Q.(a) Development of recombinant DNA technology has opened gates to many breakthroughs in the fields of medicine and agriculture. This has enabled scientists to isolate, sequence and manipulate individual genes obtained from diverse living or dead cells. Given below is a diagram showing the basic steps involved in genetically modifying an organism. Study the given diagram and answer the questions that follow : [Diagram:

Showing the 12 most recent of 24 on this concept.

1. 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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2. 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

   1. Evaluate Assertion (A): "DNA Ligase is used to join DNA fragments." …

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3. 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:

   1. (i) Isolation of genetic material (DNA) — the DNA must first be extracted in a pure form from the source cell, free of other macromolecules.
   2. (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.
   3. (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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4. CBSE 2025Set 57/6/11 markMCQ
   Q.The process of splicing in eukaryotes represents the dominance of the : (A) DNA world (B) RNA world (C) Protein world (D) Lipid world

   ›Reveal solutionSolution

   The process of splicing in eukaryotes reveals that RNA itself can act as an enzyme, providing strong evidence for the "RNA world" hypothesis — the idea that RNA was the first self-replicating molecule in early life.

   To understand why splicing points to the RNA world, we need to step back and think about what splicing actually is. In eukaryotic cells, genes are split into coding sequences (exons) and non-coding sequences (introns). After transcription, the initial RNA transcript — called pre-mRNA — contains both exons and introns. Splicing is the process that removes the introns and joins the exons together to form a mature mRNA that can be translated into protein.

   Now here is the remarkable part. For many years, biologists assumed that all biological reactions were catalysed by proteins (enzymes). But in the 1980s, Thomas Cech and Sidney Altman independently discovered that certain RNA molecules can act as catalysts. These are called ribozymes — RNA enzymes. The splicing of some introns, particularly self-splicing introns (like Group I introns), is carried out entirely by the RNA itself, without any protein help. The intron folds into a specific three-dimensional shape that brings the ends together and catalyses the cutting and rejoining reactions.

   Important

   The discovery of self-splicing RNA was a landmark because it shattered the dogma that only proteins could be enzymes. It showed that RNA can both store genetic information (like DNA) and catalyse chemical reactions (like proteins).

   This dual ability — information storage and catalysis — is the core of the RNA world hypothesis. The idea is that before DNA and proteins evolved, early life may have used RNA as the primary molecule for both heredity and metabolism. DNA is chemically more stable and better for long-term storage, while proteins are more versatile catalysts. But RNA could have been the transitional molecule that made the leap from simple chemistry to the first living systems. …

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5. 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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6. CBSE 2024Set 57/3/11 markMCQ
   Q.Which native plasmid did Stanley Cohen and Herbert Boyer use for the construction of the first recombinant DNA ? (A) Salmonella typhimurium (B) Streptococcus pneumoniae (C) Escherichia coli (D) Haemophilus influenzae

   ›Reveal solutionSolution

   The first recombinant DNA experiment used a plasmid from Salmonella typhimurium — specifically, a small, naturally occurring plasmid called pSC101 — which was cut with a restriction enzyme and spliced with foreign DNA. The correct option is (A).

   The key to this question is remembering the historical landmark experiment by Cohen and Boyer in 1973. They weren't just using any E. coli plasmid — they deliberately chose a plasmid from a different bacterial species to demonstrate that recombinant DNA could be constructed across species boundaries.

   1. Why not E. coli?

      E. coli does have plasmids (like the famous pBR322, developed later), but Cohen and Boyer wanted to prove that DNA from one organism could be stably inserted into the plasmid of another. Using an E. coli plasmid would have been less convincing — it might have been argued that the foreign DNA was just recombining with native E. coli sequences. So they picked a plasmid from a different bacterium: Salmonella typhimurium.

   2. The actual plasmid: pSC101

      This plasmid was isolated from Salmonella typhimurium by Stanley Cohen. It was small (about 9.4 kb), had a single EcoRI restriction site, and carried a tetracycline-resistance gene — perfect for selection. Boyer’s team used EcoRI to cut both pSC101 and a piece of foreign DNA (from a frog or another bacterium), then ligated them together. The resulting recombinant plasmid was introduced into E. coli, where it replicated and expressed the foreign DNA.

   3. Why the other options are wrong …

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7. CBSE 2024Set 57/3/11 markMCQ
   Q.For Questions number 13 to 16, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : Primary transcripts in eukaryotes are subjected to splicing to remove the introns. Reason (R) : Primary transcripts contain both exons and introns and the introns are non-functional in eukaryotes.

   ›Reveal solutionSolution

   The key idea is that eukaryotic primary transcripts (pre-mRNA) contain both exons and introns, and splicing removes the non-coding introns to produce functional mRNA. Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains why splicing is necessary. The correct option is (A).

   Concept and Intuition

   In eukaryotes, genes are split into coding sequences (exons) and non-coding intervening sequences (introns). When a gene is transcribed, the entire stretch — both exons and introns — is copied into a primary transcript (pre-mRNA). This pre-mRNA is not yet ready for translation because the introns would disrupt the protein-coding sequence. So, the cell performs splicing: a process that precisely cuts out the introns and joins the exons together. The reason splicing exists is exactly because introns are present in the primary transcript and are non-functional (they do not code for protein). Without splicing, the mRNA would contain useless or even harmful sequences.

   Watch out

   A common mistake is to think that introns are "junk" that are never transcribed. In fact, introns are transcribed into the primary transcript — they just get removed later. Also, note that in prokaryotes, genes generally lack introns, so splicing is not needed.

   Step-by-Step Reasoning

   1. Understand the Assertion (A):

      "Primary transcripts in eukaryotes are subjected to splicing to remove the introns."

