UPSC Prelims 2001 Q124: Science & Technology – Biotechnology & Genetics
Assertion (A): Scientists can cut apart and paste together DNA molecules at will, regardless
of the source of the molecules.
Reason (R): DNA fragments can be manipulated using restriction endonucleases and DNA ligases.
Which of the following is correct?
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (a) Both A and R are individually true and R is the correct explanation of A
Explanation
Assertion-Reason: cutting and pasting DNA molecules
| Statement | Verdict |
|---|---|
| A. Scientists can cut and paste DNA molecules at will, whatever their source | True — basis of recombinant DNA technology |
| R. DNA is manipulated with restriction endonucleases and DNA ligases | True |
| Restriction endonucleases | 'Molecular scissors' — cut DNA at specific sequences |
| DNA ligase | 'Molecular glue' — joins DNA fragments |
Restriction enzymes cut DNA at set sequences and leave matching 'sticky ends'. DNA ligase then joins pieces from different sources. These tools are exactly what make the cutting and pasting in A possible, so R explains A.
Answer: (a) Both A and R are individually true and R is the correct explanation of A.
Why each option is right or wrong
- (a) CORRECT. Restriction enzymes and ligases are how DNA is cut and pasted.
- (b) Incorrect. R directly explains A.
- (c) Incorrect. R is true; these are the enzymes of genetic engineering.
- (d) Incorrect. A is true; recombinant DNA is routine.
Key point
First restriction enzyme used widely: EcoRI from E. coli; recognises GAATTC.
Remember
Recombinant DNA: cut with restriction enzymes, join with ligase.
Common mistakes
- Doubting 'regardless of source' — DNA is chemically the same in all organisms.
- Mixing ligase (joining) with polymerase (copying).
- Marking (b) out of caution, though R explains A.
Exam tip
Scissors = restriction enzymes; glue = ligase.
Sources
- NCERT Class XII — Biology, chapter 'Biotechnology: Principles and Processes'
- Department of Biotechnology — Recombinant DNA safety guidelines