In the lac operon, the z gene codes for

Core Logic

• Statement A: Correct. A transcription unit consists of a promoter, a structural gene, and a terminator.
• Statement B: Correct. By convention, the promoter is located towards the 5'-end (upstream) of the coding strand.
• Statement C: Correct. The promoter provides a specific binding site for the RNA polymerase enzyme.
• Statement D: Correct. The presence of the promoter uniquely defines which strand acts as the template and which acts as the coding strand.
• Statement E: Correct. The terminator is located at the 3'-end (downstream) of the coding strand and defines the end of transcription.

Step 1: Final Conclusion

All statements (A, B, C, D, and E) accurately describe features of a transcription unit.

Pattern Recognition

Recall the conventions of the transcription unit: everything is described with respect to the coding strand (5' -> 3'). Promoter is 5', terminator is 3'. All provided properties match standard NCERT text.

Chapter Mix

Class 12 Biology: Molecular Basis of Inheritance

Core Logic

The technique of DNA fingerprinting involves the following sequential steps:

• Isolation of DNA and digestion by restriction endonucleases (A).
• Separation of the digested DNA fragments based on their size by gel electrophoresis (E).
• Blotting (transferring) of separated DNA fragments from the gel to synthetic membranes like nitrocellulose or nylon (C).
• Hybridisation using a labelled VNTR probe (B).
• Detection of hybridised DNA fragments under X-rays by autoradiography (D).

Step 1: Final Conclusion

The correct logical progression is A → E → C → B → D.

Pattern Recognition

Cut (Digestion) -> Run (Electrophoresis) -> Transfer (Blotting) -> Bind (Hybridisation) -> See (Autoradiography). A-E-C-B-D.

Chapter Mix

Class 12 Biology: Molecular Basis of Inheritance

Core Logic

• Statement A is correct. Histones assemble to form a histone octamer (core of 8 molecules).
• Statement B is incorrect. Histones are positively charged basic proteins. (DNA is negatively charged).
• Statement C is correct. The positive charge of histones is due to the abundance of basic amino acid residues like lysine and arginine.
• Statement D is incorrect. DNA is negatively charged (due to phosphate groups), not positively charged.
• Statement E is correct. Higher-level packaging of chromatin requires non-histone chromosomal (NHC) proteins.

Step 1: Final Conclusion

Only statements A, C, and E are correct.

Pattern Recognition

Electrostatics in genetics: DNA = Acid = Negative. Histones = Basic = Positive. Statements B and D invert these fundamental facts, making them both false.

Chapter Mix

Class 12 Biology: Molecular Basis of Inheritance