(a) (i) Identify and name the structures 'A' and 'B' marked in the image given below : (ii) State their import

Question

Q. (a)

(i) Identify and name the structures 'A' and 'B' marked in the image given below :

(ii) State their importance in various biotechnology experiments.

(OR)

(b) Explain the process by which a bacterial cell can be made 'competent' to take up foreign DNA from its surroundings, using divalent cations and temperature treatment.

CBSECBSE Class XII Board 2023Subjective· 2mImportance★★★★★

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Part (a)Concept understanding — Biotechnology Core Concepts

Biotechnology Core Concepts — A First Look

You already know biotechnology better than you think. When you eat yoghurt, that's biotechnology at work. When bread rises, that's biotechnology. When your grandmother used curd to ferment buttermilk, she was practising an ancient form of it. The core idea is simple: using living organisms (or parts of them) to make or modify products for human benefit.

The NCERT textbook defines biotechnology as "the use of living systems and organisms to develop or make useful products." That's the formal version of what you just read.

The Two Big Ideas That Hold Everything Together

Biotechnology rests on two fundamental capabilities that nature gave us, and that scientists learned to harness:

1. Genetic Engineering — the ability to change an organism's DNA directly. Think of it as editing the instruction manual of a living thing. Instead of waiting for nature to produce a trait through slow breeding, scientists can now take a specific gene from one organism and put it into another. A bacterium can be made to produce human insulin because the human insulin gene has been inserted into it.

2. Maintenance of Sterile Conditions — the ability to grow large numbers of cells or microorganisms in a controlled, contamination-free environment. This is called aseptic technique. Without it, the wrong microbes would spoil the process, and you'd get garbage instead of medicine.

Important

These two — genetic engineering and sterile maintenance — are the twin pillars of modern biotechnology. The NCERT explicitly states that biotechnology deals with techniques of using live organisms or enzymes from organisms to produce products useful to humans. The modern era of biotechnology began when we could manipulate DNA directly.

Why This Matters in Everyday Life

You don't need a lab coat to see biotechnology's impact. Consider these examples:

Medicine: Insulin for diabetes, vaccines, and gene therapy all come from biotechnology. Before genetic engineering, insulin was extracted from the pancreases of pigs and cows — expensive and sometimes caused allergic reactions. Now, bacteria make human insulin for us.
Agriculture: Crops that resist pests or tolerate drought. Bt cotton, for instance, has a bacterial gene that makes it produce a protein toxic to certain insects, reducing the need for chemical pesticides.
Environment: Microbes engineered to clean up oil spills or break down plastic waste.
Food: Cheese, beer, wine, and even the citric acid in your soft drinks are products of microbial biotechnology.

Note

The NCERT classifies biotechnology into two eras: Traditional biotechnology (fermentation, breeding) which humans have used for thousands of years, and Modern biotechnology (genetic engineering, cell culture) which began in the 1970s with the discovery of tools to cut and join DNA.

The Core Tools (What Makes It Possible)

Modern biotechnology relies on a few essential techniques. You don't need to memorise details, but understand what each does:

Recombinant DNA technology: Cutting a gene from one organism and pasting it into another. This is how we make human insulin in bacteria.
Gene cloning: Making many identical copies of a gene. If you have one copy of a useful gene, you can make millions.
Tissue culture: Growing plant or animal cells in a lab dish. A whole plant can be regenerated from a single cell.
Fermentation technology: Using microbes in large tanks (bioreactors) to produce substances like antibiotics, enzymes, or alcohol.

A Common Misunderstanding to Avoid …

Part (b)Concept understanding — Recombinant DNA Technology

Imagine you have a cookbook with recipes from all over the world. Normally, you can only cook what's in one book at a time. But what if you could cut out the best recipe from one book and paste it into another, so your new book has a dish that no single cuisine ever had before? That is the basic idea behind Recombinant DNA Technology.

At its simplest, this technology is a way to take a piece of DNA (the genetic instruction manual) from one organism and join it with the DNA of a completely different organism. The result is a new, "recombinant" DNA molecule — a hybrid that never existed in nature. Think of it as genetic tailoring: you cut a gene from a human, a bacterium, or a plant, and stitch it into the DNA of another organism, often a bacterium or yeast. That host organism then reads the new instructions and starts producing the protein the inserted gene codes for.

