Question

The phoNY operon is located on a part of the bacterial chromosome in between genes C and D. The DNA looks like: ...A B C -----D E F..... Click 5 Watson The dashed lines represent the structural genes (enzyme encoding) of the phone operon. Mutations that affect the regulation of the phoNY operon have been identified and mapped. Some of the regulatory mutations map near gene D and some map near gene A(asterisks mark approximate locations). No phoNY regulatory mutations map elsewhere. (Genes A-F are not part of the phoNY operon.)

Answer 1

1. Ans is D.

Reason : answer can be either A or D as other genes doesn't affect the operon.

There is a gap of 2 genes (B & C) between A and the operon, but promoter cannot be that far(at the gap of 2genes).Promoter is usually upstream to the reading frame of operon at a distance of 20-30 nucleotides from the first transcribed nucleotide.

2.Ans is Crick

Reason: Strands are categorised in two types during transcription.

1 Reference strand : It is the strand which is not transcribed but it's sequence to similar to the transcribed RNA (except T instead of U) hence also refer as sense strand. All the reference is done on the basis of sense strand . Promoter near D is on 5' upstream to the operon that 5' Watson. Hence Watson is reference strand.

Therefore crick is transcribe or serve as tenplate strand .

3.Ans is once on the right end of the operon.

Reason: Operon codes for polycistronic RNA. That is all structural genes are transcribed in one stretch by single promoter. In transcription unit, the first transcribed nucleotide is always methionine (present 20-30 nucleotides downstream to promoter). There is only one transcription start site in operon which is present near promoter.

4Ans is repressor gene.

As D is promoter A can't be promoter.

Operator region is present just after the promoter. Hence A can't be operator.

Hence, A can possibly be repressor gene.