Gene regulation Questions in English

(C) In the $Lac$ operon,the repressor protein is synthesized in an active form that binds to the operator region,preventing $RNA$ polymerase from transcribing the structural genes.
When an inducer (lactose/allolactose) is present,it binds to the repressor protein.
This binding causes a conformational change in the repressor,rendering it inactive.
The inactive repressor can no longer bind to the operator,allowing $RNA$ polymerase to access the promoter and initiate transcription,thus switching the operon 'on'.

(D) In the $lac$ operon,$lactose$ acts as the inducer.
When $lactose$ is present in the medium,it enters the cell and is converted into $allolactose$ by a small amount of $\beta-galactosidase$ enzyme.
$Allolactose$ binds to the repressor protein,preventing it from binding to the operator region,thereby allowing transcription to proceed.
$Glucose$ and $galactose$ do not act as inducers for the $lac$ operon.
Therefore,both $A$ and $B$ do not act as inducers.

(A) In the $Lac$ operon model,the $i$ gene codes for the repressor protein.
This repressor protein binds to the operator region and prevents transcription of the structural genes.
Therefore,the $i$ gene is known as the regulatory gene.
The $Lac \,z$,$Lac \,y$,and $Lac \,a$ genes are structural genes that code for $\beta$-galactosidase,permease,and transacetylase,respectively.

(C) In the $lac$ operon,the structural gene $lacY$ codes for the enzyme permease.
Permease increases the permeability of the cell membrane to $\beta$-galactosides like lactose.
Therefore,it is responsible for the active transport of lactose from the external medium into the cell.
$\beta$-galactosidase $(lacZ)$ is responsible for the hydrolysis of lactose into glucose and galactose.
Transacetylase $(lacA)$ transfers an acetyl group to $\beta$-galactosides.
The repressor protein is encoded by the $i$ gene and regulates the transcription of the operon.

(C) In the $lac$ operon,the inducer is $allolactose$,which is an isomer of lactose.
For an inducer to function in the $lac$ operon,it must be able to bind to the repressor protein,thereby preventing the repressor from binding to the operator region.
Glucose and galactose are monosaccharides that do not possess the structural configuration required to bind to the $lac$ repressor protein.
Therefore,they cannot induce the expression of the $lac$ operon genes.

(C) The $lac$ operon model explains gene regulation in prokaryotes:
$1$. $(i)$ Operator site $(d)$: The operator is the $DNA$ sequence where the repressor protein binds to prevent transcription.
$2$. $(ii)$ Promoter site $(c)$: The promoter is the $DNA$ sequence where $RNA$ polymerase binds to initiate transcription.
$3$. $(iii)$ Structural gene $(a)$: These genes (like $lacZ, lacY, lacA$) code for the enzymes (proteins) required for metabolic pathways.
$4$. $(iv)$ Regulator gene $(b)$: The regulator gene (or $i$-gene) codes for the repressor protein,which regulates the expression of the operon.
Therefore,the correct matching is $(i-d, ii-c, iii-a, iv-b)$.

(A) The provided figure represents the $lac$ operon model. In this model,the $i$ gene codes for the repressor protein. This repressor protein binds to the operator region to prevent transcription. When an inducer (like lactose) is present,it binds to the repressor protein,changing its conformation so it can no longer bind to the operator. Thus,'$1$' represents the inducer molecule.

(D) The provided figure represents the $lac$ operon model.
In the $lac$ operon,the structural genes are $z$,$y$,and $a$.
- Gene $z$ codes for $\beta$-galactosidase.
- Gene $y$ codes for permease.
- Gene $a$ codes for transacetylase.
In the diagram,the labels $2$,$3$,and $4$ correspond to the products of genes $z$,$y$,and $a$ respectively.
Therefore,'$2$' represents $\beta$-galactosidase.

