4 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 4
The graph shows the mRNA levels of two genes, X and Y, in a hypothetical species of bacterium before and after a sugar was added to the medium. Which claim is supported by the data?
Answer and reasoning
AGenes X and Y are both inducible, as both were expressed once sugar was in the medium. A student who takes anything that follows a signal as caused by it picks this. Gene X was expressed at the same level before the sugar was added, so the sugar did not induce it.
BGene Y is constitutive, as it reached a higher mRNA level than gene X did. A student who thinks 'constitutive' means 'expressed at the highest level' picks this. Gene Y was barely expressed until the sugar was added; gene X, expressed steadily throughout, fits a constitutive gene.
CGene Y is inducible, as its mRNA rose only after the sugar was added.Correct Gene Y's mRNA stayed at 2 units until the sugar was added at 20 min and then rose to 80 units, so its transcription was switched on by the sugar. Gene X stayed near 50 units throughout, as expected of a constitutive gene.
DThe sugar altered the DNA sequence of gene Y, which makes gene Y inducible. A student who thinks cells change gene expression by changing the DNA sequence picks this. Inducers act through regulatory proteins binding regulatory sequences; the gene's sequence is unchanged, and the data say nothing about it.
The diagram shows two regions of a chromosome in a eukaryotic cell. Gene P is in region 1 and gene Q is in region 2. Which prediction is best supported by the model?
Answer and reasoning
AGene Q is transcribed more, as its DNA is more loosely packed.Correct In region 2, acetylated histones are associated with loosely packed DNA that transcription proteins can reach. In region 1, methylated, tightly packed DNA is less accessible, so gene Q is likely to be transcribed more than gene P.
BBoth genes are transcribed equally, as both are present in the DNA. A student who thinks every gene a cell carries is expressed picks this. Having a gene does not mean it is transcribed; packing and chemical modifications affect access to it.
CGene P cannot ever be transcribed again, as these changes are permanent. A student who thinks epigenetic changes are permanent picks this. Methyl and acetyl groups can be added and removed, so these modifications, and their effects on transcription, are reversible.
DGene P is transcribed less, as its methyl groups changed its base sequence. A student who thinks changes in expression come from changes in the DNA sequence picks this. Methylation is an epigenetic modification: it changes expression without changing the base sequence.
A person's skin cells and liver cells have very different structures and functions. Which statement best explains how these differences arise?
Answer and reasoning
AEach cell type keeps the genes it uses and has lost the genes that it does not use. A student who thinks specialized cells lose unused genes picks this. Skin and liver cells generally carry the same genes; they differ in which genes are expressed.
BThe two cell types express different combinations of genes, at different levels.Correct The phenotype of a cell is determined by the combination of genes that are expressed and the levels at which they are expressed. Skin and liver cells carry the same genes but express different sets of them.
CThe genes in each cell type have mutated to suit the jobs that the cell needs to do. A student who thinks cells change their genes' sequences to suit their needs picks this. Cell types differ through gene regulation, not through mutations that arise to fit a role.
DBoth cell types express all of their genes, but their surroundings shape them. A student who thinks every gene a cell carries is expressed picks this. Each cell type expresses only some of its genes; the differences in expression produce the differences between them.
Which statement about the regulation of groups of genes is correct?
Answer and reasoning
ABoth prokaryotes and eukaryotes have groups of genes whose expression is coordinated.Correct Prokaryotes coordinate groups of genes in operons, and in eukaryotes groups of genes may be influenced by the same transcription factors, so both coordinate the regulation of groups of genes.
BBoth prokaryotes and eukaryotes coordinate groups of genes by placing them in operons. A student who applies the operon model to all organisms picks this. Operons are typical of prokaryotes; eukaryotes generally coordinate genes through shared transcription factors.
CProkaryotes coordinate groups of genes, but eukaryotes regulate each gene alone. A student who thinks only operons can coordinate genes picks this. In eukaryotes, groups of genes may be influenced by the same transcription factors.
DEukaryotes regulate their genes, but prokaryotes express every gene continuously. A student who thinks prokaryotes are too simple to regulate genes picks this. Prokaryotes regulate operons in inducible or repressible systems.
