Which of the following statements is true about gene regulation in prokaryotes
a- promoters determine whether a gene is expressed
b- expressed genes make more dna
c- dna-binding proteins determine whether a gene is expressed
d-rna polymerase regulates gene expression
2.) a lac turns off the expression of the lac genes by
a-binding to the promoters
b-binding to dna polymerase
c-binding to the operator
d-binding to the lac genes
3.) when e. coli is grown on glucose, and there is no lactose available
a-lactose molecules bind to the lac repressor
b-the lac represser binds to the operator of the lac operon
c-rna polymerase binds to the promoter of the lac operon
d-the lac genes are transcribed into messenger rna
4.) what is the function of the operator in an operon
a-provide a site on the dna where the repressor can bind and stop transcription
b-provide a site on the dna where rna polymerase can bind to begin transcription
c-code for the repressor protein
d-code for the rna polymerase protein

1. the right answer is c

coded by the regulatory gene, these allosteric proteins recognize the operator and attach themselves to it.

they have two binding sites, one for the operator and the other for the effector molecule.

the attachment of the effector molecule to the regulatory protein, will induce a conformational change of the latter inducing either a strengthening of its fixation on the operator or its detachment thereof.

2. the right answer is c

the operator is a regulatory sequence located upstream of the transcribed sequence, it is the binding site of the regulatory protein, it controls the transcription of the gene downstream.

they allow two types of controls, namely:

* a positive control, if the regulatory protein stimulates the transcription of mrna (activator),

* negative control, if the regulatory protein inhibits mrna (repressive) transcription.

3. the right answer is b

in the absence of lactose, the repressor is in its active tetrameric form.

* it binds specifically at the level of the operator blocking access of the rna polymerase to the promoter, transcription initiation site.

* there is negative regulation of transcription of lactose operon genes.

* the enzymes necessary for the metabolism of lactose are not synthesized because they are useless in the absence of lactose.

4. the right answer is a

an operator is a nucleic acid segment on which a regulatory protein that modulates downstream gene expression will bind. the binding of this protein to the operator can either stimulate or inhibit the production of the gene product thus controlled, it is called activator in the first case and repressor in the second case. initially the notion of operator has been defined for the regulation of transcription and in this case the operator is a dna sequence.