Question 1

T-DNA borders — identifying the incorrect statement about the direct repeats  Regarding the short direct-repeat sequences that flank T-DNA on the Ti-plasmid, which statement is NOT true?

Accepted Answer

Introduction / Context: T-DNA is delimited by short border repeats (about 25 base pairs) on the Ti plasmid. These border sequences are recognition elements for the virulence machinery. Clarifying what happens to the borders during transfer avoids common misconceptions. Given Data / Assumptions: Right and left borders are highly conserved across Ti plasmid classes. VirD2 nicks at the right border to initiate a single-stranded T-strand. The transferred DNA corresponds primarily to the sequence between borders. Concept / Approach: Although the borders define T-DNA boundaries and are essential for processing, the transferred T-strand generally comprises the region between the borders. The border sequences themselves are not typically integrated as intact repeats in the plant genome; rather, processing occurs at or near these sites, and the internal region becomes integrated. Step-by-Step Solution: Recall that border repeats are processing sites for VirD2/VirC.Recognize that the T-strand corresponds to the internal segment between borders.Conclude that ‘‘transferred intact repeats’’ is inaccurate.Select option stating that borders transfer intact as NOT true. Verification / Alternative check: Sequencing of T-DNA–plant junctions shows integration endpoints near but not composed of intact border repeats, supporting the conclusion. Why Other Options Are Wrong: Conservation between Ti types and importance in processing are correct; borders define processing boundaries through Vir protein recognition. Common Pitfalls: Equating ‘‘boundary definition’’ with ‘‘physical inclusion’’ of border repeats in the integrated T-DNA. Final Answer: These repeats are transferred intact into the plant genome along with T-DNA

Question 2

Terminology — what are the direct repeats flanking Agrobacterium T-DNA called?  In Agrobacterium tumefaciens Ti plasmids, the short direct repeats that flank the T-DNA and define its boundaries are known as:

Accepted Answer

Introduction / Context: Precise terminology matters in genetic engineering. The short, conserved sequences that delimit T-DNA and are recognized by Vir proteins have a specific name used throughout the Agrobacterium literature. Given Data / Assumptions: Direct repeats flank T-DNA and serve as processing sites. Recognition by VirD2/VirC occurs at these elements. Terminology distinguishes them from unrelated sequence tags in other systems. Concept / Approach: These repeats are called border sequences (left border, LB; right border, RB). ‘‘cos’’ refers to lambda phage cohesive ends; ‘‘att’’ refers to phage integration sites; ‘‘flanking sequences’’ is too generic to be precise in this context. Step-by-Step Solution: Identify the function: delimiting and processing T-DNA edges.Match to canonical term ‘‘border sequences.’’Exclude unrelated phage terms (cos, att).Choose the precise term used in plant transformation protocols. Verification / Alternative check: Vector maps and Ti plasmid schematics label LB and RB as ‘‘border’’ sequences, confirming standard usage. Why Other Options Are Wrong: ‘‘cos’’ and ‘‘att’’ belong to bacteriophage biology; ‘‘transfer sequences’’ lacks specificity; ‘‘flanking sequences’’ is non-technical here. Common Pitfalls: Using generic or phage-derived terms when the plant transformation field uses ‘‘border’’ precisely. Final Answer: Border sequences

Question 3

Classes of Agrobacterium-based plant transformation vectors  Agrobacterium-derived plant transformation vectors are generally grouped into how many fundamental categories, based on how the T-DNA region and vir functions are arranged?

Accepted Answer

Introduction / Context: Modern plant biotechnology primarily relies on Agrobacterium-based vectors that separate cloning convenience from transfer efficiency. Recognizing the two principal vector architectures helps in selecting appropriate systems for different cloning strategies. Given Data / Assumptions: T-DNA borders delimit the segment to be transferred into plants. vir genes can be retained on a helper Ti plasmid or recombined into a single molecule with T-DNA. Historically, two architectures became standard. Concept / Approach: The two major classes are: (1) co-integrate vectors, in which T-DNA sequences are recombined into a disarmed Ti plasmid residing in Agrobacterium; and (2) binary vectors, in which a small, easily manipulated plasmid carrying T-DNA borders and cargo replicates in both E. coli and Agrobacterium, while vir functions are provided in trans by a separate helper plasmid in Agrobacterium. Step-by-Step Solution: Identify organizational principle (co-integrated vs. separated functions).Relate to cloning convenience (binary) vs. single-construct (co-integrate).Conclude there are two fundamental categories.Select the answer specifying ‘‘two categories.’’ Verification / Alternative check: Plant transformation manuals and vector catalogs partition Agrobacterium systems into co-integrate and binary designs, confirming the dichotomy. Why Other Options Are Wrong: Four, six, or eight categories are not standard classifications. A single universal category ignores the clear architectural split used in practice. Common Pitfalls: Assuming many minor vector variants represent distinct categories; despite variations, they fall under the two main designs. Final Answer: Two categories (co-integrate vectors and binary vectors)