In preparation: 0 of 4 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
6.5.A.1 Regulatory sequence Fix
Regulatory sequence
A stretch of DNA, such as a promoter or an operator, that interacts with regulatory proteins to control transcription of a gene.
Regulatory protein
A protein, such as a repressor or a transcription factor, that binds a regulatory sequence and increases or decreases transcription.
Constitutive gene
A gene that is expressed continuously, at a fairly steady level, rather than in response to a particular signal.
Inducible gene
A gene whose transcription is switched on or increased in response to a specific signal, such as the presence of a particular molecule.
Students often think If one thing happens after another, or two quantities change together, the first causes the second. In fact Not necessarily. Two variables that change together, or one change that follows another in time, may be related in other ways; a causal claim needs a comparison or an experiment that rules out other explanations, and the data must show which variable depends on which.
Students often think A constitutive gene is the one expressed at the highest level, and an inducible gene is expressed at a lower level. In fact No. 'Constitutive' describes how a gene is regulated: it is expressed continuously rather than in response to a particular signal. A constitutive gene may be expressed at a lower level than an inducible gene that has been switched on.
6.5.A.2 Epigenetic change Fix
Epigenetic change
A reversible modification of DNA or of histones that changes gene expression without changing the DNA base sequence.
DNA methylation
Addition of methyl groups to DNA. Methylation of the regulatory region of a gene is generally associated with reduced transcription, and it can be reversed.
Histone acetylation
Addition of acetyl groups to histone proteins. It loosens the packing of DNA around histones and is generally associated with increased transcription; acetyl groups can be removed again.
Students often think Signals, chemical marks or the needs of a cell change the DNA sequence of a gene, and that is how its expression is switched on, switched off or adjusted. In fact No. Changes in gene expression are usually brought about by regulatory proteins binding regulatory sequences and by reversible modifications of DNA or histones; the base sequence stays the same. Mutations do change the sequence, but they arise at random with respect to the cell's needs and are not the way a cell regulates its genes.
Students often think Once a gene has been modified epigenetically, the change is permanent and the gene can never be expressed in the old way again. In fact No. Epigenetic changes are reversible modifications of DNA or histones; methyl and acetyl groups can be added and removed, so the effect on gene expression can be reversed.
6.5.A.3 Gene expression and phenotype Fix
Gene expression and phenotype
The phenotype of a cell or an organism is determined by which genes are expressed and the levels at which they are expressed.
Cell differentiation
The process by which cells become specialized. Observable differentiation results from the expression of genes for tissue-specific proteins; the different cell types of an organism generally carry the same genes.
Transcription factor
A regulatory protein that binds a regulatory sequence and activates or represses transcription of a gene.
Sequential gene expression
A cascade in which a transcription factor induced during development switches on the next gene or genes, so genes are expressed in a set order.
Gene product
The RNA or protein made from a gene. Both its function and its amount help determine the phenotype of the organism.
Students often think Every gene a cell carries is expressed, so cells with the same genes express them in the same way. In fact No. Most cells of an organism carry the same genes, but each cell expresses only some of them, at levels that change with cell type and conditions.
Students often think As cells specialize, each cell type keeps only the genes it uses and loses the rest. In fact Generally not. The different cell types of an organism generally carry the same genes; they differ because they express different genes at different levels. (A few cell types are exceptions, such as mammalian red blood cells, which lose their nucleus.)
6.5.B.1 Coordinate regulation Fix
Coordinate regulation
Control of a group of genes so that they are expressed together. Both prokaryotes and eukaryotes have groups of coordinately regulated genes.
Operon
In prokaryotes, a group of genes transcribed together as one mRNA from a single promoter and controlled by one operator.
Operator
A regulatory sequence in an operon to which a repressor binds; a bound repressor blocks transcription of the operon's genes.
Repressor
A regulatory protein that binds the operator of an operon and blocks transcription of its genes.
Inducible operon
An operon that is normally not transcribed. An inducer binds the repressor, which then releases the operator, so the genes are transcribed (for example, the lac operon).
