Question

Restriction enzymes are used in genetic engineering and
A. Cut DNA base pairs at specific sites
B. Cut DNA base pairs at variable sites
C. Join two DNA segments
D. Cut RNA base pairs at specific sites

Answer 1

Hint: In the year 1963, the two enzymes responsible for restricting the growth of bacteriophage in E.coli were isolated. One of these added methyl groups to DNA (methylase), while the other cut DNA. The latter was called restriction endonuclease.

Complete answer:
Restriction enzymes serve as chemical knives to cut genes (= DNA) into defined fragments. These may then be used (i) To determine the order of genes on chromosomes.
 (ii) To analyse the chemical structure of genes and of regions of DNA which regulate the functions of genes. To create new combinations of genes
Now let us have some information about given options :-
Cut DNA base pairs at specific sites :- Stanley Cohen and Herbert Boyer accomplished the process of gene encoding in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance. The cutting and removing of DNA at specific significant locations became possible with the discovery of the so-called molecular scissors' - restriction enzymes.
Cut DNA base pairs at variable sites :- the enzymes which cut base pairs at different locations are called as selectable marks . They generate a new DNA type or can be said as DNA recombinant .
Joins two DNA segments :- The cut piece of DNA was linked with the plasmid DNA. These plasmid DNA act as vectors to transfer the piece of DNA attached to it into the host organism. The linking of gene that shows resistance to antibiotic such as penicillin with the plasmid vector
became possible with the enzyme DNA ligase that joins two DNA segments
Cut RNA base pairs at specific sites :- no enzymes so far known can cut RNA at specific sites sometimes restrictions endonuclease cut RNA but mostly they contain DNA base at specific sites .

Our required answer is A) Cut DNA base pairs at specific sites.

Note: The first restriction endonuclease - Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. It was found that Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II.