Q.Gene manipulation is a fast emerging science. It started with the development of recombinant DNA molecules. This technology which mostly involves cutting and pasting of desired DNA fragments is based on two most important discoveries in bacteria – presence of plasmid and restriction endonucleases. The science of r-DNA technology took birth when Cohen and Boyer (1973) were able to produce a piece of gene containing foreign DNA introduced into plasmid of E.coli.
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Start your 14-day free trial to unlock the full solution →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.
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.
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. …
Part (a): the core technique is genetic engineering; a plasmid is an autonomously replicating circular extra-chromosomal DNA used as a cloning vector; Cohen & Boyer isolated an antibiotic-resistance gene from a plasmid of Salmonella typhimurium.
Part (b): an exonuclease removes nucleotides from the ends of DNA, whereas an endonuclease cuts within the DNA (restriction endonucleases at specific sites).
Recombinant DNA technology began when Cohen and Boyer (1973) introduced a piece of foreign DNA into a plasmid of E. coli. It rests on two bacterial discoveries — plasmids and restriction endonucleases. Sub-parts (a) and (b) are common; sub-part (c) is answered two ways.
- Core technique. The core technique of biotechnology that alters the genetic material is genetic engineering (recombinant DNA technology): the chemistry of the genetic material (DNA/RNA) is modified and introduced into a host to change its traits.
- Plasmid and its importance. A plasmid is a small, circular, double-stranded, extra-chromosomal DNA molecule of bacteria that replicates independently of the bacterial chromosome and often carries extra genes (e.g. antibiotic resistance). In biotechnology its importance lies in serving as a cloning vector:
- it acts as a vehicle to carry a foreign gene of interest into a host cell;
- linked to the plasmid's origin of replication, the foreign DNA is replicated and multiplied (cloned) as the host divides;
- suitable plasmids allow expression of the foreign gene to yield useful products (e.g. insulin);
- selectable markers on the plasmid (such as antibiotic-resistance genes) let researchers identify and select the transformed host cells.
(c) Cohen and Boyer's gene. Stanley Cohen and Herbert Boyer isolated an antibiotic-resistance gene (tetracycline resistance) by cutting it from a plasmid of Salmonella typhimurium, and joined it to a native plasmid of E. coli — creating and propagating the first recombinant DNA molecule.
Concept understanding — Restriction Enzyme Action
Imagine you have a long, tangled piece of string, and you need to cut it into smaller, specific pieces — not just anywhere, but exactly at the places where a certain pattern of letters appears. That is the core idea behind restriction enzyme action.
In the world of biology, the "string" is a DNA molecule — the long, thread-like chemical that carries the genetic instructions for every living thing. A restriction enzyme is a molecular "scissors" that cuts DNA, but it is incredibly precise. It does not chop randomly. Instead, it recognises a very specific, short sequence of DNA letters (usually 4 to 8 base pairs long) and cuts only at that exact spot.
Think of it like a word processor's "Find and Replace" function, but instead of replacing text, the enzyme finds a specific word and cuts the page at that word.
This ability to cut DNA at precise locations is what makes restriction enzymes the fundamental tool of genetic engineering. Without them, scientists would have no way to isolate a specific gene from a long DNA strand.
How does the enzyme "know" where to cut?
The DNA molecule is made of two strands twisted together (the famous double helix). Each strand has a sequence of four chemical "letters": A, T, G, and C. A restriction enzyme scans along the DNA until it finds its target sequence — a short, palindromic pattern (meaning it reads the same forwards on one strand and backwards on the other). For example, the enzyme EcoRI recognises the sequence GAATTC.
When it finds this exact sequence, it binds to the DNA and makes a cut in both strands. The cut can be one of two types:
- Sticky ends: The enzyme cuts the two DNA strands at different points, leaving short, single-stranded overhangs. These overhangs are like pieces of Velcro — they can easily stick to a complementary overhang from another DNA piece cut by the same enzyme. This is extremely useful for joining different DNA fragments together.
- Blunt ends: The enzyme cuts both strands straight across at the same point, leaving no overhang. These are harder to join together later, but they are still useful.
Why does this matter?
Restriction enzymes are the reason we can manipulate DNA at all. They allow scientists to:
- Cut out a specific gene from the DNA of one organism (say, the human insulin gene).
- Cut open a carrier DNA (like a plasmid from a bacterium) at the same spot.
- Insert the gene into the carrier, because the sticky ends match perfectly.
- Splice the carrier back together using another enzyme (DNA ligase), creating a recombinant DNA molecule. …
Part (a): the core technique is genetic engineering; a plasmid is an autonomously replicating circular extra-chromosomal DNA used as a cloning vector; Cohen & Boyer isolated an antibiotic-resistance gene from a plasmid of Salmonella typhimurium.
Part (b): an exonuclease removes nucleotides from the ends of DNA, whereas an endonuclease cuts within the DNA (restriction endonucleases at specific sites).
Recombinant DNA technology began when Cohen and Boyer (1973) introduced a piece of foreign DNA into a plasmid of E. coli. It rests on two bacterial discoveries — plasmids and restriction endonucleases. Sub-parts (a) and (b) are common; sub-part (c) is answered two ways.
How the action of an exonuclease differs from that of an endonuclease. Both are nucleases that break phosphodiester bonds, but they act at different places: …
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