Q.What is Plasmid ?
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Cloning Vectors and Selectable Markers (pBR322)
A cloning vector is a DNA molecule that can carry a foreign DNA fragment into a host cell and replicate there. Plasmids such as pBR322, an early and widely used E. coli cloning vector, are drawn as circular DNA maps and have several essential features that a good vector must possess.
- Origin of replication (ori): the sequence at which replication starts; it controls copy number (how many copies of the plasmid are made per cell). In pBR322 the rop gene near the ori helps regulate this copy number.
- Selectable markers: genes that allow cells carrying the vector to be distinguished from those that do not, usually by giving antibiotic resistance. pBR322 carries two such genes — ampicillin resistance (ampR) and tetracycline resistance (tetR). …
Bacteria carry a small extra loop of DNA separate from their main chromosome that can copy itself independently, a property that makes it especially useful as a tool in genetic engineering. …
Plasmids are self-replicating, circular extra-chromosomal bacterial DNA, used as gene-carrying vectors in recombinant DNA technology.
A plasmid is a small, circular, double-stranded DNA molecule that occurs naturally in the cytoplasm of bacterial cells (and in some yeasts), separate from the main bacterial (nucleoid) chromosome.
Key features:
- It carries its own origin of replication (ori), so it can replicate autonomously, independent of the chromosomal DNA, using the host cell's replication machinery.
- It is usually non-essential for normal bacterial survival but may carry genes that give the host an advantage, e.g. antibiotic resistance (R-plasmids) or the ability to degrade unusual compounds.
- Plasmids can be transferred between bacterial cells (e.g. by conjugation), spreading useful traits such as antibiotic resistance. …
Showing the 12 most recent of 25 on this concept.
- CBSE 2026Set A1 markMCQQ.Which of the following is the sequence from where replication starts in cloning vector?(a) tet^R(b) amp^R(c) ori(d) more than one
›Reveal solutionSolution
ori is the origin of replication; the correct option is (c).
A cloning vector must be able to replicate inside the host. The 'ori' (origin of replication) is the specific DNA sequence from which replication begins; any DNA linked to it can be made to replicate within the host cell. The ori also controls the copy number of the linked DNA.
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- CBSE 2025Set ANNUAL1 markMCQQ.Which one of the following is a genetic vector?(a) Phage(b) Cosmid(c) Plasmid(d) Artificial chromosome
›Reveal solutionSolution
Plasmid is the most fundamental cloning/genetic vector; it is engineered with an origin of replication, selectable marker, and cloning site to carry foreign DNA into a host cell.
A cloning/genetic vector is a DNA molecule used as a vehicle to artificially carry a foreign DNA segment into a host cell, where it can be replicated and/or expressed. Plasmids are small, circular, double-stranded, extra-chromosomal DNA molecules found naturally in bacteria that replicate independently of the main bacterial chromosome. Engineered plasmids (e.g. pBR322) are the workhorse cloning vectors — they carry an origin of replication (ori), one or more selectable marker genes (e.g. antibiotic resistance), and unique restriction sites where foreign DNA is inserted.
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- CBSE 2025Set ANNUAL1 markQ.Why is it essential to have selectable marker in cloning vector?
›Reveal solutionSolution
A selectable marker (e.g. an antibiotic-resistance gene) lets us identify and select only those host cells that have successfully taken up the vector, since transformation efficiency is naturally low.
When a recombinant DNA vector is introduced into host bacterial cells (transformation), only a small fraction of cells actually take it up — most cells remain untransformed. Since transformed and untransformed cells look identical, we need a way to selectively grow only the cells carrying the vector. This is achieved using a selectable marker gene on the vector — commonly a gene conferring resistance to an antibiotic (e.g. ampicillin resistance, tetracycline resistance) that the host bacterium is not normally resistant to. After transformation, cells are plated on medium containing that antibiotic: only cells that have taken up the vector (and hence the resistance gene) survive and grow, while untransformed cells are k …
- CBSE 2024Set E1 markMCQQ.Which of the following is not a cloning vector for bacteria?(a) Bacteriophage(b) Plasmid(c) pBR322(d) T-DNA
›Reveal solutionSolution
Plasmids, bacteriophages and pBR322 are cloning vectors used in bacteria; T-DNA is the transferred DNA of the Ti plasmid used to transform plant cells.