Why does this matter? Because it lets us manufacture things that living organisms naturally make, but in a controlled, large-scale way. For example, the human insulin gene can be inserted into E. coli bacteria. These bacteria then become tiny factories, churning out human insulin that can be purified and given to diabetic patients. Before this technology, insulin had to be extracted from the pancreases of cows and pigs — a slow, expensive, and sometimes allergenic process.

The NCERT textbook (Class 12 Biology, Chapter 11) defines it precisely: Recombinant DNA Technology is the technique of joining DNA from two different species and inserting it into a host organism to produce a new genetic combination. The textbook highlights three key tools that make this possible:

Restriction Enzymes – These are the "molecular scissors" that cut DNA at specific, predictable points. They allow scientists to cut out a desired gene cleanly.
Vectors – These are the "delivery vehicles," usually plasmids (small circular DNA in bacteria) or viruses, that carry the foreign DNA into the host cell.
Host Organisms – The living factory (like bacteria, yeast, or plant cells) that will replicate the recombinant DNA and produce the desired protein.

Important

The core principle is genetic recombination — creating a DNA molecule that contains sequences from two or more different sources. This is not the same as natural reproduction or mutation; it is a deliberate, laboratory-made hybrid.

The process itself follows a clear sequence:

Isolation of the desired gene (say, the human insulin gene) from the donor organism's DNA.
Cutting both the gene and the vector DNA with the same restriction enzyme, creating matching "sticky ends."
Ligation — using an enzyme called DNA ligase to permanently join the gene and the vector, forming the recombinant DNA.
Transformation — inserting this recombinant DNA into a host cell (like a bacterium).
Selection — identifying and growing only those host cells that successfully took up the recombinant DNA.
Expression — getting the host cells to produce the desired protein in large quantities.

Note

A common confusion is thinking this technology creates "new life." It does not. It creates a new genetic combination inside an existing living cell. The host organism remains the same species, but it now carries an extra instruction — like a factory that gets a new blueprint for a product it never made before. …

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Part (a): A = plasmid and B = bacteriophage; both are used as cloning vectors — the plasmid for smaller inserts (replicates via its ori, carries selectable markers) and the bacteriophage for larger DNA fragments and efficient transfer into bacteria. Part (b): a bacterial cell is made competent with ice-cold CaCl₂ (divalent cations) plus a brief ~42 °C heat shock, which lets the DNA enter.

Part (a)

Identifying the structures. The two structures shown are the two principal kinds of cloning vector. A is a plasmid — a small, circular, double-stranded, extrachromosomal DNA that exists and replicates independently of the main chromosome. B is a bacteriophage (phage) — a virus that infects bacteria, recognisable from its polyhedral (head) capsid enclosing the nucleic acid, a tail with a contractile sheath, a base plate and tail fibres.
Their importance in various biotechnology experiments. Both are exploited as cloning vectors — vehicles that carry a foreign gene of interest into a host cell and replicate it there.

Plasmid (A): foreign DNA is ligated into it; it has an origin of replication (ori) so it multiplies inside the host to give many copies of the insert, and it carries selectable markers (such as antibiotic-resistance genes) that allow transformed cells to be identified. It is the standard vehicle for cloning relatively small DNA fragments. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 54 on this concept.

CBSE 2026Set 57/1/11 markMCQ
$Q. Which of the following statements about plasmids is incorrect ? (A) Plasmids have the ability to replicate within the bacterial cell. (B) Their replication is controlled by chromosomal DNA. (C) They are autonomously replicating circular extra-chromosomal DNA. (D) They often carry antibiotic resistant genes.$
›Reveal solutionSolution
The incorrect statement about plasmids is that their replication is controlled by chromosomal DNA; plasmids replicate autonomously.

Plasmids are fascinating and crucial components in the world of microbiology and biotechnology. They are essentially small, circular, extra-chromosomal DNA molecules found predominantly in bacteria, but also in some eukaryotes like yeast. Think of them as accessory genetic units that carry non-essential but often beneficial genes for the host cell.

Let's break down the given statements to understand which one is incorrect:

Statement (A): Plasmids have the ability to replicate within the bacterial cell.

