The following shows the genotype of a partial diploid bacterial cell - where one
ID: 167706 • Letter: T
Question
The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E.coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/). The possible answers indicate with a'+" or a'-" whether beta -galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.) Genotype A: I + O + Z +/F'I - O + Z + KEY: 1+ = wild-type repressor I- = mutant repressor (unable to bind to the operator) Is = mutant repressor (insensitive to lactose) O+ = wild-type operator Oc = constitutive operator (insensitive to repressor) +, + -, - +, - -, + cannot tell for certain The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E.coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/). The possible answers indicate with a'+" or a'-" whether beta -galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.) Genotype B: I + Oc + Z +/F'I - O + Z + KEY: 1+ = wild-type repressor I- = mutant repressor (unable to bind to the operator) Is = mutant repressor (insensitive to lactose) O+ = wild-type operator Oc = constitutive operator (insensitive to repressor) +, + -, - +, - -, + cannot tell The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E.coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/). The possible answers indicate with a'+" or a'-" whether beta -galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.) Genotype F: I + Oc + Z +/F'I - O + Z + KEY: 1+ = wild-type repressor I- = mutant repressor (unable to bind to the operator) Is = mutant repressor (insensitive to lactose) O+ = wild-type operator Oc = constitutive operator (insensitive to repressor) +, + -, - +, - -, + cannot tellExplanation / Answer
Dear student,
Here, the key is in remembering the main principles guiding the functions of the gene and their relationships.
Repressor (I)- stops the enzyme production in absence of lactose. It also accounts for a "diffusible regulatory protein" or trans-function - meaning even if it is not present on adjacent gene Z, the protein it produces acts on adjacent strand also. So the presence of I+ (or Is) on any of the sequence, original or transfected, will lead to activity on the operator.
KEY: Look out for I+ in both plasmids-- if it is there in presence of lactose, the operator will always function; absence of lactose will lead to lots of repressor protein production, hence no function of the operator.
Operator(O)- site of repressor binding- leads to activation of lac gene Z. The Operator is a cis-type gene- meaning it will only activate the gene next to it. (and not on the other strand)
KEY: Look out for O+/Oc combined with an active gene Z (Z+) next to it for the active production of enzyme (based on lactose presence/absence)
Gene Z: Gene responsible for the final production of the enzyme. This needs to be functional for the production of the enzyme.
KEY: Look for a functional Z+, after Looking for a combination of functional for the production of the enzyme. If it is Z- in both strands, no enzyme can be produced.
Hence based on above principles, answers would be:
1. -,+
2. +,+
3. -,-
Explanation:
1. -,+
Repressor gene is of the wild type on the original bacterial strand, hence repressor protein is going to be acting naturally.
Since there are no mutations in the operator and gene on both strands, in the absence of lactose, the enzyme will not be produced. When lactose is present, the repressor protein is translated, leading to activation of the gene and production of enzyme beta- galactosidase.
2. +,+
The repressor gene is mutant and unable to bind to the operator. HENCE THERE IS NOTHING TO STOP OPERATOR FROM WORKING.
Also, here Z+ gene is active in only the original strand of DNA, so we look at that operator.
This operator is constitutive, i.e. repressor cannot bind to it.
Hence, irrespective of the absence or presence of lactose, the operator is always working and causing the gene Z+ to make the enzyme.
Note: Here, even if the repressor was +, in the original strand, it would still be +,+. This is because the operator is insensitive to repressor protein.
3. -,-
In this case, the Z sequence is Z- in both strands. Hence the gene to make the enzyme is non-functional in both cases. Therefore irrespective of the repressor or operator, there would be no production of enzyme taking place.
Extra Note: These problems do not have any "Is" type, otherwise, in that case, the operator is always stopped from activating the gene. Think of it like a stubborn kid not giving up stopping your way. Exception: Oc combination with Is. In that case, the operator is just as stubborn and activates the gene irrespective of lactose presence or absence.
Hope this helps! All the best!