing over, whereas meiosis does not

Explanation

Mitosis yields two daughter cells genetically identical to the parent, whereas meiosis yields four haploid cells that are genetically varied due to crossing over and independent assortment. This distinction underlies growth versus gamete formation.

Question 20Biology content domain (official subdomain breakdown not published in source)

In a pedigree, a trait appears in every generation and affects both males and females equally. Which mode of inheritance is most consistent with this pattern?

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

Explanation

An autosomal dominant trait typically appears in every generation, affects both sexes equally, and has an affected parent for each affected individual. X-linked recessive traits more often affect males and may skip generations.

Question 21Biology content domain (official subdomain breakdown not published in source)

A single-celled organism is observed to reproduce by dividing into two genetically identical cells without a nucleus. Which classification is most appropriate?

  1. Prokaryotic bacterium undergoing binary fission Correct answer.
  2. Eukaryotic protist undergoing mitosis
  3. Fungal cell undergoing budding
  4. Viral particle undergoing replication

Explanation

Binary fission in prokaryotes produces two identical daughter cells and occurs without a nucleus or mitotic spindle. Eukaryotic protists divide by mitosis and viruses require host cells to replicate.

Question 22Biology content domain (official subdomain breakdown not published in source)

A researcher measures the rate of photosynthesis in a leaf at increasing light intensities and finds that the rate plateaus at high intensities. Which factor most likely limits the rate at the plateau?

  1. Oxygen concentration
  2. Chlorophyll absorption spectrum
  3. Carbon dioxide concentration or temperature Correct answer.
  4. Light intensity

Explanation

At high light intensity, light is no longer limiting, so another factor such as carbon dioxide concentration or temperature limits the rate. This plateau reflects a change in the limiting factor.

Question 23Biology content domain (official subdomain breakdown not published in source)

A deletion of three nucleotides occurs near the beginning of a coding sequence. Which outcome is most likely?

  1. One amino acid is missing but the reading frame remains intact Correct answer.
  2. Transcription stops immediately at the deletion
  3. A frameshift alters all downstream amino acids
  4. The protein is always identical to the normal protein

Explanation

Because the genetic code is read in triplets, deleting three nucleotides removes one codon and preserves the downstream reading frame. The protein loses one amino acid but is not frameshifted.

Question 24Biology content domain (official subdomain breakdown not published in source)

In an ecosystem, nitrogen fixation converts atmospheric nitrogen into a form usable by plants. Which organisms carry out this process?

  1. Nitrifying bacteria that convert ammonia to nitrate
  2. Denitrifying bacteria
  3. Nitrogen-fixing bacteria such as Rhizobium Correct answer.
  4. Decomposers that release nitrogen as ammonia

Explanation

Nitrogen-fixing bacteria, including Rhizobium in root nodules, reduce atmospheric nitrogen to ammonia that plants can assimilate. Denitrifying and nitrifying bacteria act at other steps of the nitrogen cycle.

Question 25Biology content domain (official subdomain breakdown not published in source)

A population of insects shows a range of body sizes, and birds preferentially eat the largest individuals. Over time, the average body size decreases. Which type of selection is occurring?

  1. Sexual selection
  2. Directional selection Correct answer.
  3. Stabilizing selection
  4. Disruptive selection

Explanation

Directional selection shifts the population mean toward one extreme of the trait distribution. Predation against large individuals favors smaller body size, moving the average downward.

Question 26Biology content domain (official subdomain breakdown not published in source)

Which cellular structure regulates the passage of ions and small molecules into and out of a eukaryotic cell by controlling membrane permeability?

  1. The mitochondrion
  2. The plasma membrane Correct answer.
  3. The Golgi apparatus
  4. The nucleolus

Explanation

The plasma membrane is a phospholipid bilayer studded with proteins that determines which substances cross into or out of the cell, using channels, pumps, and selective permeability. The other structures are internal organelles that do not form the cell's outer boundary.

Question 27Biology content domain (official subdomain breakdown not published in source)

During which phase of the cell cycle does the nuclear envelope re-form around the separated sets of chromosomes?

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

Explanation

Telophase is the final stage of mitosis, when chromosomes decondense and a new nuclear envelope assembles around each chromosome set, followed by cytokinesis. Prophase, metaphase, and anaphase occur earlier and involve condensation, alignment, and separation rather than nuclear envelope reassembly.

