Question

Jacob and Mond proposed operon concept on the basis of their study of lactose metabolism in Escherichia coli. The concept is applicable to

• A. All prokaryotes only
• B. All prokaryotes and some eukaryotes
• C. All prokaryotes and all eukaryotes
• D. All prokaryotes and some protozoans

Answer 1

Answer: (B), (D)

The correct options are
B All prokaryotes and some eukaryotes
D All prokaryotes and some protozoans

According to Jacob and Monod model, a group of prokaryotic genes that are transcribed and is under regulation of single promoter is called operon. It consists of structural genes, promoter and additional regulatory sequences. Thus, prokaryotic mRNAs are polycistronic which means that multiple genes are present on a single transcript and the single promoter initiates transcription of all those genes and regulates their expression. They have multiple initiation and termination codons and thus translate more than one protein. Eukaryotic mRNAs are monocistronic which are transcribed from single gene and thus carry single initiation and termination codons thereby coding for single polypeptide. Some eukaryotes namely nematodes (C. elegans) and the fly, (Drosophila melanogaster) have polycistronic mRNAs for rRNA genes; option C is incorrect. Thus, most of eukaryotes have monocistronic mRNAs but few have polycistronic and follow operon model as is the case with prokaryotes; option A is incorrect and B is correct. Protozoans are unicellular eukaryotes; some of them carry operons like prokaryotes; option D is correct. Correct answers are B and D.