Class 12th Biology 2016 Set1 Delhi Board Paper Solution
(a) How are the following formed and involved in DNA packaging in a nucleus of a cell ?
(i) Histone octomer
(ii) Nucleosomes
(iii) Chromatin
(b) Differentiate between Euchromatin and Hetero-chromatin.
OR
Explain the role of lactose as an inducer in a lac operon.
(a)
(i) Eight molecules of positively charged basic proteins called histones are organised to form histone octomer.
(ii) Negatively charged DNA is wrapped around positively charged histone octomer to give rise to a nucleosome.
(iii) Nucleosomes constitute repeating unit of a structure:
| Euchromatin | Hetero-chromatin |
| It is loosly packed and stains light chromatin | More Densly packed and stains dark are called Hetero-chromatin |
| Transcriptionally active chromatin | Transcriptionally inactive chromatin |
OR
Lactose is the substrate for the enzyme beta galactosidase and it regulates switching ON and OFF of the operon. In the presence of an inducer such as lactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds.
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