Repressible operon
An operon that is normally transcribed. When a corepressor, often the end product of the pathway, is abundant, it activates the repressor, which binds the operator and blocks transcription (for example, the trp operon).
Coordinate regulation in eukaryotes
Genes, which may be on different chromosomes, that share a regulatory sequence can be switched on or off together by the same transcription factor.
Students often think Eukaryotes, like prokaryotes, coordinate groups of genes by grouping them in operons. In fact In general, no. Operons, groups of genes transcribed together as one mRNA, are typical of prokaryotes. Eukaryotes usually coordinate groups of genes, often on different chromosomes, through shared transcription factors.
Students often think Only prokaryotes coordinate genes; in eukaryotes, each gene is regulated on its own. In fact No. In eukaryotes, groups of genes may be influenced by the same transcription factors, so they are expressed together even if they are on different chromosomes.
9 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 9
In a hypothetical species of yeast, a mutation changes several nucleotides in a regulatory sequence upstream of gene R. Cells with the mutation make R protein with the normal amino acid sequence, but much less of it. Which explanation is most consistent with these observations?
Answer and reasoning
AThe regulatory sequence codes for the first amino acids of R, so fewer R molecules are completed. A student who thinks all DNA codes for protein picks this. A regulatory sequence is not translated; if it coded for part of R, the mutation would have changed R's amino acid sequence.
BThe change alters how a regulatory protein binds to the DNA, so gene R is transcribed less.Correct A regulatory sequence is a stretch of DNA that controls transcription by interacting with regulatory proteins. A change in how a regulatory protein binds there (for example, weaker binding of an activator) lowers transcription of gene R, while the coding region, and so the protein's sequence, is unchanged.
CThe regulatory sequence is the gene for a regulatory protein, so less of that protein is made. A student who confuses a regulatory sequence with the gene for a regulatory protein picks this. A regulatory sequence is a binding site for a regulatory protein; the protein is encoded by a separate gene.
DThe cells needed less protein R, so the mutation arose to lower its transcription. A student who thinks changes in DNA arise because a cell needs them picks this. Mutations arise at random; this one lowers transcription by changing a binding site, whatever the cell's needs.
Cultured cells of a hypothetical mammal were treated with a drug that blocks the removal of acetyl groups from histones. The graph shows the mean mRNA level of gene G in untreated cells, in cells with the drug, and in cells 48 h after the drug was removed (error bars, ±2 SE of the mean; n = 5). Which conclusion about the effect of the drug is supported by the data?
Answer and reasoning
AThe effect was fully reversed: after removal, expression was exactly the same as untreated. A student who thinks overlapping error bars prove equal means picks this. The overlap shows only that no significant difference was detected; the means are 1.2 and 1.0.
BThe effect was not reversible: after removal, expression stayed significantly above untreated. A student who thinks any difference between means is significant picks this. The after-removal mean (1.2) is higher than the untreated mean (1.0), but the bars overlap, so the difference is not significant.
CNo conclusion is possible, as the error bars show the measurements contained mistakes. A student who reads error bars as a record of mistakes picks this. The bars show the uncertainty in each mean from variation among the five replicates and are what allow the treatments to be compared.
DThe effect was reversible: after removal, expression was not significantly different from untreated.Correct The drug raised expression well above the untreated level (bars 2.8 to 3.6 and 0.8 to 1.2 do not overlap). After removal, the bar (0.9 to 1.5) overlaps the untreated bar but not the drug bar, so expression returned to a level not significantly different from untreated, as expected of a reversible modification of histones.
Working Intervals (mean ± 2 SE): untreated 1.0 ± 0.2, from 0.8 to 1.2; with drug 3.2 ± 0.4, from 2.8 to 3.6; 48 h after removal 1.2 ± 0.3, from 0.9 to 1.5. The drug bar does not overlap either other bar, so the drug raised expression significantly. The after-removal bar overlaps the untreated bar, so no significant difference from untreated has been shown, and it does not overlap the drug bar: the effect was reversed. Overlap does not show that the two means are exactly equal (1.0 and 1.2).