Question 28Biology content domain (official subdomain breakdown not published in source)

Which property of water allows insects such as water striders to move across the surface of a pond?

  1. Its ability to dissolve nonpolar substances
  2. Its high surface tension from hydrogen bonding Correct answer.
  3. Its low density in the liquid state
  4. Its high heat of vaporization

Explanation

Hydrogen bonds between water molecules pull them together strongly at the air-water interface, producing surface tension that can support small objects. Heat of vaporization affects cooling and evaporation, not the support of weight on the surface.

Question 29Biology content domain (official subdomain breakdown not published in source)

A student is testing a leaf for the presence of starch after the leaf has been exposed to light. Why must the leaf first be boiled in ethanol?

  1. To convert starch into glucose for detection
  2. To dissolve chlorophyll so the color change of the indicator can be seen Correct answer.
  3. To denature enzymes that would otherwise destroy the starch
  4. To remove water so iodine can penetrate the tissue

Explanation

Ethanol decolorizes the leaf by dissolving the green chlorophyll, which otherwise masks the blue-black iodine color that indicates starch. Boiling the leaf in water first kills tissue and softens it, but removing pigment is the purpose of the ethanol step.

Question 30Biology content domain (official subdomain breakdown not published in source)

Which molecule directly donates amino acids to the growing polypeptide chain during translation at the ribosome?

  1. Messenger RNA
  2. Ribosomal RNA
  3. DNA
  4. Transfer RNA Correct answer.

Explanation

Each transfer RNA carries a specific amino acid and pairs its anticodon with the mRNA codon in the ribosome, delivering the amino acid to the polypeptide. mRNA supplies the codon sequence and rRNA forms the ribosome, but neither carries amino acids.

Question 31Biology content domain (official subdomain breakdown not published in source)

In a dihybrid cross between two individuals heterozygous for both traits (AaBb x AaBb), what is the expected probability that an offspring shows both recessive phenotypes, assuming independent assortment?

  1. 1/4
  2. 3/16
  3. 9/16
  4. 1/16 Correct answer.

Explanation

Each trait has a 1/4 chance of the recessive phenotype in a monohybrid cross of heterozygotes. With independent assortment the joint probability is 1/4 x 1/4 = 1/16.

Question 32Biology content domain (official subdomain breakdown not published in source)

Which feature of eukaryotic chromosomes is responsible for preventing the loss of coding DNA at the ends during replication?

  1. Histone octamers
  2. Sister chromatids
  3. Telomeres Correct answer.
  4. Centromeres

Explanation

Telomeres are repetitive noncoding sequences at chromosome ends that buffer against shortening during replication. Centromeres are the attachment points for spindle fibers, and histones are packaging proteins, so neither protects terminal sequence.

Question 33Biology content domain (official subdomain breakdown not published in source)

Which observation would best support the conclusion that a population of lizards is undergoing natural selection?

  1. Allele frequencies at a trait locus change consistently with a measured environmental pressure Correct answer.
  2. The population interbreeds freely with a neighboring species
  3. All individuals in the population have identical body lengths
  4. The population size remains constant across several generations

Explanation

Natural selection is demonstrated when heritable variation produces differential survival or reproduction tied to an environmental pressure, shifting allele frequencies over generations. A stable population size or uniform traits would not show selection at work.

Question 34Biology content domain (official subdomain breakdown not published in source)

A cell has a solute concentration of 0.9 percent NaCl and is placed in a 5 percent NaCl solution. Which direction will water move, and what will happen to the cell?

  1. Water leaves the cell, causing it to shrink Correct answer.
  2. Water moves equally in both directions, so nothing changes
  3. Water enters the cell, causing it to swell
  4. Solutes enter the cell, causing it to burst

Explanation

The 5 percent solution is hypertonic relative to the cell, meaning it has a higher solute concentration and lower water potential. Water moves by osmosis from the cell's interior to the surrounding solution, so the cell loses volume and shrinks.

Question 35Biology content domain (official subdomain breakdown not published in source)

An enzyme catalyzes a reaction at a high rate at 37 degrees Celsius, but the rate falls sharply at 60 degrees Celsius. Which best explains the decline?