(D) The given figure represents the $lac$ operon model.
In the $lac$ operon,the structural genes are $z$,$y$,and $a$.
- Gene $z$ codes for $\beta$-galactosidase (represented by $2$ in the figure).
- Gene $y$ codes for permease (represented by $3$ in the figure).
- Gene $a$ codes for transacetylase (represented by $4$ in the figure).
Therefore,'$3$' represents permease.

(A) The given figure represents the $lac$ operon model.
In the $lac$ operon,the structural genes are $z$,$y$,and $a$.
- The gene '$z$' codes for $\beta$-galactosidase $(2)$.
- The gene '$y$' codes for permease $(3)$.
- The gene '$a$' codes for transacetylase $(4)$.
Therefore,'$4$' represents transacetylase.

(B) The $lac$ operon is regulated by a repressor protein.
This repressor protein is encoded by the regulator gene,known as the $lacI$ gene.
In the absence of an inducer (like lactose),the repressor protein binds to the operator gene,thereby preventing $RNA$ polymerase from transcribing the structural genes.
Thus,the regulator gene is responsible for switching off the $lac$ operon.

(B) The lactose operon (model proposed by Jacob and Monod) produces three structural enzymes: $b$-galactosidase $(z)$,permease $(y)$,and transacetylase $(a)$.
$b$-galactosidase $(z)$ is responsible for the hydrolysis of lactose into glucose and galactose.
Permease $(y)$ is required for the transport of lactose into the cell.
Transacetylase $(a)$ transfers an acetyl group from acetyl-$CoA$ to $b$-galactosides.

(A) The inducer for the $lac$ operon of $Escherichia \ coli$ is $lactose$ (specifically,$allolactose$,which is a metabolite of $lactose$).
Normally,the $lac$ operon remains in an inactive state because the repressor protein binds to the operator gene,preventing transcription.
When $lactose$ is present in the medium,it acts as an inducer.
It binds to the repressor protein,causing a conformational change that prevents the repressor from binding to the operator gene.
Consequently,the $RNA$ polymerase enzyme is free to bind to the promoter and transcribe the structural genes.

(C) The $Lac$ operon is an example of an inducible operon system.
An inducible operon is a genetic regulatory system that is normally switched off (repressed) because a repressor protein binds to the operator.
However,it becomes operational (switched on) in the presence of an inducer (such as lactose or allolactose),which binds to the repressor and prevents it from binding to the operator,thereby allowing transcription to proceed.

(B) An operon is a functional unit of $DNA$ containing a cluster of genes under the control of a single promoter.
It consists of structural genes,an operator,and a promoter.
Operons are primarily found in prokaryotes,where they regulate metabolic pathways by coordinating the expression of genes involved in the same process.
Therefore,it is a set of closely placed genes regulating a metabolic pathway in prokaryotes.

(A) In an inducible operon system,such as the $lac$ operon,the gene expression is normally 'off'.
This is because a repressor protein is constitutively synthesized and binds to the operator region of the $DNA$ in the absence of an inducer (the substrate).
When the inducer is present,it binds to the repressor,causing a conformational change that prevents the repressor from binding to the $DNA$,thereby allowing transcription to proceed.
Therefore,in the absence of any other factor (like an inducer),the repressor protein remains bound to the $DNA$.

(C) In the $lac$ operon,a basal or low level of $Lac Z$ expression is essential even in the absence of lactose. This low level of $\beta$-galactosidase enzyme is required to convert lactose into allolactose,which acts as the inducer. If the gene expression were completely repressed,lactose could not be converted into the inducer,and therefore,the operon could never be induced even in the presence of lactose.

(D) In the $lac$ operon,the regulator gene ($i$ gene) codes for a protein known as the repressor protein.
This repressor protein is synthesized in an active form that can bind to the operator region of the operon.
When the repressor binds to the operator,it prevents $RNA$ polymerase from transcribing the structural genes,thereby keeping the operon in an 'off' state.
Therefore,the regulator gene codes for an active repressor.

