Question

A bacterium has the following genotype:
lacIS lacP+ lacO- lacZ+ lacY- / lacI- lacP- lacO+ lacZ- lacY+

Fill in the blanks with either “on” or “off”.
a. In the presence of lactose, transcription of β-galactosidase is __ON____.
b. In the absence of lactose, transcription of β-galactosidase is ___ON___.
c. In the presence of lactose, transcription of permease is __OFF____.
d. In the absence of lactose, transcription of permease is __OFF____.

Please give a detailed explanation....the mutations such as lacI^s are throwing me off.

Answer 1

a.In the presence of lactose,transcription of beta-galactosidase is ON

b.In the absence of lactose,transcription of beta-galactosidase is OFF

c.In the presence of lactose,transcription of permease is ON

d.In the absence of lactose,transcription of permease is OFF

Mutation is the process of changing of the structure of a gene,resulting in a variant form that may be transmitted to subsequent generations,caused by the alteration of single base units in DNA,or the deletion,insertion or rearrangement of larger sections of genes or chromosomes.

If there is a mutation in the repressor protein then the protein willnot be able to bind the operator to stop transcription when there is no lactose present in the surrounding.There are two types of mutations in the repressor gene as:

*dominat mutation(d mutation) :The mutation is occuring in the dna binding site

*s mutation : The mutation is occuring in the allolactose binding site which results in the repression of transcription even if there is lactose present in the surrounding.

A operon is a set of structural genes with related functions under the control of a common regulatory region that responds to changing condition in the cells and is only occuring in prokaryotes.

The genes of the lac operon are expressed only if glucose is absent and lactose is available.

To use lactose the bacteria must express lac operon genes which encode key enzymes for lactose uptake and metabolism.However the lactose is the only sugar available,the E-coli will go right and ahead and use it as an energy source.

The lac operon contains 3 genes such as:

*lac Z (B-galactosidase):Hydrolyzes lactose into galactose and glucose

*lac Y (Permease): Facilitates entry of lactose into bacterial cells

*lac A (Transacetylase): The actual function is not known,but having the function toremove the by-products of lactose digestion from the cell

The control region of lac operon and its functions are:

*lac l (i gene) : codes for repressor protein

*lacP (promoter) : binding site of RNA polymerase

*lacO (operator) : binding site for the repressor protein

Allolactose (IPTG) is alactose metabolite that would inactivate the repressor

In case if the glucose is present then the operon will be turned off so the cell uses the glucose instead.

The genes such as lacZ,lacY and lac A transcribed as a single mRNA ,under the control of one promoter.When lactose is absent,the lac repressor binds tightly to the operator.It gets RNA polymerase's way,preventing trancription.