Researchers measured the mRNA levels of four genes, A to D, in three types of cells taken from the same individual of a hypothetical species of animal. The table shows the results. Which claim is best supported by the data?
Answer and reasoning
ACell types 2 and 3 have lost gene B, as neither has any mRNA from gene B. A student who thinks specialized cells lose unused genes picks this. The table shows mRNA, not genes; cells of one individual generally carry the same genes, and B is simply not transcribed in types 2 and 3.
BGene B is constitutive and gene A is not, since B has the higher mRNA level. A student who thinks constitutive means expressed at the highest level picks this. Constitutive describes steady expression regardless of signals, not level; gene B is not transcribed at all in two cell types.
CCell type 3 transcribes gene D because it decided that it needs protein D. A student who thinks cells choose which genes to express picks this. Expression is set by regulatory proteins acting on regulatory sequences, not by a decision of the cell.
DGenes B, C and D are tissue-specific, as each has a high mRNA level in only one cell type.Correct Genes B, C and D each have a high mRNA level in only one cell type (85, 64 and 77 units) and little or none in the others, while gene A is transcribed at similar levels (48–52) in all three, so B, C and D fit genes for tissue-specific proteins, whose expression produces the observable differentiation of the cell types.
During development of a hypothetical species of fly, a signal induces expression of gene A in certain cells. Protein A is a transcription factor that activates gene B, and protein B is a transcription factor that activates gene C. In a mutant fly, gene B is deleted. Which proteins are predicted to be made in the mutant's cells that normally make all three proteins?
Answer and reasoning
AProtein A only, because gene C needs protein B in order to be switched onCorrect The signal still induces gene A. With gene B deleted, no protein B is made, so gene C, which needs protein B to be activated, is not expressed. Sequential gene expression stops at the missing step.
BProteins A and C, because gene C switches on at its own set time in development A student who thinks genes switch on at a set time picks this. Gene C is switched on by protein B; without gene B, nothing activates gene C.
CNone of the three proteins, because losing gene B shuts down the whole pathway A student who thinks losing one gene stops a whole pathway picks this. Gene A is upstream of gene B and is induced by the signal, so protein A is still made.
DAll three proteins, because the cell makes protein B from another gene when needed A student who thinks cells switch on whatever they need picks this. There is no other source of protein B; a need does not produce a protein.
Researchers engineered plants of a hypothetical species to express gene P, which codes for an enzyme in the petal-pigment pathway, at different levels. The graph shows the pigment content of the petals. Which statement best describes the relationship shown?
Answer and reasoning
APigment rose in proportion to expression of gene P across the whole range of levels. A student who assumes a rising relationship is proportional throughout picks this. The increase per unit fell from 18 to 1 μg per g, so the relationship is not proportional.
BPigment rose up to about 4 units of expression and then started to fall. A student who reads a less steep rise as a fall picks this. Above 4 units, pigment rose from 58 to 61 μg per g; it never fell.
CPigment rose with expression of gene P and leveled off above about 4 units.Correct Pigment rose steeply from 0 to 50 μg per g between 0 and 3 units and then by only 3 μg per g from 4 to 6 units. More of the enzyme gives more pigment until another factor limits the pathway.
DPigment rose most per added unit of expression between 5 and 6, where it was highest. A student who reads the height of a curve as its slope picks this. Between 5 and 6 units pigment rose by only 1 μg per g; the largest rise per unit, 18 μg per g, was between 0 and 1 unit.
Working Increase in pigment per unit of expression: 0–1, 18; 1–2, 17; 2–3, 15; 3–4, 8; 4–5, 2; 5–6, 1. Pigment rises steeply up to about 3 units and then levels off near 60 μg per g above about 4 units; it never falls.
The diagram shows a model of an inducible operon in a hypothetical species of bacterium when the inducer is absent. In a mutant, a change in the operator prevents the repressor from binding to it. Which prediction about genes 1, 2 and 3 in the mutant is best supported by the model?
Answer and reasoning
AThey stay switched off, because the inducer can no longer bind. A student who thinks the inducer acts by binding the operator picks this. The inducer binds the repressor; with the operator changed, the repressor cannot block transcription, so the genes are on.
