THE LAC OPERON

Question 1

what is an operon

Answer 1

lenth of DNA made up of structural genes and control sites

Structural genes code f or proteins

Control sites are the promoter and operator region

Question 2

what is the function of structural genes

Answer 2

Z codes for B galactosidase

Y codes for Lactose Permease

each code can be transcibed onto mRNA and be translated on ribosomes to form proteins

Question 3

function of operator gene

Answer 3

regulates the expression of Y and Z genes via the absence or presence of the repressor protein

Question 4

function of promoter region

Answer 4

lenth of DNA to which the enzme RNA polymerase binds to begin the trascription of structural genes Z and Y

Question 5

what is strange about operator and promoter genes

Answer 5

they do not code for polypeptides

Question 6

what is the regulator gene

Answer 6

• gene that when transcibed onto mRNA and translated by ribosomes form a repressor protein
• repressor protein can bind to operator region and lactose

Question 7

explain what happens when lactose is absent from the growth medium

Answer 7

1. The regulator gene is expressed and the repressor protein produced
2. Since there is no lactose present to bind to the repressor protein, the protein binds to the operator region of the operon, which also binds to part of the promoter region (where RNA polymerase normally attaches to begin transcription)
3. As the repressor protein has bound itself to the operon, RNA polymerase cannot bind to the promoter region to transcribe the structural genes into mRNA to be expressed
4. Without the mRNA being produced, the enzymes B-galactosidase and Lactose Permease

Question 8

explain what happens when lactose is present in the growth medium

Answer 8

1. The regulator gene is expressed as normal, so the repressor protein is still produced
2. The inducer molecule lactose binds to the repressor protein, causing the shape of the repressor protein to alter so that its other binding site will not fit on the operon
3. The repressor protein therefore does not bind to the operator region (or breaks away if it is already on the operon)
4. The RNA polymerase therefore is able to bind to the promoter region and transcribe Z and Y into β- galactosidase and lactose permease, so lactose can be brought into the cell, broken down and respired,thus gaining energy from lactose

Question 9

what is theadvantage for E.Coli not manufacturing the enzymes when lactose is not present

Answer 9

• usesful resources are not wasted e.g.amino acids,making unncessary enzyemes
• energy is conserved and used for other functions.