Cloning vectors for bacteria include plasmids (option b), bacteriophages (option a) and engineered plasmids such as pBR322 (option c). T-DNA (transfer DNA) is the portion of the Ti (tumour-inducing) plasmid of Agrobacterium tumefaciens that is trans …
- CBSE 2024Set E1 markMCQQ.Which of the following is absent in pBR322?(a) Origin of replication(b) Restriction site(c) T-DNA(d) Antibiotic resistant genes
›Reveal solutionSolution
pBR322 carries ori + restriction sites + antibiotic-resistance selectable markers; T-DNA is not part of it.
pBR322 is a well-known bacterial cloning vector. Its essential features are: an origin of replication (ori) that allows it to replicate inside the host (option a), several unique restriction sites where foreign DNA can be inserted (option b), and two antibiotic-resistance genes — am …
- CBSE 2024Set E1 markMCQQ.If a foreign DNA is inserted in tetracycline resistant gene, the recombinant plasmid(a) Will lose ampicillin resistance(b) Will lose tetracycline resistance(c) Cloning would become easier(d) Ampicillin resistance will become stronger
›Reveal solutionSolution
Inserting DNA into the tetR gene disrupts it — the recombinant plasmid loses tetracycline resistance (insertional inactivation).
In pBR322, foreign DNA is often inserted at a restriction site (like BamHI) lying inside the tetracycline-resistance (tetR) gene. This insertion disrupts the gene so it can no longer function — a process called insertional inactivation. As a result, cells carrying the recombinant plasmid become sensitive to tetracycline wh …
- CBSE 2024Set D1 markQ.Match the following and write the correct pair. Column 'A':(i) Structural genes(ii) T-Lymphocytes(iii) Competition(iv) PBR322(v) Corona radiata. Column 'B':(a) bone marrow(b) vectors(c) ovum(d) detrimental interaction(e) z, y and a(f) RBCs. Find the correct match for item(iv) 'PBR322' from Column 'B'.
›Reveal solutionSolution
'PBR322' pairs with option (b) 'vectors' — it is one of the earliest and most widely used cloning vectors.
In recombinant DNA technology, a vector is a DNA molecule (commonly a plasmid or bacteriophage) used as a vehicle to artificially carry a foreign piece of DNA into a host cell, where it can be replicated and/or expressed.
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- CBSE 2024Set ZOOLOGY1 markQ.Correct the underlined portion of the sentence: The cloning vector plasmids are found in monkeys (underlined).
›Reveal solutionSolution
Plasmids used as cloning vectors are naturally occurring, self-replicating extra-chromosomal circular DNA found in bacteria (and some yeast), not in monkeys.
Plasmids are small, circular, double-stranded DNA molecules that exist independently of the bacterial chromosome and can replicate autonomously within a bacterial (or yeast) cell. Because they can carry a foreign DNA insert and replicate it along with themselves, plasmids (e. …
- CBSE 2023Set ANNUAL1 markMCQQ.Plasmids are extra-chromosomal genetic material of which of the following?(a) Viruses(b) Algae(c) Fungi(d) Bacteria
›Reveal solutionSolution
Plasmids are autonomously replicating circular DNA molecules found in bacteria, separate from the main bacterial chromosome, and they are the classic example of extra-chromosomal genetic material.
Bacterial cells (prokaryotes) contain one main circular chromosome, but many bacteria additionally carry one or more plasmids — small circular double-stranded DNA molecules that replicate independently of the chromosome. Plasmids often carry genes useful to the bacterium (e.g., antibiotic resistance) and are not essential for basic survival. Because they can be easily isolated, …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is a cloning vector?(a) Plasmid(b) RNA(c) Mesosome(d) Ribosome
›Reveal solutionSolution
A cloning vector is a DNA molecule that can carry a foreign gene into a host cell and replicate it there; plasmids are the classic and most widely used cloning vectors.
Recombinant DNA technology needs a 'vehicle' — a cloning vector — to carry a gene of interest into a host organism (commonly bacteria like E. coli) and enable its replication and, often, expression. Plasmids (e.g., pBR322) are ideal for this because they are small, circular, replicate independently of the host chromosome, and can be engineered with selectable markers (like …
- CBSE 2023Set ANNUAL1 markQ.What is a plasmid?
›Reveal solutionSolution
Plasmids are naturally occurring circular bacterial DNA molecules capable of independent replication, making them ideal vehicles (vectors) for carrying foreign genes.
A plasmid is a small, circular, double-stranded DNA molecule that exists separately from the main bacterial chromosome and can replicate independently of it, since it possesses its own origin of replication. Naturally occurring plasmids often carry genes conferring useful traits such as antibiotic resistance. In genetic engineering, plasmids (e.g., pBR322) are engineered and widely used as cloning vectors t …
- CBSE 2023Set ANNUAL1 markMCQQ.In genetic engineering, antibiotics are used(a) in selecting healthy vectors(b) as selectable markers(c) to keep the culture free from infection(d) both(a) and (c)
›Reveal solutionSolution
In genetic engineering, antibiotics are used as selectable markers.
Cloning vectors carry antibiotic-resistance genes (e.g. amp^R, tet^R in pBR322). After transformation, cells are grown on a medium containing the antibiotic: only cells that have taken up the vector (and hence the resistance gene) survive. Thus the antibiotic-resistance gene works as a selectable m …
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