Question 4

Plant gene transfer — What is the approximate size range of the virulence (Ti) plasmid carried by Agrobacterium tumefaciens that mediates T-DNA transfer to plant cells?

Accepted Answer

Introduction / Context: The tumor-inducing (Ti) plasmid of Agrobacterium tumefaciens is central to natural genetic transformation of plants and to plant biotechnology. Knowing the typical size range of this virulence plasmid helps students appreciate cloning strategies, vector design, and why binary/co-integrative systems are used rather than moving the entire plasmid into Escherichia coli. Given Data / Assumptions: The Ti plasmid carries vir genes for DNA processing/transfer and a T-DNA region integrated into plant genomes. Large plasmids are difficult to manipulate directly in standard lab hosts. Published Ti plasmids commonly fall in the low-hundreds of kilobase size range. Concept / Approach: Classical nopaline- and octopine-type Ti plasmids are typically around two hundred kilobases. Because of this size, binary vector systems split functions: a small E. coli–Agrobacterium shuttle plasmid carries T-DNA, while a resident helper plasmid in Agrobacterium supplies vir functions in trans. Step-by-Step Solution: Recall that Ti plasmids are much larger than standard cloning plasmids (e.g., pBR322 ~4.3 kb).Experimental maps and sequencing place many Ti plasmids in the ~140–235 kb window.Therefore, among the options, 140–235 kb is the most accurate typical range. Verification / Alternative check: Genetic maps in plant biotech texts consistently depict Ti plasmids ~200 kb, supporting the 140–235 kb bracket as representative across common isolates. Why Other Options Are Wrong: 40–80 kb and 80–120 kb: too small for full vir/T-DNA complement of classical Ti plasmids. >235 kb: some variants can be very large, but this is not the typical range emphasized in foundational courses. 8–20 kb: corresponds to small cloning vectors, not Ti plasmids. Common Pitfalls: Assuming all plasmids are similar in size; megaplasmids like Ti plasmids necessitate specialized vector systems. Final Answer: 140-235 kb

Question 5

Plant transformation methods — What is a key advantage set of the microprojectile (biolistic, gene gun) method compared with microinjection for delivering DNA into plant cells?

Accepted Answer

Introduction / Context: Two classic physical methods for plant transformation are microinjection and microprojectile bombardment (biolistics). Understanding their comparative advantages is essential for selecting the appropriate technique across species and tissues, especially when cell walls pose a barrier. Given Data / Assumptions: Microinjection uses glass micropipettes and typically requires large, wall-less target cells (e.g., protoplasts). Biolistics uses high-velocity DNA-coated particles to penetrate cell walls and membranes. Throughput and universality differ between these techniques. Concept / Approach: The biolistic method can transfect intact tissues (callus, embryos, leaves) without enzymatically removing cell walls. It can target many cells in a single shot and is applicable across a broad range of species, sizes, and tissues, including monocots historically recalcitrant to Agrobacterium-based methods. Step-by-Step Solution: Check whether intact, walled cells can be used: biolistics supports this.Assess universality: the method works across diverse tissues and species.Consider throughput: hundreds to thousands of cells can be hit per bombardment.Thus, all listed advantages apply to microprojectile delivery. Verification / Alternative check: Protocol literature and manufacturer guides document successful transformations in cereals, dicots, algae, and even organelles (plastid transformation), underscoring the approach’s breadth. Why Other Options Are Wrong: Each single statement (a–c) is true, but the most complete answer is the combined option. “Requires no specialized equipment” is false; gene guns and optimization are needed. Common Pitfalls: Confusing microinjection’s precision with practicality; although precise, microinjection is low-throughput and usually requires protoplasts. Final Answer: all of the above

Question 6

Plant tumor metabolism — In crown gall disease, what are opines and where are they typically found?

