Question 1

Plant mitochondrial genomics: Approximately how large is the mitochondrial genome of melon (Cucumis melo) in kilobases (kb)?

Accepted Answer

Introduction / Context: Plant mitochondrial genomes vary enormously in size and structure compared with animals. Melon (Cucumis melo) is noted for having an unusually large mitochondrial genome among plants. This question targets familiarity with approximate genome scale. Given Data / Assumptions: Sizes are presented in kilobases (kb), where 1000 kb = 1 Mb. Plant mtDNA may include repeats, multipartite structures, and foreign DNA segments. We are selecting the closest widely cited approximate size. Concept / Approach: Reports place melon mitochondrial DNA in the multi-megabase range, far exceeding animal mtDNA (~16–20 kb). A commonly referenced figure is about 2.4 Mb (≈ 2400 kb), illustrating the genomic expansion and complexity of plant mitochondria. Step-by-Step Solution: Recall that plant mtDNAs can be hundreds of kb to several Mb.Identify melon as a very large example.Choose the value ≈ 2400 kb. Verification / Alternative check: Comparative genomics literature lists Cucumis melo mtDNA around 2.4 Mb, consistent with the selected option. Why Other Options Are Wrong: 600 or 1200 kb: too small for melon's mtDNA.3000 kb: larger than commonly cited value.75 kb: animal-like scale, not plants. Common Pitfalls: Assuming plant mtDNA sizes resemble those of animals; underestimating structural complexity and repeats in plant mitochondria. Final Answer: 2400 kilobase.

Question 2

Meiosis and genetic recombination: During which meiotic substage does crossing over primarily occur?

Accepted Answer

Introduction / Context: Meiosis generates genetic diversity through recombination. Knowing the precise substage of prophase I where crossing over occurs is critical for cytogenetics and genetics. Given Data / Assumptions: Prophase I substages: leptotene, zygotene, pachytene, diplotene, diakinesis. Synaptonemal complex formation is necessary for recombination. Chiasmata become microscopically evident later than the actual exchange event. Concept / Approach: Homologous chromosomes pair during zygotene and achieve full synapsis by pachytene. The synaptonemal complex facilitates homologous recombination; the actual exchange of genetic material (crossing over) occurs during pachytene. Chiasmata are most clearly visible in diplotene as synapsis dissolves. Step-by-Step Solution: Leptotene: chromosomes condense—no recombination yet.Zygotene: synapsis initiates—setup phase.Pachytene: complete synapsis—crossing over occurs.Diplotene: chiasmata visible—evidence of prior crossing over.Diakinesis: terminalization and readiness for metaphase I. Verification / Alternative check: Electron microscopy of synaptonemal complexes shows recombination nodules during pachytene; cytology confirms chiasmata subsequently in diplotene. Why Other Options Are Wrong: Zygotene: pairing initiates but exchange has not completed.Diplotene/Diakinesis: chiasmata are visible, but the exchange already occurred.Leptotene: early condensation only. Common Pitfalls: Confusing the visibility of chiasmata (diplotene) with the timing of crossing over (pachytene). Final Answer: Pachytene stage (full synapsis and recombination).

Question 3

Transposable elements across taxa Which of the following transposable elements is NOT naturally found in plants (i.e., it is characteristic of animals/humans rather than higher plants)?

Accepted Answer

Introduction / Context: Transposable elements are DNA sequences that move within genomes. In genetics and biotechnology, it is important to distinguish plant transposons (e.g., Ac/Ds in maize, Tam elements in snapdragon) from animal-specific elements such as LINE-1 (L1), which are abundant in mammals including humans. Given Data / Assumptions: Ac (Activator) is a classic maize DNA transposon. Tam3 is a well-studied plant transposon in Antirrhinum majus. L1 (LINE-1) is a long interspersed nuclear element that is characteristic of mammalian genomes. Concept / Approach: Match each element to its native, widely documented host lineage. Plant DNA transposons like Ac/Ds and Tam3 operate via a cut-and-paste mechanism using transposase. L1 elements are non-LTR retrotransposons that mobilize via an RNA intermediate and reverse transcription; their canonical representatives are mammalian, not plant, elements. Step-by-Step Solution: Identify plant elements: Ac (maize) and Tam3 (snapdragon) are plant transposons.Identify mammalian element: L1 is a mammalian LINE retrotransposon.Conclude that L1 is not naturally found as a plant transposon. Verification / Alternative check: Intro genetics texts and transposon catalogs list Ac/Ds and Tam families in plants and L1 in mammals, confirming the mapping. Why Other Options Are Wrong: Ac and Tam3 are plant elements, so they are not “not found” in plants. “All are found in plants” is incorrect because L1 is animal-specific. “None of the above” contradicts the above evidence. Common Pitfalls: Confusing the broader concept of LINEs (which exist across taxa) with the specific L1 lineage that is mammalian. Final Answer: L1 (LINE-1 retrotransposon)

