Genetics of Prokaryotes (2B)
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MCAT Biological and Biochemical Foundations of Living Systems › Genetics of Prokaryotes (2B)
A bacterial strain becomes resistant after incubation with extracellular DNA, but only when CaCl$_2$ treatment and a brief heat shock are applied. No donor cells are present. Which statement is most consistent with the experimental requirement, given the focus on transformation?
CaCl$_2$ and heat shock are required to induce pilus synthesis for conjugation.
The treatment activates prophage excision, enabling specialized transduction.
The treatment causes targeted chromosomal recombination without requiring exogenous DNA.
The treatment likely increases membrane permeability and DNA uptake efficiency, consistent with transformation.
Explanation
The skill being tested is prokaryotic genetics. Transformation in non-naturally competent strains often requires treatments like CaCl2 and heat shock to enhance membrane permeability and DNA uptake. The requirement for these conditions with extracellular DNA and no donors points to induced transformation. Choice D is correct as it explains how the treatment facilitates uptake in transformation. Choice B fails on the misconception that CaCl2 induces pili for conjugation, but no donors are present. For similar questions, identify competence induction methods to confirm transformation. Rule out other processes by noting absence of donors or phages.
Two bacterial strains are mixed: Strain 1 is F+ and carries a plasmid encoding gentamicin resistance $(Gen^R$). Strain 2 is F− and $Gen^S$. After co-incubation, $Gen^R$ appears in Strain 2 only when cells are allowed direct contact; separating strains with a 0.22 µm filter prevents transfer. Which statement is most consistent with the observed genetic change?
Transfer likely occurs via uptake of free DNA, which should pass through the filter.
Transfer likely reflects spontaneous mutation in Strain 2 triggered by proximity to Strain 1.
Transfer likely occurs via bacteriophage particles, which cannot cross 0.22 µm filters.
Transfer likely requires cell-to-cell contact consistent with conjugation.
Explanation
The skill being tested is prokaryotic genetics. Conjugation requires direct cell-to-cell contact for DNA transfer via a pilus, which is blocked by physical barriers like filters. The transfer of $Gen^R$ only with direct contact and prevention by a 0.22 µm filter indicates contact-dependent mechanism. Choice D is correct as it links the need for contact to conjugation. Choice B fails on the misconception that free DNA can pass filters for transformation, but the filter size blocks bacteria while allowing DNA, yet transfer requires contact here. For similar questions, test physical separation to confirm contact necessity. Evaluate filter pore size to distinguish cellular from molecular transfers.
To test transformation, Bacillus subtilis was incubated with a circular plasmid encoding erythromycin resistance $(Erm^R$). In one condition, cells were heat-killed before plasmid addition; in another, live cells were used. Only the live-cell condition produced $Erm^R$ colonies. Which statement is most consistent with the data?
Transformation requires active cellular processes for DNA uptake; dead cells cannot acquire plasmid DNA.
$Erm^R$ colonies must result from conjugation because plasmids cannot enter cells by uptake.
Heat-killed cells should transform more efficiently because membranes become permeable.
$Erm^R$ colonies likely arose from transcriptional upregulation of a pre-existing chromosomal erm gene.
Explanation
The skill being tested is prokaryotic genetics. Transformation requires live, competent cells to actively uptake and integrate extracellular DNA, as dead cells lack the necessary metabolic activity and membrane integrity. In this Bacillus subtilis experiment, only live cells produce $Erm^R$ colonies, indicating active processes are essential. Choice D is correct because it emphasizes that dead cells cannot perform the energy-dependent uptake required for transformation. Choice B fails on the misconception that heat-killing enhances permeability for transformation, but actually, viability is crucial for DNA internalization. For similar questions, test cell viability to confirm active uptake mechanisms. Distinguish from passive processes by noting requirements for competence and energy.
An F+ donor carrying a plasmid with a functional tra operon is mixed with an F− recipient. A tra gene knockout is introduced into the donor, and transfer of the plasmid to recipients is no longer detected. This observation is most consistent with which statement about conjugation?
