Texas Biology STAAR EOC Hub

A nucleotide is made up of which three components?

1. AAn amino acid, a ribosome, and a protein
2. BA phosphate group, a deoxyribose sugar, and a nitrogenous base
3. CA chromosome, a gene, and an allele
4. DA capsid, genetic material, and an envelope

One Strand of a DNA Molecule

Based on complementary base pairing, which sequence represents the other strand of this DNA molecule?

1. AA-T-G-G-C-A (identical to the first strand)
2. BT-A-C-C-G-T
3. CU-A-C-C-G-U
4. DG-C-A-A-T-G

The RNA world hypothesis proposes that early self-replicating RNA molecules existed before DNA-based life. A student claims, "Since this is called a hypothesis, scientists have already proven it is correct." Which statement best evaluates this claim?

1. AThe claim is correct — once something is named a hypothesis, it has been proven.
2. BThe claim is incorrect — a hypothesis is a proposed explanation that is still being tested and refined with evidence, not a proven conclusion.
3. CThe claim is correct, because RNA has been directly observed in fossils from the time period in question.
4. DThe claim is incorrect, but only because hypotheses can never be supported by any evidence.

During which process is mRNA used as a template to assemble a chain of amino acids?

1. ATranscription
2. BTranslation
3. CDNA replication
4. DMitosis

Partial Codon Chart

An mRNA strand reads: AUG–UUU–GGC–UAA. What sequence of amino acids will this mRNA produce?

1. AGlycine–Phenylalanine–Methionine
2. BMethionine–Phenylalanine–Glycine, then translation stops
3. CMethionine–Phenylalanine–Glycine–Stop (all four codons code for amino acids)
4. DNo protein is produced because the sequence contains a stop codon.

A gene's DNA sequence is altered so that the mRNA codon at one position changes from GGC (glycine) to GAC (aspartic acid), while every other codon stays the same. Which statement best predicts the effect on the resulting protein?

1. AThe protein will be completely different, with every amino acid changed.
2. BThe protein will have the same overall length, but one amino acid (glycine) will be replaced with a different amino acid (aspartic acid) at that position, which may or may not change the protein's function depending on that amino acid's role.
3. CNo protein will be produced at all, since any change to mRNA prevents translation.
4. DThe protein will become longer because a new codon was added.

Which type of mutation is most likely to have NO effect on the resulting protein?

1. AFrameshift mutation
2. BNonsense mutation
3. CSilent mutation
4. DInsertion of ten nucleotides

Effects of Four Mutations on a Gene's Protein

Based on the table, which mutation is a frameshift mutation?

1. AMutation 1
2. BMutation 2
3. CMutation 3
4. DMutation 4

Two mutations occur in different genes. Mutation X is a substitution in the third position of a codon that does not change the amino acid produced. Mutation Y is a deletion of one nucleotide near the beginning of a gene. A student claims both mutations are equally likely to significantly affect the organism. Which statement best evaluates this claim?

1. AThe claim is correct — all mutations have the same significance regardless of type or location.
2. BThe claim is incorrect — Mutation X is silent and likely has no effect, while Mutation Y is a frameshift mutation near the start of the gene and is likely to drastically alter or destroy the protein's function.
3. CThe claim is incorrect, but only because Mutation X occurred in DNA while Mutation Y occurred in RNA.
4. DThe claim is correct, because both mutations involve only one nucleotide.

What is the main outcome of meiosis?

1. ATwo genetically identical diploid daughter cells
2. BFour genetically varied haploid gametes
3. COne diploid cell with double the original chromosome number
4. DTwo haploid cells that immediately fuse back together

Chromosome Number at Stages of Meiosis in a Cell with 8 Chromosomes (4 Pairs)

Based on the table, which statement is best supported?

1. AMeiosis II reduces the chromosome number from 4 to 2.
2. BMeiosis I is the division that reduces chromosome number from diploid (8) to haploid (4); meiosis II separates sister chromatids without further reducing the number.
3. CThe starting cell and the final gametes have the same chromosome number.
4. DMeiosis doubles the chromosome number at each stage.

A student observes that two full siblings (same parents) are noticeably different in several inherited traits. The student claims this can only be explained by new mutations occurring in each sibling. Which statement best evaluates this claim?

1. AThe claim is correct — without new mutations, siblings from the same parents would be genetically identical.
2. BThe claim is incorrect — crossing over and independent assortment during meiosis can produce gametes with many different allele combinations, so siblings can differ significantly without any new mutations.
3. CThe claim is incorrect, but only because siblings always have identical DNA regardless of meiosis.
4. DThe claim is correct, because meiosis always produces identical gametes unless a mutation occurs.

