Question

The term “Restriction” in restriction endonuclease enzyme refers to
(a) Cleaving of the phosphodiester bond in DNA by the enzyme.
(b) Cutting of DNA at a specific position only.
(c) Prevention of bacteriophage multiplication in bacteria.
(d) Cutting each of the two strands of DNA at specific points in sugar-phosphate backbones.

Answer 1

Hint: Restriction means to stop and the same applies to the name of this enzyme because it also restricts something that is very important for some unicellular organisms.

Complete answer:
Restriction endonuclease or restrictase is an enzyme bearing the same property that means it cuts DNA into fragments at or near specific recognition sites within the molecules to stop the foreign organism from causing further damage.
In the bacterial cell, restriction enzymes cleave foreign DNA, this is the way they are eliminating infecting organisms. Restriction enzymes can be isolated from bacterial cells and used in the laboratory to manipulate fragments of DNA containing genes.
A bacterium uses restriction enzymes to fight against bacterial viruses called bacteriophages, or phages. While infecting a bacterium, the phage inserts its DNA into the bacterial cell for replication. The restriction enzyme prevents the replication of the phage DNA by cutting it into many pieces. For their ability to restrict the number of strains of bacteriophage to infect a bacterium, they were named restriction enzymes.

A specific sequence of nucleotide bases was recognized by a restriction enzyme, these regions were called recognition sequences or sites, and these are randomly distributed throughout the DNA.
When a restriction endonuclease recognizes a sequence, it splits the chemical bond between the adjacent nucleotides of the DNA molecule through hydrolysis. In this manner, by disguising their recognition sequences the bacteria prevent their DNA from being degraded. Enzymes are called methylases to add methyl groups (—CH3) to adenine or cytosine bases within the recognition sequence, and thus protect itself from the endonuclease by modifying them.

So, the correct answer is, 'prevention of bacteriophage multiplication in bacteria’.

Note:
-More than 3000 restriction enzymes have been studied until now and out of these 3000 enzymes, more than 600 are commercially available.
-The restriction enzymes are mainly used in laboratories for the modification of DNA so these can also be used as a tool for molecular cloning.
-The restriction endonucleases which are naturally occurring are categorized into four groups based on their composition and requirements of cofactors of an enzyme, these are Type l, ll, lll, and lV.
-Different bacterial species also make different restriction enzymes that recognize different nucleotide sequences.