Question

How many types of restriction endonucleases are present?
A. Two types
B. Three types
C. Four types
D. Five types

Answer 1

Restriction enzyme, also called restriction endonuclease, a protein produced by bacteria that cleaves DNA at specific sites.

Complete answer:
A restriction enzyme (or restriction endonuclease) is an enzyme that cuts DNA at or near specific recognition nucleotide sequences known as restriction sites. These enzymes are isolated from the bacterial cells and used in the laboratory to manipulate fragments of DNA. It was discovered and characterized in the late 1960s and early 1970s. The restriction enzyme recognizes a short, specific sequence of nucleotide bases. These regions are called recognition sites and are randomly distributed throughout the DNA.
Option a: Traditionally, four types of restriction enzymes are recognized, designated I, II, III, and IV, which differ primarily in structure, cleavage site, specificity, and cofactors. Therefore, this is an incorrect option.
Option b: There are basically four types of restriction enzymes with specific functions. Therefore, this is an incorrect option.
Option c: Restriction endonucleases are classified into four different restriction types I, II, III, and IV. Type I enzymes cleave at sites remote from the recognition site. Type II enzymes cleave within or at short specific distances from the recognition site. Type III enzymes cleave at sites a short distance from the recognition site. Type IV enzymes target modified DNA. Therefore, this is the correct option.
Option d: There are basically four types of restriction enzymes with specific functions. Therefore, this is an incorrect option.

Thus, the correct option is (c) Four types.

Note: It is to be noted that not all the restriction enzymes produce sticky ends. Some are blunt cutters as well, they cut straight down the middle of a target sequence and leave no overhang. The restriction enzyme SmaI is an example of a blunt cutter.