Q.What does 'H', 'in', 'd' and 'III' refer to in the enzyme Hind III?
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🔒 Start your 14-day free trial to unlock the full solution →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. …
The name Hind III follows a standard naming convention for restriction enzymes. The first letter, H, comes from the genus of the bacterium from which the enzyme was isolated — in this case, Haemophilus. The next two letters, in, are taken from the species name, influenzae. So together, Hin identifies the source organism as Haemophilus influenzae. …
The letters and Roman numeral in Hind III are a systematic code that reveals the bacterial source, strain, and order of discovery of this restriction enzyme.
Restriction enzymes are the molecular scissors of biotechnology — they cut DNA at specific recognition sequences. The naming of these enzymes follows a standardised convention proposed by Hamilton Smith and Daniel Nathans, which the NCERT textbook explains clearly. Hind III is a classic example of this naming system, and each part of its name carries a precise meaning.
The first letter, H, stands for the genus of the bacterium from which the enzyme was isolated. In this case, it is Haemophilus. The next two letters, in, come from the species name of that bacterium — influenzae. So the first three letters together, Hin, identify the source organism as Haemophilus influenzae.
The fourth character, d, refers to the particular strain of that bacterial species. Here, d indicates the strain Rd of Haemophilus influenzae. Different strains of the same species may produce different restriction enzymes, so this strain designation is essential for specificity.
Finally, the Roman numeral III tells us that this was the third restriction enzyme discovered in that particular bacterial strain. The enzymes from the same source are numbered in the order of their discovery — Hind I, Hind II, Hind III, and so on. So Hind III was the third restriction enzyme isolated from Haemophilus influenzae strain Rd. …
Method 2 -- decode the name character-by-character as a lookup table
Restriction enzyme names follow a fixed code (proposed by Smith and Nathans), so the fastest way to answer is to map each character position to what it always stands for, then read off the specific values for Hind III:
| Position in name | What it always encodes | Value for Hind III |
|---|---|---|
| 1st letter (capitalised) | Genus of the source bacterium | H = Haemophilus |
| 2nd + 3rd letters (lowercase) | Species | in = influenzae |
| 4th letter (if present) | Strain | d = strain Rd |
- KCET 2026Set UNKNOWN1 markMCQQ.Given below are two statements. Statement I: Restriction enzyme BamHI has its recognition site in tetR region of pBR322. Statement II: E.coli having pBR322 with a desired DNA if inserted at BamHI site can grow in medium containing tetracycline. In the light of the above statements, choose the correct answer from the options given below. (A) Both Statement I and Statement II are true (B) Both Statement I and Statement II are false (C) Statement I is true, but Statement II is false (D) Statement I is false, but Statement II is true
›Reveal solutionSolution
pBR322 carries unique restriction sites within its two antibiotic-resistance genes; inserting DNA at one of these sites inactivates that gene rather than preserving resistance.
Step 1 — Checking Statement I
The cloning vector pBR322 has two antibiotic resistance genes, ampicillin resistance (ampR) and tetracycline resistance (tetR), each containing unique restriction sites usable for cloning. The BamHI site (along with SalI) lies within the tetR gene (PstI lies within ampR). So Statement I is true.
Step 2 — Checking Statement II …
- KCET 2026Set UNKNOWN1 markMCQQ.Match List I with List II List Ia) Visualization of DNA in gel electrophoresisb) Precipitation of DNAc) Breaking of cell wall of bacteriad) Breaking of cell wall of fungus List IIi) Chitinaseii) Lysozymeiii) Chilled ethanoliv) Ethidium bromide Codes: (A) a - iv, b - iii, c - ii, d - i (B) a - iii, b - i, c - i, d - ii (C) a - iv, b - i, c - ii, d - iii (D) a - iii, b - ii, c - iv, d - i
›Reveal solutionSolution
Matching each rDNA-technology step to its reagent: visualisation → ethidium bromide, DNA precipitation → chilled ethanol, bacterial cell wall lysis → lysozyme, fungal cell wall lysis → chitinase, giving a-iv, b-iii, c-ii, d-i.
Step 1 — (a) Visualisation of DNA in gel electrophoresis
DNA fragments separated by gel electrophoresis are stained with ethidium bromide and viewed under UV light as bright orange-coloured bands. So a → iv.
