Biology · Biotechnology: Principles and Processes · NEET
The name comes from their natural job inside bacteria, not from lab use. Bacteria make these enzymes to RESTRICT (stop) the growth of bacteriophages (viruses that infect bacteria). When a virus injects its DNA into the bacterium, the enzyme cuts up that foreign viral DNA, so the virus cannot multiply. We later borrowed these same enzymes as tools to cut DNA in the lab, but the original name stuck. For NEET, remember: the bacterium restricts the virus.
In 1963, two enzymes were isolated from E. coli that together handle bacteriophage DNA. One enzyme ADDED METHYL GROUPS to DNA (this is methylation, which protected the bacterium's own DNA so it would not get cut). The other enzyme CUT DNA (this is the restriction endonuclease that chops up the foreign viral DNA). This pairing is the 'host-controlled restriction and modification' system: modify (methylate) self, restrict (cut) invaders. NEET often frames one wrong option saying both enzymes cut DNA — only ONE cuts.
Hind II was the first restriction endonuclease whose functioning depended on a specific DNA sequence. It was isolated and characterised in 1968, about five years after the 1963 discovery of the two enzymes. Hind II always cuts DNA at a particular point by recognising a specific sequence of SIX base pairs, called its recognition sequence. This is a very common one-mark NEET fact.
It means the ability to restrict (destroy) foreign DNA is controlled by the host bacterium. Different bacterial strains produce different restriction enzymes. A virus that grows fine in one bacterial strain may be cut up and restricted in another strain, because that second strain has a restriction enzyme the first one lacked. So whether the virus survives depends on the HOST bacterium — hence 'host-controlled'.
Because the bacterium protects its own DNA using the methylating enzyme. The methyl groups added to the bacterium's recognition sites act like a shield, so the restriction endonuclease does not recognise and cut the host's own DNA. The invading viral DNA is unmethylated at those sites, so only the viral DNA gets cut. This self vs non-self recognition is the whole point of the restriction-modification system.
Which of the following is a restriction endonuclease?
Which of the following statements are not true regarding restriction endonucleases? A. They are called molecular scissors. B. These are the enzymes responsible for restricting the growth of bacteriophages in E. coli. C. They cut the DNA only at the centre of the palindromic sites. D. They remove nucleotides only from the ends of DNA fragments. E. They recognise specific palindromic base-pair sequences.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In 1963, two enzymes responsible for restricting the growth of bacteriophage in E. coli were isolated. One added methyl groups to DNA, and the other cut DNA (the restriction endonuclease).
Hind II, the first restriction endonuclease, was isolated and characterised about five years after 1963, that is in 1968. It recognises a specific sequence of six base pairs.
Hind II always cuts DNA at a particular point by recognising a specific sequence of six base pairs. This six base pair sequence is called its recognition sequence.
Bacteria use restriction enzymes as a defence system. They cut up the DNA of invading bacteriophages, restricting the virus from multiplying inside the bacterial cell.
The modification (methylating) enzyme adds methyl groups to protect the host's own DNA. The restriction enzyme (restriction endonuclease) cuts foreign DNA. Together they form the host-controlled restriction-modification system.