Question 4

Mitosis mechanics During which sub-phase do kinetochore microtubules pull chromosomes toward opposite spindle poles?

Accepted Answer

Introduction / Context: Mitosis includes precise, named movements of chromosomes. Understanding the difference between anaphase A and anaphase B helps students interpret microscopy images and drug effects that target spindle dynamics. Given Data / Assumptions: Anaphase is subdivided into A and B. Kinetochore microtubules attach chromosomes to spindle poles via kinetochores. Interpolar microtubules contribute to spindle elongation. Concept / Approach: In anaphase A, chromosome movement results from shortening of kinetochore microtubules and poleward forces at kinetochores. In anaphase B, the spindle elongates due to sliding of antiparallel interpolar microtubules and pole separation. Step-by-Step Solution: Identify the process: poleward chromosome movement via kinetochore microtubules.Map to phase: this is the hallmark of anaphase A.Select Anaphase A. Verification / Alternative check: Standard cell biology diagrams show anaphase A (chromosomes move to poles) preceding or overlapping with anaphase B (spindle elongation). Why Other Options Are Wrong: Anaphase B emphasizes spindle elongation, not primary chromosome pulling. Metaphase is alignment at the metaphase plate. Telophase involves reformation of nuclei, not poleward pulling. Prophase includes chromosome condensation and spindle assembly. Common Pitfalls: Equating all chromosome movements with “anaphase” without distinguishing A vs. B mechanics. Final Answer: Anaphase A

Question 5

Order of meiotic divisions In meiosis, daughter cells arise through two successive divisions. Which statement correctly describes the order of these divisions?

Accepted Answer

Introduction / Context: Meiosis ensures halving of chromosome number for sexual reproduction. Knowing which division reduces ploidy and which separates sister chromatids is central to interpreting genetic outcomes and nondisjunction errors. Given Data / Assumptions: Meiosis consists of meiosis I and meiosis II. Homologous chromosomes pair and recombine before segregation. Sister chromatids remain together through meiosis I (except at chiasmata). Concept / Approach: Meiosis I is reductional: homologs segregate to different daughter cells, reducing 2n to n. Meiosis II is equational: sister chromatids separate, akin to mitosis, preserving haploid number. Step-by-Step Solution: Identify the stage that halves chromosome number: meiosis I (reductional).Identify the stage that separates sister chromatids: meiosis II (equational).Choose the option that states “first reductional, second equational.” Verification / Alternative check: Most textbooks refer to meiosis I as the reduction division and meiosis II as the equational division. Why Other Options Are Wrong: Equational first then reductional is incorrect. Two reductional or two equational divisions do not describe standard meiosis. Nondisjunction exceptions exist but do not redefine the canonical scheme. Common Pitfalls: Confusing chromatid and homolog separation due to both involving “pairs.” Final Answer: First division is reductional; second is equational

Question 6

Prophase I staging During meiosis I, in which substage do homologous chromosomes undergo synapsis (pairing via the synaptonemal complex)?

