(i) (a) Hydrogen bonds.
(b) Purine base
(c) Deoxyribose sugar.
(ii) 'd' represents 5' end of the chain,

1. Lactose is the inducer for lac operon. The active form of lactose binds to the repressor protein and brings about a conformational change in the repressor.
2. As a result, the repressor is inactive, i.e. it cannot bind to the operator.
3. This provides access of the RNA polymerase to structural genes and transcription and production of enzymes continue and metabolism of lactose takes place.
4. In its absence, the repressor is active and binds to the operator, thereby switching off the process.

1. In prokaryotes, the mRNA does not require any processing to become active.
2. Transcription and translation occur in the same compartment-cytosol, as there is no well-defined nucleus. So coupling of transcription and translation is possible.
3. The mRNA has to be processed before becoming active (splicing is necessary).
4. Since nucleus and cytoplasm are well separated, in eukaryotes the RNA has to be transported to the cytoplasm for translation.
5. So coupling of transcription and translation is not possible.

1.The presence of 2'OH-group of ribose makes RNA more labile and prone to degradation.
2.In the heat-killed S-strain,it would not have remained functional and hence would not have transformed the R-cells into S-cells.