(C) The $lac$ operon is an inducible operon that regulates the catabolism of lactose in $E. coli$.
$1$. It is under both negative control (by the repressor protein) and positive control (by the $CAP-cAMP$ complex).
$2$. It controls a catabolic pathway,as it breaks down lactose into glucose and galactose.
$3$. It was discovered by $François$ $Jacob$ and $Jacques$ $Monod$ in $1961$.
$4$. The $lac$ operon does not show feedback repression; rather,it shows substrate induction (the presence of lactose induces the expression of the operon). Therefore,option $C$ is incorrect.

(A) The $lac$ operon is a polycistronic unit,meaning a single $mRNA$ molecule encodes multiple proteins. Specifically,the $lac$ $mRNA$ encodes three enzymes: $\beta$-galactosidase $(lacZ)$,permease $(lacY)$,and transacetylase $(lacA)$. Because it is polycistronic,the $lac$ $mRNA$ contains multiple initiation and termination codons to facilitate the translation of these distinct polypeptide chains.

(D) In bacterial gene regulation,specifically in the operon model,the accessibility of the promoter region to $RNA$ polymerase is controlled by the interaction of regulatory proteins (repressors) with specific $DNA$ sequences known as operators. When a repressor protein binds to the operator,it physically blocks $RNA$ polymerase from transcribing the structural genes,thereby preventing the synthesis of $mRNA$.

(A) Homeotic genes are master regulatory genes that control the body plan of an embryo along the anterior-posterior axis.
These genes encode transcription factors that contain a specific $DNA$-binding domain known as the $homeodomain$.
Mutations in homeotic genes can lead to homeotic transformations,where one body part is replaced by another (e.g.,legs growing where antennae should be in $Drosophila$).
While they are conserved across many species,they were first and most extensively studied in $Drosophila$ $melanogaster$,not humans.
They are involved in developmental patterning,not primarily in the control of oncogenesis.

(A) The repressor protein of the $lac$-operon is synthesized by the regulator gene ($i$-gene).
It functions as a tetramer,meaning it consists of four identical subunits.
Each subunit has a molecular weight of approximately $38,000$ Daltons,making the total molecular weight of the tetramer significantly higher than $16,000$.
The repressor protein contains two distinct binding sites: one for the operator region and one for the inducer (allolactose).
Therefore,the statement that it is a tetrameric protein is correct.

(D) The tryptophan $(trp)$ operon is an example of a repressible operon involved in an anabolic pathway (biosynthesis of tryptophan).
$1$. The $R$-gene (regulatory gene) produces an inactive protein called an aporepressor,which is proteinaceous,not non-proteinaceous. Thus,option $A$ is incorrect.
$2$. In the presence of tryptophan (the corepressor),the aporepressor becomes active and binds to the operator,preventing the conversion of chorismic acid to tryptophan. However,under normal conditions (when tryptophan is absent),the operon is active,and chorismic acid is converted into tryptophan. Thus,option $B$ is incorrect.
$3$. The $trp$ operon is an anabolic pathway (building up molecules),not a catabolic pathway. Thus,option $C$ is incorrect.
$4$. In the absence of tryptophan,the structural genes are transcribed and translated to produce enzymes that are normally present in the cell to facilitate the synthesis of tryptophan. Thus,option $D$ is the correct statement.

(C) The Assertion is correct because the operator gene acts as a switch; when the repressor protein binds to it,transcription is blocked. When the repressor is absent or inactivated,the operator is free,allowing $RNA$ polymerase to transcribe the structural genes.
The Reason is incorrect because the regulator gene produces a repressor protein,which can be either active or inactive depending on the operon type (inducible or repressible). It does not produce 'only active protein' that acts on the system; rather,the repressor's activity is modulated by inducers or corepressors.