This statement is correct. A defining feature of plasmids is their ability to self-replicate. They possess their own origin of replication (ori) sequence, which allows them to initiate DNA synthesis independently of the main bacterial chromosome. This autonomous replication is what makes them incredibly useful as vectors in genetic engineering, as they can multiply along with the host cell, carrying the inserted foreign DNA.

Statement (B): Their replication is controlled by chromosomal DNA.

This statement is incorrect. As mentioned above, plasmids replicate autonomously. This means their replication is independent of the bacterial cell's main chromosomal DNA replication. While the host cell's machinery (enzymes, nucleotides) is used for plasmid replication, the initiation and control of plasmid replication are governed by sequences within the plasmid itself, not by the bacterial chromosome. This independence is a key distinction between plasmids and the main chromosome.

Important

The autonomous replication of plasmids, independent of the host cell's chromosomal DNA, is a fundamental characteristic that distinguishes them and makes them invaluable tools in molecular biology.

Statement (C): They are autonomously replicating circular extra-chromosomal DNA.

This statement is correct. This is a precise definition of a plasmid.

Autonomously replicating: They have their own origin of replication and can replicate independently.

Circular: Most plasmids are found in a closed circular form. …
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CBSE 2026Set 57/2/11 markMCQ
$Q. Assertion (A) : DNA Ligase is used to join DNA fragments. Reason (R) : It catalyses the formation of glycosidic bonds between nucleotides. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not correct explanation for Assertion (A). (C) Assertion (A) is true, Reason (R) is false. (D) Assertion (A) is false, Reason (R) is true.$
›Reveal solutionSolution
DNA Ligase joins DNA fragments by catalysing phosphodiester bonds, not glycosidic bonds. So Assertion is true, Reason is false — option (C).

The question tests a very specific piece of molecular biology: what bond does DNA Ligase actually form? Many students memorise "DNA Ligase joins fragments" but forget the exact chemistry. Let's break it down cleanly.

Why the Assertion is correct

DNA Ligase is the enzyme used in DNA replication, repair, and genetic engineering to seal nicks in the DNA backbone. It joins two DNA fragments — for example, an Okazaki fragment to the growing lagging strand, or an insert into a plasmid vector. This is a standard fact in biotechnology and molecular biology.

Why the Reason is wrong

The Reason claims DNA Ligase catalyses the formation of glycosidic bonds between nucleotides. That is a critical error.

A glycosidic bond links a sugar (deoxyribose) to a nitrogenous base (A, T, G, C) — that bond is formed during nucleotide synthesis, not by DNA Ligase.

DNA Ligase catalyses the formation of a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the next nucleotide. This is the bond that holds the sugar-phosphate backbone together.

Watch out

A common mistake is confusing "joining nucleotides" with "forming the bond between sugar and base." The bond between nucleotides in a DNA strand is always phosphodiester, never glycosidic.

Step-by-step reasoning

Evaluate Assertion (A): "DNA Ligase is used to join DNA fragments." …
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CBSE 2026Set 57/3/11 markMCQ
$Q. Match Column I with Column II and select the correct option : Column I a. Primers b. Insertional inactivation c. Bioreactor d. Downstream processing Column II i. β-galactosidase ii. To separate and purify products iii. Chemically synthesized oligonucleotides iv. Large scale production of specific product of activity of microbes Options : (A) a-iii, b-i, c-iv, d-ii (B) a-iii, b-i, c-ii, d-iv (C) a-i, b-iii, c-iv, d-ii (D) a-i, b-iv, c-ii, d-iii$
›Reveal solutionSolution

Primers are chemically synthesized oligonucleotides; insertional inactivation uses β-galactosidase; bioreactors enable large-scale microbial production; downstream processing separates and purifies products.

The question tests your understanding of four fundamental tools and processes in biotechnology, each playing a distinct role in genetic engineering and industrial production. Let's work through what each term means and why it matches its partner.

Primers are short stretches of nucleotides—typically 10 to 20 bases long—that are chemically synthesized in the laboratory. They are oligonucleotides, meaning "few nucleotides." In PCR (polymerase chain reaction), primers bind to specific sequences on the DNA template and provide the starting point for DNA polymerase to begin synthesis. Without primers, the enzyme cannot initiate replication. The NCERT textbook explicitly describes primers as chemically synthesized oligonucleotides used in amplification techniques. So a matches iii.