Step 2 — (b) Precipitation of DNA
After isolating DNA in aqueous form, it is precipitated out of solution by adding chilled ethanol. So b → iii.
Step 3 — (c) Breaking the cell wall of bacteria …
- KCET 2023Set B-41 markMCQQ.The toxic substance 'haemozoin' responsible for high fever and chill, is released in which of the following diseases? (A) Typhoid (B) Dengue (C) Pneumonia (D) Malaria
›Reveal solutionSolution
Haemozoin is the toxic pigment liberated when Plasmodium-infected RBCs burst — the direct cause of malaria's periodic chill-and-fever.
Step 1 — The malarial life cycle in the human host
- An infected female Anopheles mosquito injects sporozoites while biting.
- The parasites first multiply in the liver cells.
- They then attack the red blood cells (RBCs), multiplying inside them.
- The RBCs rupture, releasing new parasites and a toxic substance called haemozoin.
Step 2 — Why haemozoin causes the fever
Haemozoin is a waste product formed as Plasmodium digests the haemoglobin of the RBC (it polymerises the toxic free haem it cannot excrete). When the RBCs burst synchronously, a bolus of haemozoin enters the circulation and triggers the immune/inflammatory response responsible for the chill and high fever recurring every three to four days.
Step 3 — Complete the cycle …
- KCET 2021Set C-31 markMCQQ.Match the following Column-I i. Cyclosporin-A ii. Streptokinase iii. Statins iv. Penicillin Column-II a. Clot busters b. Antibiotic c. Immuno suppressive agent d. Blood cholesterol lowering agent (A) i-c, ii-a, iii-d, iv-b (B) i-c, ii-d, iii-a, iv-b (C) i-a, ii-b, iii-c, iv-d (D) i-a, ii-b, iii-d, iv-c
›Reveal solutionSolution
This question tests your recall of the specific medical applications of four well-known microbial products. The correct match is Cyclosporin-A (immunosuppressive), Streptokinase (clot buster), Statins (cholesterol-lowering), and Penicillin (antibiotic), which corresponds to option (A).
The key here is not to memorise blindly, but to connect each drug to its biological function and the disease or condition it treats. Each of these is a product of microbial fermentation or extraction, and their names often hint at their use.
Let’s go through each one.
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Cyclosporin-A (i)
This is a cyclic peptide produced by the fungus Trichoderma polysporum. Its primary medical use is as an immunosuppressive agent — it suppresses the immune system to prevent rejection of transplanted organs. So it matches with c.
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Streptokinase (ii)
Produced by the bacterium Streptococcus, this enzyme dissolves blood clots by converting plasminogen into plasmin. It is used in emergency medicine to treat heart attacks and strokes — it is a clot buster. So it matches with a.
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Statins (iii)
These are produced by the fungus Monascus purpureus (and also synthesised chemically). They work by inhibiting the enzyme HMG-CoA reductase, which is key in cholesterol synthesis. Their effect is to lower blood cholesterol levels. So they match with d.
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Penicillin (iv) …
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- KCET 2021Set C-31 markMCQQ.α-1 antitrypsin is (A) an antacid (B) an enzyme (C) used to treat emphysema (D) used to treat arthritis
›Reveal solutionSolution
α-1 antitrypsin is a protease inhibitor that protects lung tissue from damage; its deficiency causes emphysema, and the purified protein is used as a therapy for emphysema. The correct option is (C).
The concept: What is α-1 antitrypsin, and why does it matter?
α-1 antitrypsin (AAT) is a serine protease inhibitor — a protein made primarily in the liver and secreted into the bloodstream. Its main job is to neutralize neutrophil elastase, an enzyme released by white blood cells during inflammation. If left unchecked, neutrophil elastase chews up the elastic fibres in lung tissue, leading to emphysema (a form of chronic obstructive pulmonary disease, COPD).
So AAT is not an enzyme itself — it inhibits an enzyme. It is also not an antacid (that neutralizes stomach acid), and it has no role in arthritis treatment. The key clinical link: people born with a genetic deficiency of AAT are at high risk of early-onset emphysema. The therapy? Purified AAT from human plasma is infused intravenously to raise blood levels and protect the lungs.
Step-by-step reasoning
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Eliminate (A) — an antacid
Antacids are simple bases like magnesium hydroxide or calcium carbonate that neutralize stomach acid. α-1 antitrypsin is a protein, not a base, and has nothing to do with gastric pH. So (A) is wrong.
