Question

The trp operon is regulated in a different way from lac operon, it is transcribed when
(a) Trypsin is needed by the cell.
(b) Trypsinogen is available to the cell.
(c) There is not much tryptophan in the cell.
(d) There is abundant tryptophan in the cell.

Answer 1

Hint: In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of one promoter. The transcription of the genes occurs forming an mRNA strand and the translation occurs either in the cytoplasm or from mRNAs with a single cistron that is formed through splicing.

Complete answer
In the trp (tryptophan ) operon, there are five coding regions present which code for the production of the tryptophan. The coding region is the DNA sequence that has codes for the protein to decode. The promoter sequence present in the operon is located upstream of the transcriptional site. The sequence of the operon is important as it helps in binding the protein and regulates the operon. The repressor protein is first bound to the DNA code as the first coding region. The tryptophan when present two in number will change their shapes to bind to the trp operator and earlier it was bound to the trp repressor. This binding of tryptophan will result in the RNA polymerase preventing its attachment and thus result in the stoppage of the operon to perform the process of transcription. The repressor will bind to the operon only when the tryptophan is present in the cells. The presence of tryptophan will switch on the operon which leads to the start of the transcription. Without the tryptophan present in the cell, the operon can’t be switched off. Thus in this type of operon model tryptophan acts as a corepressor.
The word operon means the group. In the case of the Lac operon model, which is responsible for the gene regulation is important for the process of lactose metabolism and transportation in the case of the E. coli and other enteric bacteria. In the Lac operon, the most important source of carbon is the molecule of glucose. This glucose in its absence during beta-galactosidase activity will allow the digestion of the lactose very effectively.

So, the correct answer is ‘There is not much tryptophan in the cell’.

Note:
The lac operon model of the bacterium Escherichia coli was the first operon to be discovered. It provides a typical example of operon function. In fact, the trp operon in E. coli was the first repressible operon to be discovered and studied.