(D) The $Lac$ operon exhibits both negative and positive control.
In negative control,the repressor protein binds to the operator to inhibit transcription.
In positive control,the $CAP$ (Catabolite Activator Protein) binds to the promoter to enhance transcription.
Therefore,Assertion $A$ is incorrect because it states that the $Lac$ operon exerts only negative control.
Reason $R$ is also incorrect because the operator is occupied by the active repressor protein,not the aporepressor (which is an inactive form of the repressor).

(B) In the context of gene regulation,an operon is a cluster of genes under the control of a single promoter.
An $Inducer$ is a small molecule that triggers the transcription of an operon by binding to the repressor protein.
When the $Inducer$ binds to the repressor,it changes the shape of the repressor,preventing it from binding to the operator region.
This allows $RNA$ polymerase to bind to the promoter and initiate transcription.
For example,in the $lac$ operon,lactose acts as the $Inducer$.

(B) Differentiation of organs and tissues in a developing organism is associated with the differential expression of genes.
In the regulation of gene expression,chromosomal proteins play an important role.
Chromosomal proteins are of two types: histones and non-histones.
The regulation of gene expression involves a complex interaction between histones and non-histone proteins,which determines which genes are transcribed in specific cell types.

(B) The correct labeling is: $A$-Proinsulin,$B$-$A$ peptide,$C$-$B$ peptide,$D$-free $C$-Peptide.
Proinsulin is synthesized as a pro-hormone containing an extra stretch called the $C$-peptide. During the maturation process,this $C$-peptide is removed to form mature insulin,which consists of two short polypeptide chains,$A$ and $B$,linked together by disulfide bridges.

(A) Transposons,also known as 'jumping genes',are $DNA$ sequences that can change their position within a genome.
They are widely used in molecular biology for 'transposon tagging' or 'transposon-mediated mutagenesis'.
In the context of gene function analysis,transposons are used to disrupt specific genes to study their effects,which is a fundamental technique in gene silencing (specifically,insertional mutagenesis leading to loss-of-function).
Therefore,they are effectively used in gene silencing studies to determine the function of unknown genes.

(B) In the $lac$ operon,the $i$ gene codes for a repressor protein.
Normally,the repressor protein binds to the operator region and prevents $RNA$ polymerase from transcribing the structural genes $(z, y, a)$.
When lactose (the inducer) is present,it binds to the repressor protein,causing a conformational change that prevents the repressor from binding to the operator.
In this specific case,the mutated $i$ gene produces a repressor that cannot bind the inducer (lactose).
Therefore,the repressor remains bound to the operator regardless of the presence of lactose.
As a result,the $RNA$ polymerase is blocked from transcribing the structural genes $(z, y, a)$.
Since transcription does not occur,the genes will not be translated into proteins.

(A) In eukaryotes,structural genes are typically monocistronic,meaning one gene encodes one polypeptide chain. $Macaca$ (macaque monkey) and $Macropus$ (kangaroo) are eukaryotes.
In prokaryotes,structural genes are typically polycistronic,meaning one gene encodes multiple polypeptide chains. $Bacillus$ is a prokaryotic bacterium.
$Saccharomyces$ (yeast) is a unicellular eukaryote,and thus its structural genes are also monocistronic.
Therefore,$P$ (monocistronic) can be $Saccharomyces$ or $Macaca$,and $Q$ (polycistronic) must be a prokaryote like $Bacillus$.
Matching the options,option $A$ represents $P$ as $Macaca$ (eukaryote) and $Q$ as $Bacillus$ (prokaryote),which is correct.

(B) In eukaryotic organisms,gene expression is regulated at several levels:
$1$. Transcriptional level (formation of primary transcript).
$2$. Processing level (regulation of splicing).
$3$. Transport of $mRNA$ from the nucleus to the cytoplasm.
$4$. Translational level.
Replication is the process of $DNA$ duplication and is not a level of gene expression regulation.