Conjugation transfers only chromosomal DNA, not plasmids, so tra genes are irrelevant.
Conjugation requires recipient competence genes, so donor mutations should not matter.
Conjugation depends on donor-encoded transfer machinery, including pilus formation and DNA processing.
Conjugation is mediated by bacteriophages, so donor tra genes are unnecessary.
Explanation
The skill being tested is prokaryotic genetics. Conjugation relies on donor-encoded tra genes that facilitate pilus formation, DNA processing, and transfer to the recipient. The tra knockout in the donor abolishes plasmid transfer, highlighting the donor's role in providing transfer machinery. Choice A aligns because it correctly states that conjugation depends on donor tra functions for pilus and DNA handling. Choice B is a distractor based on the misconception that conjugation is phage-mediated, but it is a direct cell-to-cell process independent of viruses. In similar questions, identify if mutations affect donor or recipient to pinpoint machinery origin. Check for pilus involvement to confirm conjugation over other transfers.
A plasmid carrying a toxin-antitoxin (TA) module is introduced into a bacterial population. After several generations without antibiotic selection, most cells still retain the plasmid. Which explanation is most consistent with plasmid maintenance under these conditions?
DNase in the environment prevents plasmid loss by protecting extracellular DNA.
Plasmids are always replicated faster than chromosomes, so they cannot be lost.
Only bacteriophages, not plasmids, can encode stable inheritance systems.
TA systems can create post-segregational killing, enriching for plasmid-containing cells.
Explanation
The skill being tested is prokaryotic genetics. Toxin-antitoxin (TA) systems on plasmids promote maintenance by killing daughter cells that lose the plasmid, as the stable toxin persists while the labile antitoxin degrades. In this population, plasmid retention without selection suggests TA-mediated post-segregational killing enriches for plasmid carriers. Choice A aligns because it describes how TA systems ensure inheritance by eliminating plasmid-free cells. Choice B is incorrect based on the misconception that plasmids replicate faster than chromosomes, but replication rates vary and do not prevent loss without mechanisms like TA. In similar questions, look for absence of selection and high retention as indicators of addiction modules. Verify if the system targets plasmid-free cells post-division.
A lab observes that after mixing an F+ donor with an F− recipient, the recipient gains a plasmid marker but the donor does not lose the marker. The investigator concludes that the donor must have transferred a single-stranded DNA copy while retaining the original plasmid. Which statement is most consistent with this conjugation model?
Rolling-circle replication during transfer can allow the donor to retain a plasmid copy while exporting a strand to the recipient.
The recipient must first become competent and then secrete pili to pull DNA from the donor.
The donor must lyse to release plasmid DNA, explaining why it retains the marker.
The marker persistence in the donor indicates transfer occurred by transduction rather than conjugation.
Explanation
This question tests knowledge of prokaryotic genetics, specifically the mechanisms of horizontal gene transfer in bacteria. Conjugation is a process where genetic material, often in the form of a plasmid, is transferred from a donor bacterium to a recipient via direct cell-to-cell contact, typically mediated by a sex pilus. In this scenario, an F+ donor transfers a plasmid marker to an F− recipient without losing the marker itself, consistent with the transfer of a single-stranded DNA copy while retaining the original plasmid. Choice A aligns with this by describing rolling-circle replication, which allows the donor to replicate and export a single strand of the plasmid DNA to the recipient while keeping a copy. A common misconception, as in choice B, is confusing conjugation with processes involving cell lysis, like transduction, but conjugation does not require the donor to lyse. For similar questions, verify if the mechanism preserves the donor's genetic material, as in conjugation via rolling-circle replication. Additionally, distinguish conjugation from transformation or transduction by noting the requirement for cell contact and absence of competence or viral vectors.
A researcher notes that a resistance gene spreads rapidly through a bacterial population only when the gene is located on a conjugative plasmid, but spreads slowly when located on the chromosome. Which statement is most consistent with the observed difference in spread, focusing on one process?
Plasmid transfer is vertical only, so chromosomal genes should spread faster than plasmids.