  1. Substrate molecules lose kinetic energy as temperature rises
  2. The enzyme's tertiary structure is disrupted, reducing active site function Correct answer.
  3. The enzyme begins to catalyze the reverse reaction exclusively
  4. The activation energy of the reaction increases without limit

Explanation

High temperature breaks the hydrogen bonds and other interactions holding the enzyme's tertiary shape, denaturing the active site so substrate no longer binds properly. Rising temperature initially increases molecular motion and reaction rate, so loss of kinetic energy is not the cause.

Question 36Biology content domain (official subdomain breakdown not published in source)

Chlorophyll absorbs light most strongly in the blue and red regions of the spectrum. Which color of light would be expected to drive the lowest rate of photosynthesis in a green plant?

  1. Violet
  2. Red
  3. Green Correct answer.
  4. Blue

Explanation

Chlorophyll reflects and transmits green wavelengths, so green light is absorbed least and supports the lowest photosynthetic rate. Blue and red are absorbed strongly and drive higher rates.

Question 37Biology content domain (official subdomain breakdown not published in source)

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

  1. Deletion of the entire chromosome segment
  2. A silent substitution at that position
  3. A frameshift that alters every downstream codon Correct answer.
  4. No change in the polypeptide because the code is degenerate

Explanation

Inserting one nucleotide shifts the reading frame, so all codons after the insertion are read differently and the polypeptide is usually truncated or nonfunctional. Degeneracy protects against some substitutions but cannot preserve the frame after an insertion.

Question 38Biology content domain (official subdomain breakdown not published in source)

In a cross between a heterozygous individual and a homozygous recessive individual for a single gene, what phenotypic ratio is expected among the offspring?

  1. 1:2:1
  2. 1:1 Correct answer.
  3. 3:1
  4. 9:3:3:1

Explanation

A heterozygote produces two types of gametes in equal frequency, while the homozygous recessive produces only one. Half the offspring inherit the dominant allele and half do not, giving a 1:1 phenotypic ratio.

Question 39Biology content domain (official subdomain breakdown not published in source)

Two species of birds live in the same forest and eat insects from the same tree branches, but one feeds mainly at dawn and the other mainly at dusk. This is an example of which ecological interaction?

  1. Parasitism
  2. Competitive exclusion
  3. Resource partitioning Correct answer.
  4. Mutualism

Explanation

By dividing the available resource in time, the two species reduce direct competition and can coexist, which is resource partitioning. Competitive exclusion would instead involve one species eliminating the other from the shared niche.

Question 40Biology content domain (official subdomain breakdown not published in source)

Which process returns carbon from living organisms to the atmosphere in the carbon cycle?

  1. Transpiration
  2. Photosynthesis
  3. Cellular respiration Correct answer.
  4. Nitrogen fixation

Explanation

Cellular respiration oxidizes organic molecules and releases carbon dioxide as a byproduct, returning carbon to the atmosphere. Photosynthesis removes carbon dioxide, and the other listed processes involve nitrogen or water rather than carbon return.

Question 41Biology content domain (official subdomain breakdown not published in source)

A human genetic disorder is caused by a dominant allele on an autosome, and an affected heterozygous parent has children with an unaffected partner. What is the probability that a child is affected?

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

Explanation

The affected parent is heterozygous and passes the dominant allele to half of the gametes, while the unaffected parent contributes only recessive alleles. Each child therefore has a 50 percent chance of inheriting the dominant allele and being affected.

Question 42Biology content domain (official subdomain breakdown not published in source)

A researcher finds that a certain enzyme works efficiently at pH 2 in the stomach but loses activity at pH 7. Which conclusion is best supported?

  1. The enzyme requires a cofactor that is present only at neutral pH
  2. The enzyme's structure and charge depend on pH, with an optimum near pH 2 Correct answer.
  3. The enzyme is denatured by low pH and functions only near neutrality
  4. The enzyme is a lipid that dissolves at low pH

Explanation

Enzyme activity depends on pH because hydrogen ion concentration alters the charges and hydrogen bonding that maintain the active site. An optimum near pH 2 indicates the enzyme is adapted to acidic conditions, and shifting to pH 7 disrupts its functional shape.

Question 43Biology content domain (official subdomain breakdown not published in source)

In a study, mice that lack a functional gene for leptin become severely obese. Which interpretation of this result is most accurate?

