Which of the following statements are correct with reference to packaging of DNA helix? A. Histones are organized to form a unit of eight molecules called histone octamer. B. Histones are negatively charged basic proteins. C. Histones are rich in the basic amino acid residues - lysine and arginine. D. The positively charged DNA is wrapped around the histone octamer to form nucleosome. E. The packaging of chromatin at higher levels requires an additional set of proteins called non-histone chromosomal proteins. Choose the correct answer from the options given below :

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

In the lac operon model, the structural genes code for specific enzymes involved in lactose metabolism:

• z gene: Codes for beta-galactosidase (β$\beta$-gal), which hydrolyzes lactose into galactose and glucose.
• y gene: Codes for permease, increasing cell permeability to β$\beta$-galactosides.
• a gene: Codes for transacetylase.
• i gene: (Regulatory gene) codes for the repressor protein of the lac operon.

Step 1: Final Conclusion

The z gene codes for beta-galactosidase.

Pattern Recognition

Acronym to remember order and function: ZYA = β$\beta$-gal, Permease, Transacetylase.

Chapter Mix

Class 12 Biology: Molecular Basis of Inheritance