(A) The enzyme $\beta$-galactosidase is encoded by the $lacZ$ gene in the $lac$ operon.
It catalyzes the hydrolysis of the disaccharide lactose into its constituent monosaccharides,glucose and galactose.
The chemical reaction is: $H_2O + \text{Lactose} \xrightarrow{\beta\text{-galactosidase}} \text{Glucose} + \text{Galactose}$.

(B) In the $lac$ operon,the $Lac \, y$ gene codes for the enzyme $permease$.
$Permease$ is a membrane-bound protein that facilitates the transport of $lactose$ from the external environment into the cell.
Therefore,the product of the $Lac \, y$ gene is located in the cell membrane.

(A) In the $lac$ operon,the regulator gene produces a repressor protein that binds to the operator region,preventing transcription.
When lactose is present in the medium,it acts as an inducer.
Lactose binds to the repressor protein,causing a conformational change that prevents the repressor from binding to the operator.
This allows $RNA$ polymerase to transcribe the structural genes,thereby initiating the expression of the $lac$ operon.

(A) In the $Lac$ operon,the $Lac \,i$ gene codes for the repressor protein.
This repressor protein binds to the operator region and prevents transcription of the structural genes in the absence of the inducer (lactose).
Therefore,the correct option is $Lac \,i$.

(A) The $lac$ operon consists of three structural genes: $lac\, z$,$lac\, y$,and $lac\, a$.
$lac\, z$ codes for $\beta$-galactosidase,$lac\, y$ codes for permease,and $lac\, a$ codes for transacetylase.
$A$ nonsense mutation in a gene introduces a premature stop codon,leading to the termination of translation for that specific polypeptide.
Since $lac$ operon genes are transcribed as a single polycistronic mRNA,a mutation in $lac\, y$ will prevent the synthesis of functional permease.
However,the translation of the upstream gene $lac\, z$ is unaffected,so $\beta$-galactosidase will still be synthesized.
The downstream gene $lac\, a$ translation is typically affected by the nonsense mutation in the upstream $lac\, y$ gene due to the nature of polycistronic translation.
Therefore,only $\beta$-galactosidase will be synthesized.

(A) In the $lac$ operon model,the repressor protein is synthesized in an active form that binds to the operator region,preventing $RNA$ polymerase from transcribing the structural genes.
When an inducer (such as lactose or allolactose) is present,it binds to the repressor protein.
This binding causes a conformational change in the repressor protein,rendering it inactive.
As a result,the inactive repressor can no longer bind to the operator,allowing $RNA$ polymerase to access the promoter and initiate transcription.

(A) In the context of the $lac$ operon or any operon model,the promoter,operator,and structural genes are specific segments of the $DNA$ molecule.
$1$. The promoter is the site where $RNA$ polymerase binds to initiate transcription.
$2$. The operator acts as a switch that controls the access of $RNA$ polymerase to the structural genes.
$3$. Structural genes are the segments of $DNA$ that code for specific polypeptides or enzymes.
Therefore,all these components are sequences of $DNA$.

(D) Complementarity refers to the specific base-pairing or structural interaction where two molecules fit together like a lock and key or through hydrogen bonding.
$1$. $DNA$ template and $mRNA$ exhibit complementarity through base pairing $(A-U, T-A, C-G, G-C)$.
$2$. The operator sequence and repressor protein exhibit complementarity,as the repressor protein binds specifically to the operator site.
$3$. The inducer and repressor protein exhibit complementarity,as the inducer binds to the repressor to change its conformation.
$4$. $mRNA$ and protein do not exhibit complementarity. $mRNA$ is a sequence of nucleotides that codes for amino acids,while the protein is a sequence of amino acids. The relationship between them is governed by the genetic code (codons),not by physical complementarity or base pairing.

(A) The provided figure shows the $lac$ operon in a repressed state.
In the absence of an inducer (lactose),the repressor protein produced by the $i$ gene binds to the operator $(o)$ region.
This binding prevents $RNA$ polymerase from transcribing the structural genes $(z, y, a)$.
Since the transcription is blocked,this state is referred to as the 'switch off' state of the $lac$ operon.

