Question

Restriction endonucleases was discovered in-
a. 1942
b. 1966
c. 1950
d. 1970

Answer 1

Hint: Francis Crick, Sydney Brenner, Leslie Barnett, and Richard Watts-Tobin first discovered the three bases of DNA code for one amino acid in 1961. That was the time when scientists cracked the code of life. So, only after the decoding of the genetic code, restriction endonuclease can be discovered.

Complete answer:
Restriction endonucleases are the particular types of nucleases that cut the DNA internally at the specific sites called the restriction sites. Restriction enzymes are one class of the broad-ranging endonuclease group of enzymes.

They are generally categorized into five types, which vary in their structure and whether they cut their DNA substrate at their recognition site, or if the recognition and cleavage sites are segregated from one another.

All restriction enzymes make two incisions, to cut DNA, once through each sugar-phosphate backbone (i.e. each strand) of the DNA double helix. These were discovered in 1970 by Hamilton O. Smith, Thomas Kelly, and Kent Wilcox isolated from the bacterium Haemophilus influenzae. The enzymes are present in the cells of bacteria and cleave the DNA of any bacteriophage that infects the bacterium. Owing to their specific cleaving activity, they are mostly used in the rDNA processes.

Hence, the correct answer is option (D).

Note: Restriction enzymes usually recognize a specific sequence of nucleotides and produce a double-stranded cut in the DNA. The recognition sequences can also be categorized by the number of bases in its recognition site, usually between 4 and 8 bases, and the number of bases in the sequence will decide how frequently the site will appear by chance in any given genome.