Accepted Answer

Introduction / Context: Prophase I of meiosis is subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis. Each stage features distinct chromosome behaviors that enable recombination and proper segregation. Given Data / Assumptions: Synapsis is the intimate pairing of homologs mediated by the synaptonemal complex. Crossing over occurs after synapsis is established. Concept / Approach: Leptotene: chromosomes begin to condense. Zygotene: homologs synapse. Pachytene: synapsis is complete, and crossing over occurs. Diplotene: synaptonemal complex disassembles; chiasmata visible. Diakinesis: further condensation and terminalization. Step-by-Step Solution: Identify stage of initial homolog pairing: zygotene.Recognize pachytene as the stage when recombination is prominent but after synapsis is established.Select zygotene as the correct answer. Verification / Alternative check: Standard meiosis schematics label zygotene as the synapsis phase. Why Other Options Are Wrong: Leptotene precedes pairing; chromosomes are thread-like. Pachytene follows synapsis and features crossing over. Diplotene/diakinesis involve desynapsis and preparation for metaphase I. Common Pitfalls: Confusing synapsis (pairing) with recombination (exchange), which mainly occurs in pachytene. Final Answer: Zygotene

Question 7

Monohybrid cross with incomplete dominance What phenotypic ratio is expected in the F2 generation when a trait shows incomplete dominance?

Accepted Answer

Introduction / Context: Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes (e.g., red x white snapdragons producing pink F1). Understanding the resulting F2 ratios is foundational to classical genetics. Given Data / Assumptions: Single gene with two alleles. Incomplete dominance: heterozygote has an intermediate phenotype. Random mating or selfing of F1 heterozygotes. Concept / Approach: From two true-breeding parents (AA and aa), F1 are all Aa and intermediate. Selfing F1 (Aa x Aa) yields genotypes AA, Aa, aa in a 1:2:1 ratio. With incomplete dominance, phenotypes mirror genotypes: 1 homozygote phenotype : 2 intermediate : 1 other homozygote. Step-by-Step Solution: List F2 genotype ratios: AA:Aa:aa = 1:2:1.Map genotype to phenotype under incomplete dominance (three distinct phenotypes).Conclude F2 phenotypic ratio is also 1:2:1. Verification / Alternative check: Classic examples (flower color) reproduce the 1:2:1 phenotypic ratio in lab crosses. Why Other Options Are Wrong: 3:1:1 and 1:3:1 are not standard monohybrid outcomes. 1:1:2 misorders counts and is nonstandard. 9:3:3:1 applies to a dihybrid cross with independent assortment, not monohybrid. Common Pitfalls: Confusing incomplete dominance with codominance; both differ from strict dominance but have different phenotype interpretations. Final Answer: 1:2:1

Question 8

Genetic testing of an F1 hybrid Crossing an F1 individual with a homozygous recessive tester is referred to as which type of cross?

Accepted Answer

Introduction / Context: Breeders and geneticists use specific crosses to reveal genotype. A test cross helps determine whether an individual with a dominant phenotype is homozygous dominant or heterozygous. Given Data / Assumptions: F1 is typically heterozygous when derived from two true-breeding parents. The tester is homozygous recessive for the locus of interest. Concept / Approach: In a test cross, mating with a homozygous recessive reveals the gametes contributed by the tested parent, because the tester contributes only recessive alleles. Offspring phenotypes therefore directly reflect the tested parent’s genotype. Step-by-Step Solution: Define the cross: F1 (e.g., Aa) x tester (aa).Predict offspring: 1 Aa (dominant phenotype) : 1 aa (recessive phenotype) if F1 is heterozygous.Name of such cross: test cross. Verification / Alternative check: Any genetics manual describes test crosses for mapping and genotype assessment. Why Other Options Are Wrong: “Recessive cross” is not standard terminology. “Criss-cross” refers to sex-linked transmission patterns, not this cross. “Para-cross” is not a standard genetic term. Backcross to a dominant parent is a different design and does not reveal hidden recessive alleles. Common Pitfalls: Confusing backcross (to either parent) with a test cross (specifically to recessive). Final Answer: Test cross

Question 9

Timing of mitotic stages Which mitotic phase is typically the shortest in duration during a normal somatic cell cycle?

