Sample 1Easy
The first restriction endonuclease whose functioning depended on a specific DNA nucleotide sequence to be isolated and characterised was
- A
EcoRI
- B
BamHI
- C
Sal I
- D
Hind II
Show the answer
The answer is D.The first restriction endonuclease, Hind II, whose functioning depended on a specific DNA nucleotide sequence, was isolated and characterised. Hind II always cuts DNA at a particular point by recognising a specific sequence of six base pairs.
- A.
EcoRI is a restriction enzyme from Escherichia coli RY 13, but it was not the first one characterised.
- B.
BamHI is a restriction enzyme used as a recognition site in pBR322, not the first one characterised.
- C.
Sal I is a restriction enzyme with a site in pBR322, not the first one characterised.
NCERT: Class 12 Biology, Chapter 9 (Biotechnology: Principles and Processes), Tools of Recombinant DNA Technology, Restriction Enzymes
Sample 2Medium
A map of the plasmid pBR322 shows a circle with ori and rop, and two antibiotic resistance genes, ampR and tetR. BamHI and SalI sites lie inside tetR, PstI and PvuI sites inside ampR, and an EcoRI site elsewhere. A foreign gene is ligated into the BamHI site and the plasmid is put into E. coli. How will cells carrying the recombinant plasmid behave?
- A
They grow on tetracycline but not on ampicillin
- B
They grow on both ampicillin and tetracycline
- C
They grow on neither antibiotic
- D
They grow on medium with ampicillin but not on medium with tetracycline
Show the answer
The answer is D.Ligating foreign DNA at the BamHI site inactivates the tetracycline resistance gene, so recombinant plasmids lose tetracycline resistance. They can still be selected on ampicillin because ampR is intact.
- A.
The insert inactivates tetR, while ampR stays intact.
- B.
Insertion at BamHI disrupts tetR, so tetracycline resistance is lost.
- C.
ampR is untouched, so the cells remain ampicillin resistant.
NCERT: Class 12 Biology, Chapter 9, 9.2
Sample 3NEET level
A recombinant DNA is inserted within the coding sequence of the beta-galactosidase gene of a vector, and the transformed bacteria are grown in the presence of a chromogenic substrate. How do the colonies with recombinant plasmids appear?
- A
Colourless, because the insert inactivates the beta-galactosidase gene
- B
Blue, because the insert activates the beta-galactosidase gene
- C
Blue, because the plasmid without insert cannot produce beta-galactosidase
- D
Colourless, because the insert destroys the antibiotic resistance gene
Show the answer
The answer is A.Insertion of the alien DNA into the beta-galactosidase coding sequence inactivates the enzyme (insertional inactivation). Colonies whose plasmid has no insert produce the enzyme and turn blue with the chromogenic substrate, while recombinant colonies produce no colour.
- B.
The insert inactivates, not activates, the gene; blue colonies are the non-recombinants.
- C.
A plasmid without insert does produce beta-galactosidase, which is why non-recombinant colonies turn blue.
- D.
Colourless is right but the reason is wrong: the selection here rests on beta-galactosidase, not on an antibiotic resistance gene.
NCERT: Class 12 Biology, Chapter 9 (Biotechnology: Principles and Processes), Tools of recombinant DNA technology: cloning vectors