Accepted Answer

Introduction / Context: Agrobacterium tumefaciens induces crown gall tumors by transferring T-DNA into plant cells. The transformed cells synthesize unusual metabolites called opines, which reveal a fascinating plant–microbe metabolic relationship and serve as diagnostic markers of transformation. Given Data / Assumptions: T-DNA encodes enzymes that redirect plant metabolism. Opines are typically condensation products of amino acids with keto acids or sugars. Different Ti plasmids specify different opine types (e.g., nopaline, octopine, agropine). Concept / Approach: Opines are synthesized in tumor tissues (not in healthy tissues) and can be catabolized by the infecting Agrobacterium as exclusive carbon/nitrogen sources. This creates a selective ecological niche benefiting the bacterium that initiated the tumor. Step-by-Step Solution: Identify biochemical nature: amino acid-derived compounds.Locate occurrence: produced in gall (tumor) tissues harboring integrated T-DNA.Connect function: provide specific nutrients for opine-catabolizing Agrobacterium strains.Hence, opines are amino acid derivatives found in tumor tissues. Verification / Alternative check: Paper chromatography and enzymatic assays historically differentiated opine types and correlated them with Ti plasmid classes. Why Other Options Are Wrong: Normal tissues do not synthesize opines in the absence of T-DNA genes. “Both normal and tumor tissues” contradicts the hallmark of transformation. “Simple sugars in healthy leaves” mischaracterizes opines. Common Pitfalls: Assuming opines are general plant metabolites; they are transformation-specific markers. Final Answer: amino acid derivatives found in tumor tissues

Question 7

Basic microbiology — Agrobacterium tumefaciens, the agent of crown gall disease and a key tool in plant genetic engineering, is classified as what type of microorganism?

Accepted Answer

Introduction / Context: Agrobacterium tumefaciens transfers T-DNA to plant cells, causing crown gall tumors and enabling engineering of transgenic plants. Understanding its microbial classification underpins choices of antibiotics, culture conditions, and vector design in the lab. Given Data / Assumptions: Agrobacterium belongs to Alphaproteobacteria. Gram reaction influences outer membrane characteristics and sensitivity to certain drugs. It is a bacterium, not a eukaryote or virus. Concept / Approach: Agrobacterium tumefaciens (now also referred to within the Rhizobium/Agrobacterium complex) is Gram-negative, possessing an outer membrane and thin peptidoglycan layer. This aligns with its staining properties and genetic toolkits used for binary vector systems. Step-by-Step Solution: Identify domain: Bacteria.Determine Gram status: Gram-negative cell envelope.Exclude fungi/yeast/viruses which are unrelated organismal groups.Select Gram-negative bacteria as the correct classification. Verification / Alternative check: Standard microbiology references and lab protocols list Agrobacterium as Gram-negative rods growing on specific media like LB or YEB. Why Other Options Are Wrong: Gram-positive: would show thick peptidoglycan and lack an outer membrane, which is incorrect here. Fungi/yeast/virus: wrong domains and biology. Common Pitfalls: Confusing plant pathogens broadly with fungi; Agrobacterium is bacterial. Final Answer: gram (-) bacteria

Question 8

Vector strategy — In co-integrating transformation systems used with Agrobacterium, where must a region of DNA homology be present to allow recombination?

Accepted Answer

Introduction / Context: Co-integrating vector systems predated today’s binary vectors. They rely on homologous recombination between an E. coli-derived vector and the resident Ti plasmid in Agrobacterium, forming a larger co-integrate that carries the engineered T-DNA for plant transformation. Given Data / Assumptions: Homology is required for stable recombination events. The Ti plasmid provides virulence functions needed for T-DNA processing and transfer. The engineered segment must be captured into a transferable context. Concept / Approach: To form a co-integrate, the small cloning vector and the Ti plasmid share a homologous region. Recombination integrates the engineered T-DNA into the Ti backbone inside Agrobacterium, enabling subsequent mobilization of T-DNA into plant cells. Step-by-Step Solution: Identify the two replicons to be recombined: the E. coli vector and the Ti plasmid.Provide a shared sequence to direct homologous recombination.After recombination, the co-integrate harbors T-DNA plus vir functions.Thus, homology must exist between the vector plasmid and the Ti plasmid. Verification / Alternative check: Historic co-integrative systems (e.g., pRK290 derivatives) used defined homology regions to capture engineered inserts into Ti backbones. Why Other Options Are Wrong: Homology restricted to only one plasmid cannot mediate inter-molecular recombination. “None of these” ignores the fundamental requirement of shared sequence. Plant genome homology is irrelevant for plasmid–plasmid recombination in Agrobacterium. Common Pitfalls: Confusing co-integrative vectors with binary vectors; binary systems do not require recombination with the Ti plasmid because vir genes act in trans. Final Answer: between vector plasmid and Ti-plasmid

Question 9

Physical DNA delivery — The microprojectile (gene gun) method introduces DNA into plant cells by what means?