BThey are transcribed whether or not the inducer is present.Correct In the model, the repressor bound to the operator blocks RNA polymerase. If the repressor cannot bind the operator, nothing blocks transcription, so genes 1–3 are transcribed with or without the inducer.
CThey are switched on just when the inducer is present, as before. A student who thinks only changes in coding regions matter picks this. The operator is a regulatory sequence; changing it changes how the genes are regulated.
DThey respond to the inducer as before but make altered proteins. A student who thinks any change in an operon alters its proteins picks this. The coding regions of genes 1–3 are unchanged, so their proteins are normal, and with the operator changed the repressor cannot block transcription.
In a hypothetical species of bacterium, the enzymes that make the amino acid histidine are encoded by one operon. A student hypothesizes that this operon is a repressible operon. The student will grow the bacteria in medium with and without added histidine and measure the operon's mRNA. If the hypothesis is correct, which result is predicted?
Answer and reasoning
AMore operon mRNA in bacteria grown with added histidine than in those without it A student who thinks a substance always switches on genes related to it picks this. In a repressible operon, an abundant end product switches the operon off.
BThe same amount of operon mRNA in both cultures, as bacterial genes are on all the time A student who thinks prokaryotes do not regulate their genes picks this. A repressible operon is switched off when its end product is abundant, so the cultures should differ.
CLess operon mRNA in the bacteria grown with added histidine than in those without itCorrect In a repressible operon, an abundant end product, here histidine, activates the repressor, which binds the operator and blocks transcription. Bacteria given histidine should therefore have less operon mRNA than bacteria that must make their own.
DNo operon mRNA in either culture, because a repressible operon is kept switched off A student who reads 'repressible' as 'repressed' picks this. A repressible operon is transcribed until its corepressor is abundant, so the culture without added histidine should make the mRNA.
A student tests whether an operon for breaking down a sugar, S, is inducible in a hypothetical species of bacterium. The student grows the bacteria in a medium containing sugar S and measures the activity of the enzymes encoded by the operon. Which additional culture is the most appropriate control?
Answer and reasoning
AA second culture of the same bacteria in the same medium containing sugar S A student who thinks repeating the treatment provides a control picks this. A replicate shows whether the result is reproducible but cannot show what happens without sugar S.
BThe same bacteria in the medium containing sugar S, kept at a higher temperature A student who thinks a control changes some other condition picks this. Changing temperature adds a second variable and does not show the effect of sugar S.
CA sample of the same species of bacterium taken directly from its natural habitat A student who thinks a control is the organism in its natural conditions picks this. Wild bacteria differ from the cultured ones in many ways, so the comparison cannot isolate the effect of sugar S.
DThe same bacteria grown in the same medium and conditions, but with no sugar S addedCorrect This culture differs from the experimental culture only in the presence of sugar S, the independent variable. If enzyme activity is much higher with sugar S, the operon is inducible by S.
The diagram shows three genes of a hypothetical species of plant that are expressed together when the plant is exposed to high temperature. Which statement best relates the model to the regulation of gene expression in eukaryotes?
Answer and reasoning
ASequence H is the gene for transcription factor T, so each of the three genes makes its own T. A student who confuses a regulatory sequence with the gene for its regulatory protein picks this. Sequence H is a binding site for T; T is encoded elsewhere.
BHigh temperature changes the base sequence of Sequence H, which switches the genes on. A student who thinks expression is changed by changing the DNA sequence picks this. The model shows the same Sequence H bound by an active transcription factor; regulation does not alter the base sequence.
COne transcription factor coordinates genes on different chromosomes, as each gene has Sequence H.Correct In eukaryotes, groups of genes may be influenced by the same transcription factors. Genes J, K and L lie on different chromosomes, but each has Sequence H, so activating T switches all three on together.
DEach gene senses the high temperature and switches itself on because the plant needs it. A student who thinks genes or cells act according to need picks this. The genes are switched on by transcription factor T binding Sequence H, not by the genes sensing a need.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