Chromosomal genes cannot be replicated, so they spread only by transformation.
Conjugative plasmids can transfer horizontally between cells, accelerating dissemination relative to chromosomal inheritance.
Conjugative plasmids require bacteriophages to move, so spread depends on phage abundance.
Explanation
The skill being tested is prokaryotic genetics. Conjugative plasmids enable horizontal transfer between cells, spreading genes faster than vertical chromosomal inheritance. Rapid spread on plasmids vs. slow on chromosomes highlights conjugation's efficiency. Choice A is correct as it contrasts horizontal conjugation with vertical replication. Choice D fails due to the misconception that plasmids are only vertical, ignoring their horizontal mobility. For similar questions, compare spread rates between locations to infer transfer mode. Assess conjugation potential to explain dissemination differences.
A bacterium becomes resistant to an antibiotic after exposure to extracellular DNA isolated from a resistant strain. When the same experiment is repeated with DNA isolated from a bacteriophage lysate instead of purified cellular DNA, resistance appears even without competence induction. Which statement best accounts for the second result while avoiding confusion with transformation?
The second result still requires competence because phages can only bind competent cells.
The second result implies spontaneous mutation because extracellular DNA cannot alter genotype.
The second result indicates conjugation because lysates contain pili that mediate transfer.
The second result is consistent with transduction, where phage particles deliver DNA independent of competence.
Explanation
The skill being tested is prokaryotic genetics. Transduction involves phage-mediated DNA transfer, which can occur without competence as phages inject DNA directly. The resistance from phage lysate without competence distinguishes it from transformation, which requires competence for uptake. Choice D is correct as it explains transduction's independence from competence. Choice B fails on the misconception that phages need competent cells, but phage infection bypasses uptake machinery. For similar questions, compare competence requirements to differentiate transduction. Check DNA source (lysate vs. purified) to identify phage involvement.
A conjugative plasmid carrying a resistance gene enters a new host species. The plasmid is maintained, but resistance is not observed, and sequencing shows the gene is intact. Which statement is most consistent with a prokaryotic gene expression explanation that does not invoke additional processes?
Conjugation must have failed because plasmids cannot cross species boundaries.
The resistance gene should automatically express at high levels in any bacterium because all sigma factors are identical.
Promoter or regulatory elements on the plasmid may not be recognized efficiently by the new host, reducing transcription.
The resistance gene must be on the chromosome to be translated, so plasmid location prevents expression.
Explanation
The skill being tested is prokaryotic genetics. Prokaryotic gene expression can be host-specific, with promoters or regulators on plasmids not always compatible across species, leading to low expression. The intact gene but absent resistance after interspecies transfer suggests regulatory incompatibility. Choice A is correct as it explains potential failure of host machinery to recognize plasmid elements. Choice D fails due to the misconception that all sigma factors are identical, but species differences affect expression. For similar questions, assess cross-species expression to detect compatibility issues. Sequence integrity to rule out mutations vs. regulation.
A hospital isolate carries a plasmid encoding an extended-spectrum beta-lactamase (ESBL). When grown without antibiotics, the plasmid copy number decreases over time, and the fraction of ESBL-positive cells drops. Which interpretation is most consistent with plasmid function and selection?
Without selective pressure, plasmid carriage can impose a fitness cost, allowing plasmid-free cells to outcompete.
Loss of ESBL indicates that chromosomal DNA was deleted by conjugation.
The trend requires bacteriophage transduction because plasmids cannot be lost during division.
Plasmids are always essential, so their frequency should increase without antibiotics.
Explanation
The skill being tested is prokaryotic genetics. Plasmids can impose fitness costs without selection, leading to loss as plasmid-free cells outcompete carriers during growth. The decreasing ESBL-positive fraction without antibiotics reflects this cost-driven segregation. Choice A aligns because it describes the fitness burden allowing loss under non-selective conditions. Choice D is incorrect based on the misconception that plasmids require transduction for maintenance, but loss occurs via segregation during division. In similar questions, monitor plasmid frequency without selection to detect costs. Compare to selected conditions to quantify stability differences.