Accepted Answer

Introduction / Context: Biolistics, also known as the microprojectile method or gene gun, is a widely used physical technique for plant transformation, especially when cell walls or species traits make Agrobacterium methods inefficient. Understanding the principle clarifies when and why to choose this method. Given Data / Assumptions: DNA can be precipitated onto dense microcarriers (gold or tungsten). High-velocity acceleration enables particles to cross cell walls and membranes. Delivery can target many cells or tissues simultaneously. Concept / Approach: In biolistics, DNA-coated particles are accelerated using pressurized helium, gunpowder charges, or spring-driven devices. The particles penetrate tissues, releasing DNA inside cells where it can integrate or be transiently expressed. This is fundamentally a ballistic (kinetic) delivery mechanism rather than micromanipulation with needles or electric field–mediated pore formation. Step-by-Step Solution: Prepare microcarriers coated with plasmid DNA.Accelerate particles toward target tissue using a gene gun.Particles traverse cell walls/membranes, depositing DNA intracellularly.Regenerate transformed cells on selective media. Verification / Alternative check: Fluorescent reporter assays (e.g., GUS, GFP) show rapid transient expression post-bombardment, confirming ballistic entry. Why Other Options Are Wrong: Micromanipulator/needles: describe microinjection, not biolistics, and are low-throughput. Electroporation of intact leaves without particles is a different technique and less common for whole tissues. “Any of the above” conflates distinct methods. Common Pitfalls: Using the term “bolistics” colloquially; the standard term is “ballistics/biolistics,” but the concept is the same: kinetic delivery with microprojectiles. Final Answer: with the help of bolistics

Question 10

Plasmid tools — In Agrobacterium-mediated plant transformation workflows, which statement is NOT true about helper plasmids?

Accepted Answer

Introduction / Context: Helper plasmids are central to both co-integrative and binary vector systems used with Agrobacterium. Distinguishing their real properties from misconceptions helps avoid design errors in cloning and conjugation steps. Given Data / Assumptions: Helper plasmids typically carry transfer (tra) functions and sometimes vir genes. They replicate in Agrobacterium and can mediate mobilization of non-self-transmissible vectors. Intermediate vectors may be conjugation-deficient and need a helper. Concept / Approach: By definition, helper plasmids are compatible replicons in Agrobacterium that provide necessary functions for conjugation and/or virulence in trans. Therefore, the claim that they cannot replicate in Agrobacterium is incorrect. Step-by-Step Solution: Identify required traits: replication in Agrobacterium and provision of transfer functions.Intermediate vectors lacking tra genes require helper plasmids to mobilize into Agrobacterium.Hence, statements about replication and conjugation assistance are true; the statement that they cannot replicate is false. Verification / Alternative check: Widely used helpers like pRK2013 function in Agrobacterium, enabling triparental mating and plasmid mobilization. Why Other Options Are Wrong: Replicate in Agrobacterium: correct. Mediate conjugation: correct. Provide vir in trans (in some systems): correct. “All of the above”: cannot be correct because one choice is false. Common Pitfalls: Confusing helper plasmids with E. coli-only cloning plasmids that lack replication origins compatible with Agrobacterium. Final Answer: These can't replicate in Agrobacterium

Question 11

Conjugation support — Intermediate vectors carrying T-DNA are often conjugation-deficient. Which helper plasmid is classically used to mediate conjugation into Agrobacterium?