  1. Leptin is required for normal regulation of appetite and energy balance Correct answer.
  2. Leptin directly digests fat stored in adipose tissue
  3. The leptin gene is unrelated to body weight regulation
  4. Obesity in these mice is caused entirely by lack of exercise

Explanation

Loss of leptin function producing obesity indicates that leptin normally signals satiety and helps regulate energy balance. The result demonstrates a regulatory role, not a digestive or exercise-related mechanism.

Question 44Biology content domain (official subdomain breakdown not published in source)

A species of flowering plant depends on a single species of moth for pollination, and the moth larvae feed only on that plant. Which term best describes this relationship?

  1. Predation
  2. Interspecific competition
  3. Obligate mutualism Correct answer.
  4. Commensalism

Explanation

Both partners benefit and each depends on the other, making the relationship an obligate mutualism. Commensalism benefits only one partner, and competition or predation harms at least one participant.

Question 45Biology content domain (official subdomain breakdown not published in source)

A bacterial population is grown in a closed culture, and the number of cells is plotted over time. After a period of rapid growth, the curve levels off. Which factor best explains the plateau?

  1. All bacteria have mutated into a non-dividing form
  2. Nutrients become depleted and toxic waste products accumulate Correct answer.
  3. The cells have lost their ribosomes permanently
  4. The bacteria have switched from asexual to sexual reproduction

Explanation

In a closed culture, exponential growth slows as nutrients are consumed and metabolic wastes build up, producing the stationary phase where growth and death balance. Mutation to a nondividing form is not the general cause of the plateau.

Question 46Biology content domain (official subdomain breakdown not published in source)

A segment of DNA has the template strand sequence 3'-TAC GGA-5'. Assuming standard genetic code pairing, what mRNA sequence is transcribed from this template?

  1. 5'-ATG CCT-3'
  2. 5'-TAC GGA-3'
  3. 5'-AUG CCU-3' Correct answer.
  4. 5'-UAC GGA-3'

Explanation

Transcription builds mRNA complementary to the template strand and antiparallel to it, so T pairs with A, A with U, C with G, and G with C. Reading the template 3' to 5' gives the mRNA 5'-AUG CCU-3'.

Question 47Biology content domain (official subdomain breakdown not published in source)

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

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

Explanation

With q = 0.3, p = 0.7, and the heterozygote frequency is 2pq = 2(0.7)(0.3) = 0.42. The values 0.09 and 0.49 are q squared and p squared, the homozygous frequencies.

Question 48Biology content domain (official subdomain breakdown not published in source)

Two enzymes in a metabolic pathway catalyze sequential reactions, and the product of the first reaction inhibits the first enzyme. What is this control mechanism called?

  1. Competitive inhibition
  2. Irreversible denaturation
  3. Feedback inhibition Correct answer.
  4. Allosteric activation

Explanation

Feedback inhibition occurs when the end product of a pathway binds an earlier enzyme and reduces its activity, preventing overaccumulation of the product. Competitive inhibition involves a molecule resembling the substrate competing at the active site, which is not described here.

Question 49Biology content domain (official subdomain breakdown not published in source)

A gene is located on the X chromosome and shows X-linked recessive inheritance. An affected father and an unaffected homozygous mother have a daughter. Which statement about the daughter is correct?

  1. She will be affected only if she also inherits an affected allele from her mother
  2. She cannot inherit the affected allele because her mother is homozygous normal
  3. She will be affected because she inherits the father's X chromosome
  4. She will be an unaffected carrier because she inherits one affected X and one normal X Correct answer.

Explanation

A daughter receives one X from her father and one from her mother. The affected father passes his affected X, and the homozygous normal mother passes a normal X, so the daughter is heterozygous and an unaffected carrier.

Question 50Biology content domain (official subdomain breakdown not published in source)

A lake receives runoff rich in nitrogen and phosphorus, and within weeks algal growth surges, then collapses as dissolved oxygen drops sharply. Which sequence best explains the oxygen decline?

  1. The algae convert oxygen into nitrogen gas
  2. Decomposers consuming dead algae deplete dissolved oxygen Correct answer.
  3. Phosphorus binds oxygen chemically and removes it from water
  4. Algal photosynthesis consumes oxygen faster than it is produced

Explanation

Nutrient enrichment causes an algal bloom; when the algae die, bacteria decompose the organic matter and consume dissolved oxygen, creating hypoxia. Photosynthesis by algae adds oxygen, and phosphorus does not chemically bind oxygen.