Previous year questions: Biotechnology: Principles and Processes

Board pattern for this chapter

Questions from this chapter come in a predictable set: identify a tool, state a function, explain a process in order or read a flow diagram. All ten questions below are rewritten from actual CBSE board papers between 2022 and 2025. The marks in each heading tell you how long the answer should be, and the year tells you which paper to open if you want the original.

Q1 (CBSE Board 2022, 3 marks)

Once DNA has been cut by restriction endonucleases, the fragments have to be separated and isolated. Name and explain the steps that follow the restriction digest.

Answer: The cut DNA is loaded into wells at one end of an agarose gel and an electric field is applied. DNA is negatively charged, so the fragments move towards the anode. Agarose, a natural polymer from seaweeds, acts as a sieve, so smaller fragments travel farther while larger ones stay closer to the wells. The bands are then stained with ethidium bromide and viewed under UV light, since plain DNA cannot be seen in visible light. The chosen band is cut out of the gel and its DNA extracted, a step called elution.

Q2 (CBSE Board 2022, 3 marks)

Explain the various steps of the polymerase chain reaction.

Answer: PCR copies a target DNA segment in vitro. Two sets of short synthetic primers bind to the regions that flank the target. DNA polymerase extends the primers using the nucleotides in the mixture and the genomic DNA as template. Each cycle has three stages: the double stranded DNA is denatured by heat, the primers anneal to their complementary sequences, and the primers are extended by the polymerase. A thermostable DNA polymerase from Thermus aquaticus survives the denaturation temperature. Repeating the cycles amplifies the segment to about a billion copies.

Q3 (CBSE Board 2022, 3 marks)

A host cell must be made competent before it can receive recombinant DNA. Justify the statement and explain one method of making a bacterial cell competent.

Answer: DNA is a hydrophilic molecule and cannot pass through the cell membrane on its own, so the host membrane and wall first have to be opened up. A measured dose of a divalent cation such as calcium improves the odds that DNA will cross the cell wall through its pores. The cells are then kept with the recombinant DNA on ice, given a brief heat shock at 42 degrees Celsius, and transferred back to ice, which allows the DNA to be taken up.

Q4 (CBSE Board 2022, 1 mark)

This is part (i) of a five-part five-mark question from the 2022 paper. In a cloning experiment, must two different restriction endonucleases be used, one to cut the vector and another to cut the desired DNA? Support your answer with a reason.

Answer: No, one enzyme is used for both. When the vector and the source DNA are cut with the same restriction endonuclease, the cut ends carry the same sticky ends, which can form hydrogen bonds with each other. DNA ligase can then seal the ends and produce a stable recombinant molecule. If different enzymes were used, the sticky ends would not match and the vector could not be joined to the desired fragment.

Q5 (CBSE Board 2023, 1 mark)

The organism used in the construction of the first artificial recombinant DNA by Cohen and Boyer in 1972 was

(A) E. coli (B) Salmonella typhimurium (C) Agrobacterium tumefaciens (D) Bacillus thuringiensis

Answer: Option (B), Salmonella typhimurium. The antibiotic resistance gene was linked to a native plasmid of Salmonella typhimurium. The recombinant DNA was then put into Escherichia coli, which is closely related to Salmonella, where it replicated and multiplied.

Q6 (CBSE Board 2023, 3 marks)

Explain the process of isolating DNA from a plant cell for use in recombinant DNA technology.

Answer: The plant cells are broken open so that the DNA is released together with RNA, proteins, polysaccharides and lipids. The cell walls are digested with cellulase. RNA is removed by treatment with ribonuclease, and proteins are removed by treatment with protease. Genes lie on long DNA molecules wound around histones, so removing the proteins is necessary for pure DNA. After the other molecules are removed by suitable treatments, chilled ethanol is added and the purified DNA separates out as fine threads that can be removed by spooling.

Q7 (CBSE Board 2024, 1 mark)

Hind II reads a fixed sequence before making its cut. How many base pairs long is that recognition sequence?

(A) six base pairs (B) four base pairs (C) seven base pairs (D) three base pairs

Answer: Option (A), six base pairs. Hind II was the first restriction endonuclease to be isolated and characterised, and it recognises a specific sequence of six base pairs in the DNA.

Q8 (CBSE Board 2024, 2 marks)

Explain the convention followed while naming the restriction enzyme EcoRI.

Answer: The name starts with the genus initial, then two letters from the species name of the source prokaryote. A letter before the Roman numeral marks the strain, and the numeral records the order of isolation from that strain. In EcoRI, E stands for the genus Escherichia, co for the species coli, R for strain RY 13, and I shows that it is the first enzyme isolated from that strain.

Q9 (CBSE Board 2025, 1 mark)

The cloning site present in the tetracycline resistance gene of the E. coli cloning vector pBR322 is

(A) EcoR I (B) Pvu II (C) Sal I (D) Pst I

Answer: Option (C), Sal I. The marking scheme for this paper keys Sal I. Figure 9.4 also places BamH I inside the tetracycline resistance gene, and either site can be used to inactivate that gene, but for this printed question the expected option is Sal I.

Q10 (CBSE Board 2025, 2 marks)

A flow diagram of recombinant DNA technology cuts vector DNA and alien DNA with EcoR I and joins them. (a) Name the enzyme that joins the cut plasmid and the alien DNA. (b) State the technical term for the step in which the recombinant DNA is transferred into the host cell. (c) Justify the use of the same restriction enzyme EcoR I on both DNAs.

Answer: (a) The enzyme is DNA ligase. (b) The step is called transformation. (c) EcoR I cuts both molecules at its recognition sites and leaves the same sticky ends on the vector and on the alien DNA. Matching single stranded ends pair by hydrogen bonds, so DNA ligase can seal them and form the recombinant DNA molecule. Different enzymes would leave incompatible ends and no recombinant molecule would form.

What the board rewards

This chapter rewards precise tool names and ordered processes. Keep three lists ready: the three vector features, the six steps of recombinant DNA technology, and the three tools (restriction enzyme, ligase, vector). A one-mark question wants a name, a two-mark answer wants a name plus a reason, and a three-mark answer wants a process in order. When a flow diagram is given, read the labels first and answer only what each part asks.

Where this chapter links to the next one

The tools described here are the machinery; the chapter that follows uses the same machinery in agriculture and medicine. When a question asks for an application of recombinant DNA technology, connect it back to a tool from this chapter: Bt cotton comes from a gene transferred with a vector, insulin comes from a gene expressed in a host, and gene therapy depends on a vector that can enter human cells. Seeing the link turns a memory question into a reasoning question, and reasoning answers hold up better when the wording of a question is new.

One more habit helps in the exam hall. When a question says "with the help of a diagram" and no diagram is provided in the answer, describe the diagram instead: name the parts in order from left to right, then state what each part does. Examiners accept a clear description because the mark is for understanding the arrangement, not for artistic accuracy. Read the question stem twice before choosing an option; half the mistakes in this chapter come from answering a neighbouring question.