Accepted Answer

Introduction / Context: Triparental mating is a classical way to move plasmids from E. coli into Agrobacterium. A mobilizable but non-self-transmissible plasmid (the intermediate or binary vector) needs transfer functions supplied in trans by a helper plasmid. Given Data / Assumptions: Intermediate vectors often lack tra genes. Helper plasmids provide mob/tra functions. Efficient conjugation is routine in plant transformation pipelines. Concept / Approach: pRK2013 is a widely used helper plasmid that carries transfer genes allowing mobilization of vectors bearing an origin of transfer (oriT). In a triparental setup, donor (vector), helper (pRK2013), and recipient (Agrobacterium) strains are mixed, enabling conjugative transfer of the vector into Agrobacterium. Step-by-Step Solution: Identify the need: intermediate vector lacks tra.Select helper with tra functions compatible with both hosts.Implement triparental mating, mobilizing the vector into Agrobacterium.Thus, pRK2013 is the correct helper plasmid. Verification / Alternative check: Method sections in plant transformation protocols consistently cite pRK2013 as the standard helper for conjugation. Why Other Options Are Wrong: pRN 3: not a standard helper plasmid in this context. “Either (a) or (b)” fails because only (a) is correct. “None of these” and pUC19: pUC19 is a cloning vector without tra functions. Common Pitfalls: Assuming any plasmid can act as a helper; transfer requires specific tra/mob machinery and oriT compatibility. Final Answer: pRK 2013

Question 12

Microinjection basics — Which description best captures a defining characteristic of DNA microinjection into eukaryotic cells?

Accepted Answer

Introduction / Context: Microinjection delivers nucleic acids directly into individual cells using a glass micropipette under a micromanipulator. The method is precise but technically demanding and is most practical for relatively large target cells (e.g., oocytes, zygotes, or large cultured cells). Given Data / Assumptions: Cell size constrains micropipette positioning and survival after injection. Typical DNA volumes are small (picoliters), not “large amounts.” The micropipette tip must be much smaller than the cell diameter to avoid catastrophic damage. Concept / Approach: Because microinjection requires visual targeting and controlled penetration of the cell membrane (and sometimes the nuclear envelope), the method is largely limited to larger cells where manipulation is feasible. This practical limitation is the key characteristic in many exam-style questions, distinguishing microinjection from high-throughput methods like biolistics or electroporation. Step-by-Step Solution: Evaluate each statement against standard practice.Volume: amounts are tiny; “large amount of DNA” is misleading.Needle size: pipette tips are smaller—not larger—than the cell diameter.Cell size: the technique is indeed most feasible in larger cells.Thus, among the choices provided, “injection of DNA into bigger cells” best captures the defining characteristic. Verification / Alternative check: Microinjection protocols in developmental biology routinely use frog or fish oocytes/embryos because of their large size and resilience to micromanipulation. Why Other Options Are Wrong: Large DNA amounts: incorrect; microinjection uses minute volumes. Needle larger than cell: impossible and destructive. All of the above: cannot be true since two statements are false. The detailed nucleus-targeting statement is accurate in practice but was not among the original legacy answer keys; in the given set, the canonical distinguishing point is cell size. Common Pitfalls: Equating microinjection with high-throughput transformation; it is precise but low-throughput and size-limited. Final Answer: injection of DNA into bigger cells

Question 13

T-DNA insertion pattern — Which statement about the genomic integration of T-DNA into plant nuclear DNA following Agrobacterium-mediated transfer is correct?

Accepted Answer

Introduction / Context: Agrobacterium-mediated transformation delivers T-DNA into the plant nucleus, where it integrates into the host genome. Understanding the insertion pattern is essential for interpreting position effects, gene disruption risks, and transgene expression variability in transformants. Given Data / Assumptions: T-DNA border sequences and Vir proteins mediate DNA transfer and integration. Host repair pathways (e.g., non-homologous end joining) are involved in integration. Insertions can be single or multiple, and orientation can vary. Concept / Approach: Empirical mapping of insertion sites in numerous transformants shows a quasi-random distribution across the nuclear genome, with certain biases (e.g., towards gene-rich regions) depending on species and protocol. There is no single, fixed locus for T-DNA insertion under standard methods. Step-by-Step Solution: Recognize that integration is primarily host-repair mediated rather than homology-directed to a specific locus.Assess population data: independent events map to many distinct loci.Therefore, insertions occur at many different, apparently random sites. Verification / Alternative check: T-DNA flanking sequence recovery across large transformant populations (TAIL-PCR, genome walking) reveals broad genomic distributions with no single mandatory target site. Why Other Options Are Wrong: Single or two specific sites: contradicted by extensive mapping data. “One site that may be random”: self-contradictory and incorrect in practice; multiple sites are observed across events. Chloroplast integration: standard Agrobacterium pathways target the nucleus; plastid transformation uses different methods. Common Pitfalls: Assuming precise targeting without engineered homology; site-specific integration requires specialized tools (e.g., CRISPR HDR), not standard Agrobacterium. Final Answer: Integration of T DNA can occur at many different, apparently random, sites in the plant nuclear DNA