      This is a factual statement. In eukaryotic cells, the immediate product of transcription (pre-mRNA) contains both exons and introns. Splicing is the process that removes introns and joins exons, producing mature mRNA. So, Assertion (A) is true.

   2. Understand the Reason (R):

      "Primary transcripts contain both exons and introns and the introns are non-functional in eukaryotes." …

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8. CBSE 2024Set ANNUAL1 markMCQ
   Q.DNA or RNA segment tagged with radioactive molecule is called –

   (a) Probe

   (b) Vector

   (c) Clone

   (d) Plasmid

   ›Reveal solutionSolution

   A radioactively labelled DNA/RNA segment used to identify a complementary target sequence is called a molecular hybridisation probe.

   In recombinant DNA technology, a probe is a single-stranded DNA or RNA sequence tagged with a radioactive isotope (e.g., ³²P), used to detect the presence of a complementary nucleotide sequence in a sample through molecular hybridisation (e.g., in Southern blotting, or colony hybridisation for screening a clone carrying the gene of interest).

   …

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9. CBSE 2023Set 57/1/11 markMCQ
   Q.Identify the region 'X', the factor 'Y' and the enzyme 'Z' involved in the process of transcription in prokaryote as shown in the schematic representation given below. Region 'X' Factor 'Y' Enzyme 'Z'

   (a) Terminator Sigma (σ) RNA polymerase

   (b) Promoter Rho (ρ) RNA polymerase

   (c) Promoter Sigma (σ) RNA polymerase

   (d) Promoter Sigma (σ) DNA polymerase

   ›Reveal solutionSolution

   Prokaryotic transcription begins at the promoter region (X), where the sigma (σ) factor (Y) guides RNA polymerase (Z) to bind and initiate RNA synthesis. The correct option is (c).

   Transcription is the fundamental process where genetic information from a DNA template is copied into an RNA molecule. In prokaryotes, this process is carried out by a single type of RNA polymerase, but it requires specific signals and accessory factors to ensure accuracy and regulation. Understanding the roles of these components is key to grasping how genes are expressed.

   Here's a step-by-step breakdown of the components involved:

   1. Identifying Region 'X': The Promoter

      • Transcription does not begin randomly on the DNA. It starts at a specific sequence called the promoter. The promoter acts as a recognition site for RNA polymerase, indicating where transcription should begin and which strand should be used as the template.
      • In prokaryotes, common promoter sequences include the -35 region (e.g., TTGACA) and the -10 region (Pribnow box, e.g., TATAAT), located upstream from the transcription start site.
      • Therefore, the region 'X' where transcription initiates is the promoter.

   2. Identifying Factor 'Y': The Sigma (σ) Factor

      • While RNA polymerase is the enzyme that synthesizes RNA, it cannot, on its own, efficiently recognize and bind to the promoter sequence. This is where accessory factors come into play.
      • In prokaryotes, the sigma (σ) factor is a dissociable subunit of the RNA polymerase holoenzyme. Its primary role is to recognize and bind specifically to the promoter sequences (like the -35 and -10 regions). This binding guides the core RNA polymerase enzyme to the correct initiation site.
      • Once transcription has initiated and a short RNA strand is synthesized, the sigma factor typically dissociates from the core enzyme, allowing the core enzyme to proceed with elongation.
      • The Rho (ρ) factor, mentioned in option (b), is involved in termination of transcription in some cases, not initiation.
      • Therefore, the factor 'Y' involved in initiating transcription by recognizing the promoter is the sigma (σ) factor.

      RNA Polymerase Holoenzyme = Core Enzyme (α₂ββ'ω) + Sigma (σ) Factor

   3. Identifying Enzyme 'Z': RNA Polymerase …

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10. CBSE 2023Set TERM21 markMCQ
    Q.A single strand of Nucleic acid tagged with a radioactive molecule is called:

    (a) Plasmid

    (b) Vector

    (c) Probe

    (d) Selectable marker

    ›Reveal solutionSolution

    A single-stranded nucleic acid molecule tagged with a radioactive isotope, used to detect a complementary sequence, is called a probe.

    In recombinant DNA technology, to confirm the presence of a specific gene or DNA sequence among many DNA fragments, a single-stranded DNA or RNA molecule complementary to that sequence is labelled with a radioactive isotope (commonly ³²P) — this labelled molecule is called a (radioactive) probe. It is used in techniques such as Southern/Northern hybridisation to specifically bind (hybridise) with its complementary sequence, which can then be detected by autoradiography. …

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11. CBSE 2023Set ANNUAL1 markMCQ
    Q.At what level does the control of gene expression occur?

    (a) transcription

    (b) translation

    (c) DNA replication

    (d) both

    (a) and (b)

    ›Reveal solutionSolution

    Gene expression is controlled at transcription and at translation, so both (a) and (b).

    Regulation of gene expression is the control of the amount and timing of gene products. In prokaryotes it is chiefly at the level of transcription (e.g. the lac operon). In eukaryotes control can occur at transcription, RNA processing/splicing, transport of mRNA, and translati …

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12. CBSE 2023Set ANNUAL1 markMCQ
    Q.The reagent that is used in the ELISA test is

    (a) polymerase

    (b) peroxidase

    (c) ligase

    (d) endonuclease

    ›Reveal solutionSolution

    ELISA uses an enzyme such as peroxidase.

    ELISA (Enzyme-Linked Immunosorbent Assay) detects an antigen or antibody by attaching an enzyme to a specific antibody. When the substrate is added, the enzyme catalyses a colour-producing reaction whose intensity indicates the amount of antigen present. Common reporter enzymes are horseradish peroxidase and alkaline phosph …

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