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Eliminate (B) — an enzyme
AAT is a protease inhibitor, not a protease. It binds to and inactivates the enzyme neutrophil elastase. Calling it an enzyme would be like calling a lock a key — it blocks the action, not catalyses a reaction. So (B) is wrong.
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Consider (D) — used to treat arthritis
Arthritis (especially rheumatoid arthritis) involves inflammation, but AAT therapy is not a standard treatment for it. Some research has explored AAT for other inflammatory conditions, but the established, approved use is for emphysema due to AAT deficiency. So (D) is not correct in the exam context.
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Confirm (C) — used to treat emphysema …
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- KCET 2019Set A-11 markMCQQ.In cloning vectors, antibiotic resistant genes are helpful for (A) Selection of recombinants (B) Cleaving of vector by REN (C) Transfer of foreign gene to the host (D) Making the host cells competent
›Reveal solutionSolution
Antibiotic-resistance genes act as selectable markers, letting us pick out the few transformed/recombinant cells from a huge background of untransformed ones.
Step 1 — The problem a selectable marker solves.
Transformation is very inefficient: only a tiny fraction of the host cells actually take up the plasmid. We need a way to make only those cells grow.
Step 2 — How the resistance gene does it.
The vector carries, say, ampR (ampicillin resistance). Plating the transformation mixture on ampicillin-containing medium kills every cell that did not take up the vector; survivors necessarily carry it. This is selection.
Step 3 — Selecting recombinants specifically.
pBR322 carries two markers, ampR and tetR. A foreign gene ligated into the tetR site inactivates it (insertional inactivation). So:
- grows on ampicillin and tetracycline ⇒ non-recombinant (vector re-circularised),
- grows on ampicillin but not tetracycline ⇒ recombinant.
Step 4 — Reject the others. …
- KCET 2018Set A-11 markMCQQ.The technique of bombarding plant cells with high velocity microparticles of gold or tungsten, coated with DNA, is (A) Microinjection (B) Biolistic method (C) Heat shock method (D) By disarmed pathogen vector
›Reveal solutionSolution
"Bombarding with high-velocity DNA-coated gold/tungsten microparticles" is the textbook definition of the biolistic (gene gun) method of direct gene transfer.
Step 1 — The problem the method solves.
Plant cells have a rigid cellulose cell wall, so naked DNA cannot simply diffuse in. Direct gene-transfer methods must physically breach the wall and membrane. The biolistic method does exactly that: DNA is precipitated onto inert, dense microparticles of gold or tungsten, which are then accelerated to high velocity by a gene gun so they pierce the cell wall and deliver the DNA into the cell/nucleus.
Step 2 — Why the other three are not this method.
- (A) Microinjection — DNA is injected directly into the nucleus with a fine glass micro-needle; used for animal cells (e.g. producing transgenic animals). No particles, no bombardment.
- (C) Heat-shock method — competent bacterial cells held in divalent Ca²⁺ are given a brief 42 °C shock so plasmid DNA passes through transient pores. Chemical/thermal, not mechanical. …
- KCET 2018Set A-11 markMCQQ.Choose the bacterium which is not a source of REN: (A) \textit{Haemophilus influenzae} (B) \textit{Escherichia coli} (C) \textit{Agrobacterium tumefaciens} (D) \textit{Bacillus amyloliquefaciens}
›Reveal solutionSolution
Read the restriction-enzyme naming convention: the first letter is the genus, the next two the species — Eco, Hin, Bam all trace back to bacteria in this list; Agrobacterium does not name any RE, it supplies the Ti-plasmid vector.
Step 1 — What REN means and how REs are named.
A restriction endonuclease cuts DNA at a specific palindromic recognition sequence. Its name is built from its bacterial source: first letter = genus (capital), next two letters = species, then the strain and a Roman numeral for the order of isolation.
Step 2 — Test each option.
- (A) Haemophilus influenzae → HindII (the first restriction endonuclease ever isolated) and HindIII. A source. ✓
- (B) Escherichia coli → EcoRI, the workhorse enzyme cutting 5′-G↓AATTC-3′ to give sticky ends. A source. ✓
- (D) Bacillus amyloliquefaciens → BamHI. A source. ✓ …
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