Which of the following bacteria is not a source of restriction endonuclease? (a) Haemophilus influenzae (b) Es

NCERT Exemplar · Q13

Q. Which of the following bacteria is not a source of restriction endonuclease?

(a) Haemophilus influenzae

(b) Escherichia coli

(d) Bacillus amyloliquefaciens

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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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Entamoeba coli is a protozoan, not a bacterium, and therefore is not a source of restriction endonucleases, which are enzymes primarily found in bacteria.

Restriction endonucleases are a fascinating class of enzymes that have revolutionised molecular biology and biotechnology. Often referred to as "molecular scissors," these enzymes play a crucial role in genetic engineering by precisely cutting DNA molecules at specific recognition sequences. Their discovery was a pivotal moment, enabling scientists to manipulate DNA with unprecedented accuracy.

The natural habitat for restriction endonucleases is within bacteria. These enzymes evolved as a defence mechanism for bacteria against invading bacteriophages (viruses that infect bacteria). When a bacteriophage injects its DNA into a bacterial cell, the restriction endonuclease recognises and cuts the foreign viral DNA, thereby inactivating it and protecting the bacterial host. To prevent the enzyme from cutting its own DNA, bacteria modify their own DNA, typically by methylation, at the recognition sites.

Let's examine the options provided in light of this understanding:

Haemophilus influenzae: This is a well-known bacterium. Several restriction enzymes, such as HindII and HindIII, were isolated from different strains of Haemophilus influenzae. The 'H' in HindII stands for Haemophilus, 'in' for influenzae, and 'd' for the strain Rd.
Escherichia coli: Commonly known as E. coli, this is one of the most extensively studied bacteria. The restriction enzyme EcoRI, a cornerstone in genetic engineering, was isolated from Escherichia coli strain RY 13. The 'E' stands for Escherichia, 'co' for coli, 'R' for the strain RY, and 'I' indicates it was the first enzyme isolated from that strain.
Entamoeba coli: This organism is a species of amoeba, which is a type of protozoan. Protozoans are single-celled eukaryotic organisms, distinct from bacteria, which are prokaryotes. Restriction endonucleases are characteristic of bacteria and archaea, not eukaryotes like protozoans. Therefore, Entamoeba coli would not be a source of these enzymes. …

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Q.Which of the following bacteria is not a source of restriction endonuclease?

Concept understanding — Restriction Enzyme Action

How does the enzyme "know" where to cut?

Why does this matter?

Alternative Approach: Classify by Domain of Life BEFORE Considering Enzymes at All

Q.Which of the following bacteria is not a source of restriction endonuclease?

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