An organism with the genotype "Bb" is described as which of the following?

1. AHomozygous dominant
2. BHeterozygous
3. CHomozygous recessive
4. DCodominant

Punnett Square for a Cross Between Two Heterozygous Pea Plants (Tt × Tt), where T (Tall) is dominant over t (short)

Based on the Punnett square, what is the expected ratio of tall to short offspring?

1. A1 tall : 1 short
2. B3 tall : 1 short
3. C1 tall : 3 short
4. DAll offspring will be tall

A parent with blood type A (genotype AO) and a parent with blood type B (genotype BO) have a child. A student claims the child cannot have blood type AB, because neither parent has type AB blood. Which statement best evaluates this claim?

1. AThe claim is correct — a child's blood type can never differ in category from both parents' blood types.
2. BThe claim is incorrect — the child could inherit the A allele from one parent and the B allele from the other, resulting in genotype AB and, due to codominance, blood type AB.
3. CThe claim is incorrect, but only because blood type is determined by a single allele, not multiple alleles.
4. DThe claim is correct, because the O alleles from each parent would dominate and the child would have type O blood.

The bones in a human arm, a bat wing, and a whale flipper have a similar arrangement but different functions. What type of evidence does this represent?

1. ABiogeography
2. BAnatomical homology
3. CThe fossil record
4. DMolecular homology

Percent DNA Sequence Similarity to Species X

Based on the data, which conclusion is best supported?

1. ASpecies W shares the most recent common ancestor with Species X.
2. BSpecies Y likely shares a more recent common ancestor with Species X than Species W does.
3. CDNA similarity provides no information about evolutionary relationships.
4. DSpecies X and Species Y cannot be related because their DNA is not identical.

Whales have small, non-functional pelvic bones embedded in their bodies, similar in position to the pelvic bones that support hind limbs in other mammals. A student claims these bones serve no purpose and therefore provide no scientific information. Which statement best evaluates this claim?

1. AThe claim is correct — structures with no current function cannot be used as evidence.
2. BThe claim is incorrect — as vestigial structures, these reduced pelvic bones are evidence that whales share ancestry with land mammals that used such bones to support hind limbs.
3. CThe claim is incorrect, but only because the bones are actually still used for swimming.
4. DThe claim is correct, because vestigial structures always disappear completely within one generation.

A lineage shows little change for millions of years in the fossil record. What term best describes this pattern?

1. AGradualism
2. BAbrupt appearance
3. CStasis
4. DSpeciation

Average Shell Thickness (mm) of a Fossil Mollusk Lineage at Different Time Intervals

Based on the data, which rate-of-change pattern is best supported?

1. AStasis, because shell thickness never changed
2. BGradualism, because shell thickness increased slowly and steadily over the 10-million-year span
3. CAbrupt appearance, because shell thickness jumped suddenly between two time points
4. DNo pattern can be determined from this data

A fossil lineage shows almost no change for 5 million years, then a new, distinctly different form appears in rock layers dated only 50,000 years later. A student claims this proves evolution does not occur gradually and must always happen in sudden jumps. Which statement best evaluates this claim?

1. AThe claim is correct — this single example proves all evolutionary change is abrupt.
2. BThe claim overgeneralizes — this example shows one lineage exhibiting stasis followed by an apparent abrupt appearance, but other lineages in the fossil record show gradualism; both patterns occur, and the apparent suddenness could also partly reflect gaps in fossil preservation.
3. CThe claim is incorrect, but only because 50,000 years is actually a very long time.
4. DThe claim is correct, because gradualism has never been observed in any fossil lineage.

Natural selection most directly results in a change in which of the following?

1. AThe traits of a single individual during its lifetime
2. BThe frequency of traits or alleles in a population across generations
3. CThe number of chromosomes in a species' cells
4. DThe location of genes on a chromosome within one generation

Frequency of Dark-Colored Moths in a Population Over Time

Based on the data, which statement is best supported?

1. AIndividual moths are changing color as they age.
2. BThe frequency of the dark-color trait in the moth population is increasing across generations, suggesting it provides a survival or reproductive advantage in this environment.
3. CThe total number of moths is decreasing over time.
4. DDark coloration provides no advantage, since the percentage changed.

A student writes: "Giraffes evolved long necks because individual giraffes kept stretching their necks to reach high leaves, and this stretching was passed on to their offspring." Which statement best evaluates this claim?