Accepted Answer

Introduction / Context: Mitosis is subdivided into prophase, prometaphase, metaphase, anaphase, and telophase. The relative duration of each informs how frequently cells are observed in each stage under the microscope and reflects underlying mechanics. Given Data / Assumptions: Chromosome alignment at metaphase requires time for tension and checkpoint satisfaction. Anaphase is a rapid separation and movement event. Interphase is not part of mitosis and is much longer. Concept / Approach: Once all kinetochores are correctly attached and the spindle assembly checkpoint is satisfied, separase triggers cohesin cleavage, and chromatids move swiftly poleward. This phase—anaphase—typically lasts the shortest interval compared with the set-up (metaphase) and wrap-up (telophase). Step-by-Step Solution: Consider durations: prophase/prometaphase (setup), metaphase (checkpoint), anaphase (execution), telophase (reassembly).Identify the rapid event: sister chromatid separation and poleward movement.Select anaphase as the shortest phase. Verification / Alternative check: Time-lapse microscopy in model cells consistently shows anaphase as brief relative to other mitotic stages. Why Other Options Are Wrong: Interphase is not a mitotic phase and is much longer. Metaphase persists until checkpoint satisfaction. Prophase and telophase involve structural assembly/disassembly that takes longer. Common Pitfalls: Including interphase when asked specifically about mitotic phases. Final Answer: Anaphase

Question 10

Design of a plant transformation plasmid for Bt toxin Which statement best describes a necessary feature of a plasmid used to introduce the Bacillus thuringiensis (Bt) toxin gene into plant cells for transgenic expression?

Accepted Answer

Introduction / Context: Engineering Bt crops requires expressing a bacterial insecticidal protein in plant tissues. Proper expression in plants depends on plant-compatible regulatory sequences, codon optimization, and transformation/selection strategies. Given Data / Assumptions: Bt toxin genes originate from Bacillus thuringiensis. Plants use different promoters, introns, and polyadenylation signals than bacteria. Selectable markers (e.g., kanamycin resistance) are used in tissue culture but are not intended for field antibiotic resistance. Concept / Approach: A successful plant transformation vector positions the Bt coding sequence under a strong plant promoter (e.g., CaMV 35S for constitutive expression, or tissue-specific promoters) with plant terminators and often codon-optimized sequence. This ensures transcription/translation in the plant nucleus/cytoplasm. Tumor-inducing functions (opines) are removed in disarmed Ti vectors. Step-by-Step Solution: Identify the key requirement: plant-compatible gene expression control.Choose the description that mentions a bacterial gene under a plant promoter.Reject options about opines (tumor phenotype) and field antibiotic resistance misstatements. Verification / Alternative check: Standard binary vectors (e.g., pBI121 derivatives) demonstrate plant promoters driving transgenes, validating this approach. Why Other Options Are Wrong: Antibiotic resistance markers are for selection in culture, not agronomic antibiotic resistance. Opine synthesis is from oncogenic T-DNA; disarmed vectors lack these genes. Toxic vectors would kill transformants. Lack of selectable markers hinders recovery of true transformants. Common Pitfalls: Assuming bacterial promoters work efficiently in plants; they generally do not without plant regulatory elements. Final Answer: Carries a bacterial gene placed under a plant promoter and regulatory signals

Question 11

Alternation of generations in plant life cycles In organisms with alternation of generations, which phase represents the haploid, multicellular stage?

Accepted Answer

Introduction / Context: Many plants and some algae alternate between multicellular haploid and diploid generations. Knowing which generation is haploid versus diploid is crucial for understanding meiosis, fertilization, and inheritance in plant biology. Given Data / Assumptions: Sporophyte produces spores by meiosis. Gametophyte produces gametes by mitosis. Fertilization creates a diploid zygote that grows into a sporophyte. Concept / Approach: The gametophyte is haploid (n) and multicellular; its cells carry a single set of chromosomes. The sporophyte is diploid (2n). Gametes (n) are single cells, not a multicellular generation. The zygote is diploid and develops into the sporophyte. Step-by-Step Solution: Define the two generations: haploid gametophyte and diploid sporophyte.Identify which is multicellular and haploid: the gametophyte.Select “Gametophyte.” Verification / Alternative check: Mosses emphasize the gametophyte; flowering plants emphasize the sporophyte, but the male and female gametophytes (pollen and embryo sac) are still haploid multicellular structures. Why Other Options Are Wrong: Zygote is diploid and unicellular initially. Gamete is haploid but unicellular, not a “generation.” Sporophyte is diploid. Endosperm is typically triploid (in angiosperms) and not the haploid generation. Common Pitfalls: Equating gametes with the gametophyte; the gametophyte produces gametes but is itself a multicellular haploid organism or tissue. Final Answer: Gametophyte

Plant Genetics Questions