Question 14

Agrobacterium tumefaciens (Ti plasmid) — Virulent strains carry large tumor-inducing (Ti) plasmids that mediate DNA transfer to plant cells. How many distinct border sequence sets required for gene transfer are present on a classical Ti plasmid (i.e…

Accepted Answer

Introduction / Context: Agrobacterium tumefaciens transfers a defined DNA segment, called transfer DNA (T-DNA), from its Ti plasmid into the nuclear genome of wounded plant cells. This process depends on short, specific border sequences that mark the left and right ends of the T-DNA to be mobilized. Understanding the number and role of these borders is foundational in plant genetic engineering and binary vector design. Given Data / Assumptions: A Ti plasmid harbors a T-DNA region destined for transfer. Transfer requires precise recognition sites (border repeats) by the Vir protein machinery. Only DNA between the borders is transferred and integrated. Concept / Approach: The essential cis-acting signals for T-DNA excision and transfer are two short, largely imperfect direct repeats known as the left border (LB) and right border (RB). VirD1/VirD2 endonuclease complex nicks these borders to generate the transferable single-stranded T-strand. Therefore, there are two sets of required border sequences per T-DNA: one at each end. Step-by-Step Solution: Identify the cis signals → left and right border repeats.Relate to mechanism → VirD1/VirD2 nick at borders to form T-strand.Infer count → two border sequence sets per T-DNA. Verification / Alternative check: Binary vector systems retain LB and RB around cargo genes; removing either border abolishes transfer, confirming the requirement for two borders flanking the insert. Why Other Options Are Wrong: One set: insufficient; T-DNA must be delimited at both ends.Three or four sets: not part of the canonical architecture.Sequence-independent: false; transfer is border-dependent. Common Pitfalls: Confusing vir gene locations (in trans) with cis borders; vir genes are not transferred, while the bordered segment is. Final Answer: Two sets of sequences necessary for gene transfer.

Question 15

Cloning strategy — Because native Ti plasmids are very large, intermediate (shuttle) vectors were developed by subcloning T-DNA into which commonly used plasmid backbone for E. coli propagation?

Accepted Answer

Introduction / Context: Early Ti plasmids were too large and unwieldy for standard cloning in E. coli. The solution was to construct intermediate vectors that carry T-DNA regions within small, well-characterized bacterial plasmids, enabling routine cloning, selection, and amplification before plant transformation via Agrobacterium. Given Data / Assumptions: Goal: manipulate T-DNA cargo in E. coli first, then deliver it into plants. pBR322 is a classic, small, multi-copy plasmid with amplicon, ori, and selectable markers. pRK2013 is typically a mobilization/helper plasmid, not the standard cloning backbone for subcloning T-DNA cassettes. Concept / Approach: Intermediate vectors (and later, binary vectors) utilize pBR322-derived backbones for stability in E. coli. These constructs are then introduced into Agrobacterium along with vir functions (on the same plasmid or a helper), allowing T-DNA transfer to plants. Step-by-Step Solution: Identify common small plasmid used for cloning → pBR322 derivatives.Differentiate helper vs. cloning plasmids → pRK2013 provides transfer functions but is not the typical subcloning backbone.Select pBR322-based vector as correct. Verification / Alternative check: Standard binary vectors (e.g., pBIN19 lineage) trace design features to pBR322-like origins of replication and antibiotic markers for E. coli growth. Why Other Options Are Wrong: b) pRK2013 is a helper/mobilizer; not the canonical cloning backbone.c) pRN3 is not the common historical backbone here.d,e) “All/None” contradict the specific, well-documented use of pBR322-based vectors. Common Pitfalls: Assuming any conjugal helper plasmid is also the cloning vector; roles are distinct in Agrobacterium systems. Final Answer: pBR322-based plasmid vector.

Question 16

Vir gene induction (tobacco) — Which plant phenolic signal molecules regulate expression of virB, virC, virD, and virE in Agrobacterium during infection of tobacco?

