Identify the correct sequence of steps in each cycle of Polymerase Chain Reaction :

Core Logic

Let's review the statements about restriction endonucleases:

• A is true: They are commonly known as molecular scissors.
• B is true: They form part of the defense mechanism in bacteria (like E. coli) to restrict bacteriophage growth by cutting viral DNA.
• C is false: They typically cut the DNA slightly away from the centre of the palindrome sites, but between the same two bases on the opposite strands, which creates sticky ends.
• D is false: Removing nucleotides from the ends of DNA is the function of restriction exonucleases, not endonucleases. Endonucleases cut at specific positions within the DNA.
• E is true: They do recognize specific palindromic nucleotide sequences.

Step 1: Final Conclusion

The statements that are NOT true are C and D.

Pattern Recognition

Endo = inside. Exo = outside (ends). Therefore, D is clearly false for an endonuclease. They create sticky ends by cutting off-center, making C false.

Chapter Mix

Class 12 Biology: Biotechnology Principles and Processes

Core Logic

• Genetically modified organism (A): Bt cotton is a famous transgenic (GMO) crop. (A-II)
• Thermostable DNA polymerase (B): Taq polymerase, used in PCR, is isolated from the hot spring bacterium Thermus aquaticus. (B-III)
• Ti plasmid (C): Tumor-inducing plasmid is extracted from the plant pathogen Agrobacterium tumefaciens. (C-I)
• pBR322 (D): The classic, artificially engineered plasmid vector used in Escherichia coli. (D-IV)

Step 1: Final Conclusion

The exact mapping is A-II, B-III, C-I, D-IV.

Pattern Recognition

Matching Taq (Thermostable) with Thermus aquaticus (B-III) and Ti Plasmid with Agrobacterium (C-I) are the most direct routes to the answer.

Chapter Mix

Class 12 Biology: Biotechnology Principles and Processes Class 12 Biology: Biotechnology and its Applications

Core Logic

• Statement A is correct. DNA cutting is performed by restriction endonucleases, often referred to as molecular scissors.
• Statement B is correct. During gel electrophoresis, DNA fragments (which are negatively charged) move towards the anode and separate strictly according to their size due to the sieving effect of the agarose gel.
• Statement C is incorrect. Pure DNA fragments are invisible and cannot be seen under UV light without prior staining.
• Statement D is incorrect. Even when stained with ethidium bromide, DNA fragments are invisible in visible (normal) light; they only fluoresce bright orange when exposed to UV radiation.

Step 1: Final Conclusion

Only statements A and B are correct.

Pattern Recognition

Visualizing DNA requires BOTH staining (Ethidium bromide) AND UV light. Statements C and D break this rule by missing one of the prerequisites.

Chapter Mix

Class 12 Biology: Biotechnology Principles and Processes

Core Logic

The cloning vector pBR322 possesses two main antibiotic resistance genes: ampR$amp^{R}$ (ampicillin resistance) and tetR$tet^{R}$ (tetracycline resistance). The restriction site for BamHI is located directly within the tetR$tet^{R}$ gene. If a foreign DNA is ligated at the BamHI site, insertional inactivation of the tetR$tet^{R}$ gene occurs.

Step 1: Final Conclusion

Due to this insertional inactivation, the recombinant plasmid loses its resistance to tetracycline.

Pattern Recognition

BamHI and SalI are in the Tetracycline resistance gene. PvuI and PstI are in the Ampicillin resistance gene.

Chapter Mix

Class 12 Biology: Biotechnology Principles and Processes