Q.The bacterium which causes a plant disease called crown gall is ................

pBR322 is a classic engineered bacterial plasmid used as a cloning vector; the other options are restriction enzymes or a different type of vector.

• EcoRI and AluI are restriction endonucleases (molecular scissors that cut DNA at specific recognition sequences), not plasmids.

• BAC (Bacterial Artificial Chromosome) is a cloning vector, but it is derived from the bacterial F-plasmid and used for cloning very large DNA fragments — it is not the standard small circular plasmid referred to here.

• pBR322 is a classic, widely used small circular double-stranded bacterial plasmid engineered as a cloning vector, carrying an origin of replication, selectable antibiotic-resistance marker genes (ampR, tetR), and unique restriction sites for inserting foreign DNA.

Agrobacterium tumefaciens is nature's own genetic engineer, widely used as a vector to introduce foreign genes into plant cells.

Agrobacterium tumefaciens is a soil bacterium that naturally infects wounded plant tissue and causes crown gall disease by transferring a segment of its Ti (tumour-inducing) plasmid, called T-DNA, into the plant genome.

• Biotechnologists exploit this natural gene-transfer ability: the tumour-causing genes are removed from the Ti plasmid (disarmed) and replaced with the gene of interest, which is then delivered into the plant cell and integrates into its chromosome.

• This makes Agrobacterium tumefaciens the most widely used vector for creating transgenic (dicot) plants.

• Bacillus thuringiensis supplies the Bt toxin gene (a source gene, not a vector); E. coli is a standard cloning/expression host for recombinant DNA work but not typically used to transform plant cells directly.

The Ti (tumour-inducing) plasmid is naturally found in the soil bacterium Agrobacterium tumefaciens and, disarmed of its tumour-causing genes, is widely used as a vector to introduce foreign genes into plant cells.

Agrobacterium tumefaciens is a plant-pathogenic bacterium that naturally causes crown gall disease by transferring a segment of its Ti (tumour-inducing) plasmid, called T-DNA, into the genome of the host plant cell, inducing tumour formation. Genetic engineers exploit this natural gene-transfer ability:

• The tumour-inducing genes on the Ti plasmid are removed (disarmed), and the gene of interest is inserted into the T-DNA region instead.
• The disarmed Ti plasmid then acts as a natural vector to deliver the desired gene into plant cells, which is the basis of many transgenic plant technologies (e.g., Bt cotton uses Agrobacterium-mediated transformation in some protocols).

Of the given options, Bacillus thuringiensis is the source of the Bt (cry) toxin genes (not the Ti plasmid), E. coli is the classic bacterial host for cloning (source of plasmids like pBR322), and Mycobacterium bovis is used to make the BCG vaccine — none of these carry the Ti plasmid.

Agrobacterium tumefaciens causes crown gall disease in plants and is also exploited as a natural genetic-engineering tool.

Agrobacterium tumefaciens is a soil-dwelling bacterium that naturally infects wounded plant tissue (typically near the crown/root-stem junction) and transfers a segment of its own Ti (tumour-inducing) plasmid DNA — the T-DNA — into the plant cell's genome. Genes carried on this T-DNA cause uncontrolled proliferation of the infected plant cells, producing a characteristic tumour-like swelling called a crown gall. Because of its natural ability to transfer DNA into plant genomes, Agrobacterium's Ti plasmid (with its tumour-causing genes removed and replaced by a gene of interest) is widely used in biotechnology as a vector for genetically engineering plants.

Plasmids are small circular extrachromosomal bacterial DNA molecules that are the most common vectors used to carry and clone foreign genes.

Among the options, a plasmid — a small, circular, self-replicating extra-chromosomal DNA molecule found in bacteria — is the structure most directly connected with genetic engineering. Plasmids like pBR322 are widely used as cloning vectors: the gene of interest is inserted into the plasmid (using restriction enzymes and DNA ligase) to form recombinant DNA, which is then introduced into a bacterial host cell for cloning/expression. Ribosomes, mitochondria and Golgi bodies are normal cell organelles involved in protein synthesis, energy production, and packaging respectively, and are not used as vectors in genetic engineering.

In pBR322, "BR" honours the plasmid's creators, Bolivar and Rodriguez.

pBR322 is a classic E. coli cloning vector constructed in 1977 by Francisco Bolivar and Raymond Rodriguez, and its name follows a common plasmid-naming convention: the lower-case "p" denotes plasmid, "BR" commemorates the initials of its two creators (Bolivar and Rodriguez), and "322" is simply a laboratory reference/serial number distinguishing it from other constructs made in the same lab. pBR322 carries an origin of replication and two antibiotic-resistance genes (for ampicillin and tetracycline resistance) that allow researchers to select for bacteria that have taken up the plasmid, along with unique restriction sites within these resistance genes that allow insertional inactivation to be used to screen for successful gene insertion -- features that made it one of the foundational vectors of early recombinant DNA technology. "BR" is therefore neither a description of bacterial recombination or bacterial replication in general, nor a reference to the virologist David Baltimore (who discovered reverse transcriptase but had no role in constructing this plasmid).

Plasmids are extra-chromosomal bacterial DNA, useful naturally and as cloning vectors.

In addition to their single main circular chromosome, many bacteria carry one or more small, circular, double-stranded DNA molecules called plasmids, which replicate independently of the main chromosome. Plasmids often carry accessory genes that are not essential for normal bacterial growth but can confer important survival advantages — most notably, genes for resistance to antibiotics or heavy metals, and genes (in F-plasmids) enabling conjugation, the transfer of DNA between bacterial cells. Because plasmids are small, easy to isolate, and can carry and replicate foreign DNA fragments inserted into them, they are extensively exploited in biotechnology as cloning vectors to introduce and multiply desired genes inside host bacterial cells (recombinant DNA technology).

pBR322 is a classic engineered plasmid cloning vector — the answer is (a).

pBR322 is a well-known artificial cloning vector. It is a small circular double-stranded DNA plasmid derived from E. coli, engineered to carry:

• an origin of replication (ori) so it can multiply inside the host,
• two antibiotic-resistance selectable markers (ampicillin resistance, ampᴿ, and tetracycline resistance, tetᴿ), and
• several unique restriction sites for inserting foreign DNA.

Because it is a plasmid (not a virus/bacteriophage, not a cosmid, not an artificial chromosome), the correct answer is (a).