Insertional inactivation is a clever selection technique used to identify recombinant DNA. The principle relies on disrupting a functional gene when foreign DNA is inserted into a plasmid vector. The classic example involves the lacZ gene, which codes for the enzyme β-galactosidase. When a foreign DNA fragment is successfully inserted into the cloning site within lacZ, the gene is inactivated—the enzyme is no longer produced. Colonies containing recombinant plasmids remain white when grown on medium with a chromogenic substrate (like X-gal), while non-recombinant colonies turn blue because their intact β-galactosidase cleaves the substrate. This visual distinction makes screening straightforward. Thus b matches i.

Note

The blue-white screening method is one of the most widely used techniques in molecular cloning labs because it provides immediate visual confirmation of successful insertion.

Bioreactors are large vessels designed for the controlled, large-scale cultivation of microorganisms, plant cells, or animal cells. They maintain optimal conditions—temperature, pH, oxygen supply, nutrient availability—to maximize the production of desired products like enzymes, antibiotics, vaccines, or recombinant proteins. The NCERT textbook describes bioreactors as systems that provide the ideal environment for growing cultures in volumes ranging from 100 to 1000 liters or more. The entire purpose is large-scale production of a specific product through microbial or cellular activity. Therefore c matches iv. …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Given below are the steps carried out to construct a recombinant DNA.$
(i) Isolation of genetic material
(ii) Insertion of recombinant DNA in the host cell /organism
(iii) Obtaining the foreign gene product
(iv) Amplification of gene of interest
(v) Downstream processing. Which one of the following gives the correct sequences of these steps?
(a)
(i)
(iii)
(iv)
(ii)
(v)
(b)
(i)
(iv)
(ii)
(iii)
(v)
(c)
(ii)
(i)
(iii)
(iv)
(v)
(d)
(ii)
(iv)
(v)
(iii) (i)
›Reveal solutionSolution
Constructing and using recombinant DNA follows a fixed logical sequence: first isolate and amplify the gene of interest, then insert the recombinant DNA into a host, let the host express the gene product, and finally process that product for use — i.e., (i) → (iv) → (ii) → (iii) → (v).

Recombinant DNA technology (genetic engineering) involves several sequential processes:

(i) Isolation of genetic material (DNA) — the DNA must first be extracted in a pure form from the source cell, free of other macromolecules.

(iv) Amplification of the gene of interest — using restriction enzymes to cut out the desired gene and, typically, PCR to amplify it, then ligating it into a suitable vector to form the recombinant DNA.

(ii) Insertion of the recombinant DNA into the host cell/organism — the recombinant DNA (vector + gene of interest) is introduced into a competent host cell (transformation) and the host is then cultured/multiplied so the gene is expressed. …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Which is not a step of PCR cycle?$
(a) Denaturation
(b) Primers Annealing
(c) Extension of primers
(d) Identification of DNA with genes
›Reveal solutionSolution
A PCR cycle has three steps: denaturation, annealing and extension; identifying DNA is not one of them.

The Polymerase Chain Reaction (PCR) amplifies a specific DNA segment through repeated cycles, each with three steps:

Denaturation: heating (about 94-95 C) separates the double-stranded DNA into single strands.

Annealing: cooling allows two primers to bind (anneal) to their complementary sequences flanking the target. …
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CBSE 2026Set ANNUAL1 mark
$Q. Write answer in one word/sentence: Which type of charge found on DNA fragments?$
›Reveal solutionSolution

DNA fragments carry a negative charge because of their phosphate backbone.

The backbone of a DNA molecule is made of alternating sugar and phosphate groups, and the phosphate groups carry negatively charged oxygen atoms. As a result, DNA fragments are negatively charged. This is the basis of gel electrophoresis: when an electric field is applied, the negati …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The enzyme required for isolating DNA from fungi is -$
(a) Cellulase
(b) Chitinase
(c) Lysozyme
(d) Endonuclease
›Reveal solutionSolution
Fungal cell walls are made of chitin, so the enzyme chitinase is used to break them open and release the DNA.

