What does 'H', 'in', 'd' and 'III' refer to in the enzyme Hind III?

NCERT Exemplar · Q21

Q. What does 'H', 'in', 'd' and 'III' refer to in the enzyme Hind III?

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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.

Note

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. …

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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. …

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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

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Q.What does 'H', 'in', 'd' and 'III' refer to in the enzyme Hind III?

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