1. AThe claim is correct — traits acquired through an individual's behavior during its lifetime are passed directly to offspring.
2. BThe claim is incorrect — neck-stretching during an individual's lifetime does not change its offspring's genes; instead, giraffes with genes for longer necks would have had a survival/reproductive advantage, increasing the frequency of those genes in the population over generations.
3. CThe claim is incorrect, but only because giraffes do not stretch their necks.
4. DThe claim is correct, as long as enough giraffes stretch their necks at the same time.

Which three elements must be present together for natural selection to occur?

1. AMutation, migration, and genetic drift
2. BInherited variation, overproduction of offspring, and finite environmental resources
3. CSpeciation, extinction, and adaptation
4. DPredation, parasitism, and competition

Fish Population: Eggs Laid vs. Offspring Surviving to Adulthood

Based on the data, which conclusion is best supported?

1. AAll eggs laid survive to adulthood each generation.
2. BThe population consistently produces far more offspring than survive, consistent with overproduction of offspring and limited resources driving competition.
3. CThe number of eggs laid has no relationship to resource availability.
4. DSurvival rate increases every generation.

A student claims, "If a population had unlimited food, water, and space, natural selection would happen even faster, because more offspring could survive." Which statement best evaluates this claim?

1. AThe claim is correct — natural selection always speeds up when resources increase.
2. BThe claim is incorrect — if resources were truly unlimited, there would be no competition for survival, so differential reproductive success based on inherited traits would not occur; finite resources are a required element of natural selection, not an obstacle to it.
3. CThe claim is incorrect, but only because organisms cannot survive without competition.
4. DThe claim is correct, because more surviving offspring always means faster natural selection.

What is required for two populations of a common ancestor to become separate species?

1. AThey must live in completely different climates.
2. BThey must become reproductively isolated and accumulate enough genetic differences that they can no longer interbreed.
3. CThey must each have a different number of total individuals.
4. DThey must look visibly different from each other.

Interbreeding Success Between Two Populations Separated by a Mountain Range

Based on the data, which conclusion is best supported?

1. AInterbreeding success increases the longer two populations are separated.
2. BAs time since separation increases, the populations become progressively more reproductively isolated, consistent with ongoing speciation.
3. CThe two populations were never able to interbreed, even at 10 generations.
4. DGeographic separation has no effect on interbreeding success.

A river changes course and splits a single fish population into two groups living in separate channels. A student claims that simply being physically separated automatically and immediately makes them two different species. Which statement best evaluates this claim?

1. AThe claim is correct — physical separation alone instantly creates a new species.
2. BThe claim is incorrect — geographic separation only sets the stage for speciation; the populations must accumulate genetic differences over many generations, through natural selection and other mechanisms, before they become reproductively isolated and qualify as separate species.
3. CThe claim is incorrect, but only because fish cannot undergo speciation.
4. DThe claim is correct, because reproductive isolation is not actually required for speciation.

Which evolutionary mechanism involves random changes in allele frequency that are especially significant in small populations?

1. ANatural selection
2. BGenetic drift
3. CGene flow
4. DRecombination

Allele Frequency Change After One Generation in Two Populations

Based on the data, which conclusion is best supported?

1. ABoth populations show the same type and magnitude of allele frequency change.
2. BThe large change in Population 2 is consistent with genetic drift, which has a much greater effect in small populations than in large ones.
3. CPopulation 1's small change shows it is more affected by genetic drift than Population 2.
4. DAllele frequency cannot change in a single generation.

A small population of beetles, isolated on a remote island for one generation, shows a large increase in the frequency of a particular shell-color allele. There is no evidence this allele provides any survival or reproductive advantage on the island. Which evolutionary mechanism best explains this change?

1. ANatural selection, because any allele frequency change must be due to a fitness advantage.
2. BGenetic drift, because the change occurred in a small population and is not linked to any fitness advantage — consistent with a random shift, possibly amplified by a founder effect if the island population began from few individuals.
3. CGene flow, because the beetles migrated to a new island.
4. DMutation, because a new allele must have just appeared.

A tapeworm lives inside the digestive tract of a dog, absorbing nutrients and harming the dog's health. Which type of relationship does this describe?

1. AMutualism
2. BParasitism
3. CCommensalism
4. DCompetition

Predator and Prey Population Sizes Over Time

Based on the data, which statement is best supported?

1. APredator and prey populations change completely independently of each other.
2. BIncreases in the prey population tend to be followed by increases in the predator population, and predator increases tend to be followed by prey declines — a typical predator-prey cycle.
3. CThe predator population is always larger than the prey population.
4. DBoth populations only ever increase over time.

A bird species regularly removes and eats ticks from the backs of large grazing mammals. A student claims this must be parasitism because the bird benefits from the relationship. Which statement best evaluates this claim?