The first step in isolating genetic material (DNA) is to break open the cell and its wall so that the DNA is released along with other macromolecules. Because different organisms have different cell-wall chemistry, different enzymes are used:

Bacteria → lysozyme (digests peptidoglycan) …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Assertion : Ligase enzyme is used to join a foreign gene with a cloning vector. Reason : Ligase forms phosphodiester bonds between complementary base pairs.$
(a) If both Assertion and Reason are true and Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.
(c) If Assertion is true but Reason is false.
(d) If both Assertion and Reason are false.
›Reveal solutionSolution

Assertion (ligase joins gene to vector) is true, but the Reason is worded wrongly — ligase makes phosphodiester bonds in the backbone between adjacent nucleotides, not between base pairs — so the correct choice is (C).

Evaluating the Assertion: DNA ligase is indeed the enzyme that joins a foreign DNA fragment (gene) to a cloning vector, sealing them into a recombinant DNA molecule. So the Assertion is TRUE.

…

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CBSE 2026Set ANNUAL1 mark
$Q. Give reason for moving DNA fragments towards the anode in gel electrophoresis.$
›Reveal solutionSolution

DNA is negatively charged because of its phosphate backbone, so it migrates towards the positive electrode (anode) during gel electrophoresis.

In gel electrophoresis, DNA fragments are separated according to size by forcing them through an agarose gel in an electric field. Every nucleotide of DNA carries a negatively charged phosphate group in its sugar-phosphate backbone, so the whole DNA molecule is negatively charged.

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The enzyme Taq polymerase used in PCR has been isolated from the bacterium :$
(a) Agrobacterium tumefaciens
(b) Thermus aquaticus
(c) Streptomyces albus
(d) Escherichia coli
›Reveal solutionSolution

Taq polymerase, the thermostable DNA polymerase used in PCR, comes from the hot-spring bacterium Thermus aquaticus, so (b) is correct.

In the CBSE/NCERT Biotechnology: Principles and Processes chapter, PCR (Polymerase Chain Reaction) heats DNA to ~94°C to denature it. An ordinary DNA polymerase would be destroyed at that temperature, so PCR uses a thermostable polymerase that survives repeated high-temperature cycles. This enzyme, Taq polymerase, is isolated from *Thermus aquaticus …

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CBSE 2025Set ANNUAL1 markMCQ
$Q.Match the process in column I with their uses in column II and choose the correct option: Column I \tColumn II A) ELISA \ti) Direct introduction of rDNA B) PCR \tii) Gene amplification C) Biolistic \tiii) Antigen-antibody interaction D) Micro-injection \tiv) Gold coated DNA$
(a) A(iv), B(iii), C(ii), D(i)
(b) A(i), B(ii), C(iii), D(iv)
(c) A(ii), B(i), C(iv), D(iii)
(d) A(iii), B(ii), C(iv), D(i)
›Reveal solutionSolution
ELISA uses antigen-antibody interaction, PCR amplifies genes, Biolistic uses gold-coated DNA particles, and Micro-injection directly introduces rDNA — matching gives A(iii), B(ii), C(iv), D(i).

Correct matching of each technique with its principle/use:

A) ELISA (Enzyme-Linked ImmunoSorbent Assay) — based on the specific antigen-antibody interaction (iii); used to detect the presence of an antigen (e.g., a pathogen protein) or antibody (e.g., in disease diagnosis, pregnancy tests) in a sample.

B) PCR (Polymerase Chain Reaction) — used for gene amplification (ii); makes multiple copies of a gene/DNA segment of interest in vitro, using the enzyme Taq polymerase. …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Which one of the following is not required for rDNA technology ?$
(i) DNA polymerase
(ii) DNA ligase
(iii) Restriction endonuclease
(iv) Reverse transcriptase
›Reveal solutionSolution
Basic recombinant DNA technology always needs a cutting enzyme (restriction endonuclease), a joining enzyme (DNA ligase) and DNA-copying capability (DNA polymerase, e.g. for PCR); reverse transcriptase is only needed in the special case of starting from an RNA/mRNA template, so it is the one "not required" in general.

The core enzyme toolkit of rDNA technology includes:

Restriction endonucleases: "molecular scissors" that cut DNA at specific recognition sequences to isolate the gene of interest and to open up the vector for insertion — always required.

DNA ligase: joins ("glues") the cut gene of interest into the vector DNA to form the recombinant DNA molecule — always required.

DNA polymerase: needed for amplifying DNA (e.g. via PCR) and is also the enzyme the host cell itself uses to replicate the recombinant plasmid — required in essentially every rDNA procedure. …
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Q.(a)

Part (a)

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