Accepted Answer

Introduction / Context: Agrobacterium senses plant wound signals to activate its virulence regulon. Phenolic compounds exuded at wound sites bind the VirA/VirG two-component system, upregulating operons such as virB–virE that are essential for T-DNA processing and transfer. Given Data / Assumptions: Tobacco and many dicots release phenolics upon wounding. Acetosyringone is the classic inducer of Agrobacterium vir genes in lab protocols. Alpha-hydroxy-syringone is also an effective inducer in species like tobacco. Concept / Approach: Vir gene activation is not restricted to a single phenolic. Multiple related syringyl compounds can trigger VirA autophosphorylation, which then activates VirG to turn on the downstream vir operons. Therefore, both listed molecules act as regulators/inducers in this context. Step-by-Step Solution: Identify known vir inducers → acetosyringone and related phenolics.Connect to VirA/VirG signaling → transcription of virB–virE.Conclude both compounds are active → choose “Both”. Verification / Alternative check: Plant transformation media often include acetosyringone to enhance T-DNA delivery; literature shows α-hydroxy-syringone is active especially in tobacco systems. Why Other Options Are Wrong: a or b alone: incomplete.d) False; at least these phenolics do regulate expression.e) Salicylic acid is more associated with defense signaling and can inhibit Agrobacterium infection. Common Pitfalls: Assuming a single universal inducer; induction spectrum varies with plant species and Agrobacterium strain. Final Answer: Both (a) and (b).

Question 17

DNA delivery by microinjection — Which statement is NOT true about microinjecting DNA into plant protoplasts?

Accepted Answer

Introduction / Context: Microinjection is a direct physical method for introducing DNA, RNA, or proteins into individual plant cells or protoplasts. The technique requires precision instrumentation and gentle handling to avoid lysis, especially because protoplasts lack a cell wall and are osmotically fragile. Given Data / Assumptions: Protoplasts are spherical, wall-less cells enclosed only by a plasma membrane. Micromanipulators and microcapillaries are standard equipment. Cells must be stabilized during injection to prevent movement and damage. Concept / Approach: The microinjection needle must be substantially finer than the target cell to penetrate the membrane with minimal disruption. A needle larger than the cell diameter would catastrophically damage the cell. Therefore, the statement claiming a larger-than-cell needle is used is not true. Step-by-Step Solution: Confirm core setup → micromanipulator and microcapillary are correct.Confirm immobilization → true for positional control.Evaluate needle size → must be smaller than the cell; a larger needle is incorrect → choose option (c) as NOT true. Verification / Alternative check: Protocols specify tip diameters in the submicron to a few microns range, far below typical protoplast diameters (20–50 microns). Why Other Options Are Wrong: a,b) Both accurately describe the procedure.d) Cannot be true because (c) is false.e) Reaffirms correct practice (fine needle smaller than cell diameter). Common Pitfalls: Using excessive injection pressure or a too-wide tip causes lysis; osmotic stabilization and gentle manipulation are essential. Final Answer: It employs a needle with a diameter greater than the cell diameter.

Question 18

Nature of T-DNA integration — In plant tumor (crown gall) cells induced by Agrobacterium, what portion of the Ti plasmid is stably integrated into plant nuclear DNA?

Accepted Answer

Introduction / Context: Crown gall formation is driven by the stable integration and expression of T-DNA genes in the plant genome. Distinguishing between transferred cargo (T-DNA) and bacterial machinery (vir region) is central to understanding and applying Agrobacterium-mediated transformation in plant biotechnology. Given Data / Assumptions: T-DNA is delimited by left and right border repeats on the Ti plasmid. Vir genes act in trans to process and transfer T-DNA but are not themselves transferred. Plant tumor traits arise from T-DNA oncogenes and opine synthase genes. Concept / Approach: Only the DNA between the LB and RB is nicked, generated as a T-strand, and transferred to the plant nucleus for integration. The rest of the Ti plasmid remains in the bacterium. Hence, the correct statement is that only a specific, small T-DNA segment integrates. Step-by-Step Solution: Identify borders → define the transferable segment.Relate vir function → acts in bacterium, not transferred.Select the option specifying “only the T-DNA segment”. Verification / Alternative check: Binary vectors confirm that any gene placed between LB and RB can be transferred and integrated; sequences outside borders are not transferred. Why Other Options Are Wrong: a,b,d) Contradict the border-defined mechanism.e) vir genes remain bacterial; they do not integrate into plant DNA under normal processes. Common Pitfalls: Assuming plasmid replication equals transfer; transfer is targeted and border-dependent. Final Answer: Only a small, specific T-DNA segment bounded by left/right borders.

Gene Transfer in Plants Questions