Question

Restriction enzymes are used in genetic engineering because they
A. Can join DNA fragments
B. Cut DNA at specific base sequences
C. Cut DNA at variable sites
D. Are proteolytic enzymes that degrade harmful proteins

Answer 1

Restrictions enzymes are important tools of recombinant DNA technology. They are protein molecules that are produced by bacteria as a defense against viral genomes. They are specialized to cut at specific restriction sites. Thus, they protect bacteria from Bacteriophages.

Complete answer:
The restriction enzymes are a class of proteins that are produced by bacteria as part of their defense mechanisms against Bacteriophages. These are able to cleave the DNA very specifically by recognizing certain base pairs called the restriction site. The restriction enzymes cut the viral genome of Bacteriophages that infects the bacterial cells thus protecting them.

Restriction enzymes are recognized as essential tools of genetic engineering. They can be easily isolated from the bacterial cells and used in experiments. In laboratories, the restriction enzymes can recognize short sequences of specific nucleotides. Once, they recognize the restriction sites, they make a blunt or sticky end cut on that site. They do so by hydrolyzing the bond between the two adjacent nucleotides.
There are three main types of restriction enzymes: Type I, Type II, and Type III.
The type I restriction enzymes recognize and cut the DNA from restriction sites but produce discrete fragments. Hence, they are not of practical use in genetic engineering. The type II restriction enzymes are very specific to recognize and cut the desired DNA segments. They recognize the palindromic sequences which read the same from left and right. They are very specific and reliable and thus are used exclusively for genetic engineering experiments. EcoRI is one example of Type II restriction enzymes. It is isolated from E.coli bacterial strain. If they cut at the center of the recognition sequence, blunt ends are produced. The type III restriction enzymes are methyltransferase proteins.
So, from the above discussion, we can conclude that Restriction enzymes are used in genetic engineering because they cut at specific recognition sequences of base pairs in DNA.

Thus, option B) is the right answer.

Note: It has been noticed that bacteria contain restriction enzymes but how it can protect their DNA is of interest to scientists. It is believed that bacteria add methyl groups at the cytosine or adenine bases present within recognition sequences of the bacterial genome. Methylases enzymes are used for this purpose. This causes alteration of recognition sequence and thus restriction enzymes do not cut the bacterial genome.