(B) In the lac operon model,the gene '$i$' codes for the repressor protein $(P)$.
The inducer is lactose (or allolactose),which binds to the repressor protein to prevent it from binding to the operator region.
In the provided diagram,'$Q$' represents the inducer molecule that interacts with the repressor protein '$P$' to inhibit its binding to the operator.
Therefore,'$Q$' is the inducer.

(A) During the process of gene regulation,an $RNA$ molecule that is complementary to the $mRNA$ strand is known as antisense $RNA$.
This antisense $RNA$ binds to the complementary $mRNA$ sequence,forming a double-stranded structure.
This binding prevents the ribosome from accessing the $mRNA$,thereby blocking the process of translation.
Therefore,the correct option is $A$.

(A) In the $lac$ operon,the structural genes are $z$,$y$,and $a$.
$1$. The $z$ gene codes for $\beta$-galactosidase,which is responsible for the hydrolysis of lactose into galactose and glucose.
$2$. The $y$ gene codes for permease,which increases the permeability of the cell to $\beta$-galactosides.
$3$. The $a$ gene codes for transacetylase.
Therefore,the $z$ gene codes for $\beta$-galactosidase.

(B) In the $lac$ operon model, the genes have specific functions:
$1$. The $i$ gene codes for the repressor protein, which regulates the operon.
$2$. The $z$ gene codes for $\beta$-galactosidase, which is responsible for the hydrolysis of lactose into galactose and glucose.
$3$. The $y$ gene codes for permease, which increases the permeability of the cell to $\beta$-galactosides.
$4$. The $a$ gene codes for transacetylase, which transfers an acetyl group to $\beta$-galactosides.
Therefore, the correct matching is: $a-iv, b-iii, c-ii, d-i$.

(A) In the $lac$ operon model,the structural genes and the regulator gene have specific functions:
$1$. Gene $z$: Codes for the enzyme $\beta$-galactosidase,which is responsible for the hydrolysis of lactose into galactose and glucose.
$2$. Gene $y$: Codes for the enzyme permease,which increases the permeability of the cell to $\beta$-galactosides.
$3$. Gene $a$: Codes for the enzyme transacetylase.
$4$. Gene $i$: Codes for the repressor protein,which binds to the operator region to prevent transcription in the absence of an inducer.
Therefore,the correct matching is: $A-II, B-III, C-IV, D-I$.

(B) The $y$ gene of the $lac$ operon codes for the enzyme permease.
Permease increases the permeability of the bacterial cell membrane to $\beta$-galactosides,including lactose.
Therefore,the lactose present in the growth medium is transported into the cell by the action of permease.

(C) In the $lac$ operon model,the repressor protein is synthesized by the $i$-gene.
In the absence of the inducer (lactose),the repressor protein binds to the operator region and prevents $RNA$ polymerase from transcribing the operon.
When lactose is present,it acts as an inducer.
Lactose binds to the repressor protein,causing a conformational change that prevents the repressor from binding to the operator.
This allows $RNA$ polymerase to access the promoter and transcribe the structural genes.
Therefore,lactose functions as an inducer that binds with the repressor protein.

(D) The regulatory gene $(i)$ in the $lac$ operon codes for the repressor protein.
This gene is constitutive,meaning it is expressed continuously at a low level regardless of the presence or absence of the inducer (lactose).
Therefore,statement $(I)$ is correct because $\text{mRNA}$ is transcribed from the $i$ gene in both conditions.
Statements $(II)$ and $(III)$ are incorrect because they suggest the gene is regulated by the presence of lactose,which is false.
Statement $(IV)$ is incorrect because lactose acts as an inducer that binds to the repressor protein,not by inhibiting the translation of the $i$ gene $\text{mRNA}$.
Since the question asks for the incorrect statements,$(II)$,$(III)$,and $(IV)$ are the incorrect ones.