1. AThe claim is correct — any relationship where one species clearly benefits is parasitism.
2. BThe claim is incorrect — the bird benefits by gaining food, and the grazing mammal also benefits by having parasitic ticks removed, making this mutualism rather than parasitism.
3. CThe claim is incorrect, but only because birds cannot be parasites.
4. DThe claim is correct, because the grazing mammal is unaffected by the bird's actions.

Which statement best distinguishes matter and energy as they move through an ecosystem?

1. ABoth matter and energy flow in one direction and are not reused.
2. BMatter cycles repeatedly through the ecosystem, while energy flows one-directionally and is progressively lost as heat.
3. CEnergy cycles repeatedly, while matter flows one-directionally and is lost.
4. DNeither matter nor energy moves through an ecosystem.

Effect of a Decomposer Die-Off on Soil Nutrient Levels and Producer Growth Over Time

Based on the data, which conclusion is best supported?

1. ADecomposers have no effect on matter cycling in this ecosystem.
2. BThe decomposer die-off disrupted the cycling of matter (nutrients), which in turn reduced energy flow to producers and lowered their biomass.
3. CProducer biomass increased after the decomposer die-off.
4. DSoil nutrient levels are unrelated to producer growth.

A student claims, "Since energy keeps entering an ecosystem from the sun every day, energy must cycle through the ecosystem just like matter does." Which statement best evaluates this claim?

1. AThe claim is correct — energy is recycled the same way carbon or nitrogen atoms are.
2. BThe claim is incorrect — although new energy continually enters as sunlight, that energy flows through the ecosystem in one direction and is progressively lost as heat at each trophic level rather than being reused, unlike matter, which is conserved and cycles repeatedly.
3. CThe claim is incorrect, but only because sunlight is not a source of energy for ecosystems.
4. DThe claim is correct, because heat lost from organisms is recaptured by producers.

Which process converts atmospheric nitrogen gas into a form that plants can use?

1. APhotosynthesis
2. BNitrogen fixation
3. CCellular respiration
4. DDecomposition

Nitrogen Runoff and Dissolved Oxygen Levels in a Coastal Bay Over Time

Based on the data, which conclusion is best supported?

1. ANitrogen runoff and dissolved oxygen levels are unrelated.
2. BAs nitrogen runoff increases, dissolved oxygen decreases, consistent with excess nitrogen disrupting the nitrogen cycle and destabilizing the bay's ecosystem.
3. CDissolved oxygen increases as nitrogen runoff increases.
4. DNitrogen runoff has remained constant over the time period shown.

A student claims, "Burning fossil fuels can't really disrupt the carbon cycle, because the carbon released was already part of the natural cycle millions of years ago when those organisms were alive." Which statement best evaluates this claim?

1. AThe claim is correct — since the carbon was once part of a living organism, releasing it now has no effect on the cycle's balance.
2. BThe claim is incorrect — that carbon had been removed from active cycling and stored for millions of years; releasing it rapidly by burning fossil fuels adds it back to the atmosphere far faster than natural processes can absorb it, disrupting the cycle's current balance.
3. CThe claim is incorrect, but only because fossil fuels do not contain carbon.
4. DThe claim is correct, because the carbon cycle has no maximum rate of carbon absorption.

Which statement best describes the relationship between biodiversity and ecosystem stability?

1. ABiodiversity has no effect on ecosystem stability.
2. BHigher biodiversity generally increases ecosystem stability through ecological redundancy.
3. CHigher biodiversity always decreases ecosystem stability.
4. DEcosystem stability depends only on abiotic factors, not biodiversity.

Number of Native Plant Species and Ecosystem Recovery Time After Drought in Three Prairie Plots

Based on the data, which conclusion is best supported?

1. APlots with fewer species recovered fastest from the drought.
2. BPlots with higher native species counts (greater biodiversity) recovered more quickly from the drought, consistent with biodiversity supporting ecosystem stability.
3. CSpecies count has no relationship to recovery time.
4. DAll three plots recovered at the same rate.

A wetland ecosystem loses one species of frog to disease, but a student notes that 30 other amphibian and insect species still remain, and concludes the wetland's stability was not affected at all because "one species barely matters." Which statement best evaluates this claim?

1. AThe claim is always correct — losing any single species out of many never affects ecosystem stability.
2. BThe claim oversimplifies the situation — whether stability is affected depends on the lost species' ecological role; if it was a keystone species with a disproportionately large effect, even one loss among many could meaningfully reduce stability, while loss of a more redundant species might have little effect.
3. CThe claim is incorrect, but only because frogs cannot go extinct.
4. DThe claim is correct, because ecosystem stability is determined only by the total number of species present.