Read the following paragraph carefully and answer the given questions.
In the year 1963 the two enzymes responsible for restricting the growth of bacteriophage in E. coli were isolated. The first restriction endonuclease - Hind II, whose functioning depended on a specific DNA nucleotide sequence was characterized five years later. It was found that Hind II always cut DNA molecules at a particular point by recognizing a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria, each of which recognize different recognition sequences.
When DNA from different sources is cut by the same restriction enzymes, the resultant DNA fragments have the:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Same kind of sticky-ends
Option (1) Same kind of sticky-ends — Correct. One restriction enzyme always recognises the same palindrome and cuts it between the same two bases. So it leaves the same overhang on every DNA it cuts, whatever the source of the DNA. Such ends can pair with each other, and DNA ligase then joins them into recombinant DNA.
Option (2) Different kind of sticky-ends — Incorrect. Different ends would come only from different restriction enzymes. Such ends cannot pair. That is why the vector and the source DNA must be cut with the same enzyme.
Option (3) Sticky ends with adenine and thymine only — Incorrect. The bases on a sticky end depend on the recognition sequence of the enzyme. EcoRI does leave the bases AATT, but other enzymes leave other bases. Nothing in the question limits the ends to A and T.
Option (4) Sticky ends rich in guanine and cytosine only — Incorrect. For the same reason, an enzyme does not choose G and C. The bases of the overhang come from the cut site, and they may be any of the four.
Note: This passage is taken from NCERT as it is. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Restriction Enzymes – …
Figure 9.2 Diagrammatic representation of recombinant DNA technology