Question
In the process of transcription in eukaryotes, the RNA polymerase I transcribes [NEET (Odisha) 2019]
(a) mRNA with additional processing, capping and tailing
(b) tRNA, 5 srRNA and snRNAs
(c) rRNAs-28 S, $18 \mathrm{~S}$ and $5.8 \mathrm{~S}$
(d) precursor of mRNA, hnRNA
Answer/Explanation
Ans. (c)
In the process of transcription (i.e. copying of genetic information from one strand of the DNA into RNA) in eukaryotes, the RNA polymerase I transcribes rRNA – 28S, 18S and 5.8S. On the other hand, tRNA, 5srRNA and snRNAs are transcribed by RNA polymerase III. RNA polymerase II transcribes precursor of mRNA, hnRNA.
Question
Match the following genes of the Lac operon with their respective products[NEET (National) 2019]
A. igene (i) $\beta$-galactosidase
B. z gene (ii) Permease
C. a gene (iii) Repressor
D. ygene (iv) Transacetylase
Select the correct option.
A B C D
(a) (iii) (i) (ii) (iv)
(b) (iii) (i) (iv) (ii)
(c) (iii) (iv) (i) (ii)
(d) (i) (iii) (ii) (iv)
Answer/Explanation
Ans. (b)
(A)-(iii), (B)-(i), (C)-(iv), (D)-(ii)
In a Lac operon, $i$ gene is a regulator gene which produces a repressor that binds to operator gene and stops its functioning. $z, y$ and $a$ are the three structural genes in the lac operon of $E$. coli. z gene produces $\beta$-galactosidase for hydrolysing galactoside. y gene produces permease for allowing the entry of lactose from outside.
A gene produces transacetylase which helps to transfer an acetyl group from acetyl Co-A to beta- galactoside.
Question
Select the correct match. [NEET 2018]
(a) Matthew Meselson : Pisum sativum and F. Stahl
(b) Alfred Hershey and: TMV Martha Chase
(c) Alec Jeffreys : Streptococcus pneumoniae
(d) Francois Jacob: Lac operon and Jacques Monod
Answer/Explanation
Ans. (d)
Jacob and Monod (1916) discovered the lac operon. An operon is a part of genetic material or DNA which acts as a single regulated unit. It possesses one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor gene and an inducer or corepressor.
Matthew Meselson and F Stahl discovered the semi-conservative mode of DNA replication in E. coli. Alfred Hershey and Martha Chase use $T_2$ Bacteiriophage in their experiments to infect $E$. coli and proved that DNA is the genetic material. Alec Jeffreys (1984) invented the DNA fingerprinting technique. This technique determines nucleotide sequences of certain areas of DNA which are unique to each individual.
Question
All of the following are parts of an operon except [NEET 2018]
(a) an enhancer
(b) structural genes
(c) an operator
(d) a promoter
Answer/Explanation
Ans. (a)
Except enhancer, all the given components are parts of an operon. Enhancer sequences are present in eukaryotes that, when bound by specific proteins or transcription factors, enhance the transcription of an associated gene. On the other hand, operon is a regulatory unit of DNA containing a cluster of genes in prokaryotes.
Question
Which of the following is required as inducer(s) for the expression of lac operon? [NEET 2016, Phase I]
(a) galactose
(b) lactose
(c) lactose and galactose
(d) glucose
Answer/Explanation
Ans. (b)
Lac operon is an inducible operon. Lactose is the substrate for the enzyme $\beta$-galactosidase and it also regulates switching on and off of the operon. Hence, it is termed as inducer.
Question
Which one of the following is wrongly matched? [CBSE AIPMT 2014]
(a) Transcription-Writing information from DNA to $t$ RNA
(b) Translation-Using information in mRNA to make protein
(c) Repressor protein-Binds to operator to stop enzyme synthesis
(d) Operon-Structural genes, operator and promoter
Answer/Explanation
Ans. (a)
Statement (a) is wrongly matched because transcription is a process of mRNA synthesis from a DNA template. It involves three main events, i.e. initiation (binding of RNA polymerase to as DNA), elongation (development of a short stretch of DNA) and termination (recognition of the transcription termination sequence and the release of RNA polymerase).
Question
Which enzyme/s will be produced in a cell in which there is a non-sense mutation in the lac Y-gene? [NEET 2013]
(a) $\beta$-galactosidase
(b) Lactose permease
(c) Transacetylase
(d) Lactose permease and transacetylase
Answer/Explanation
Ans. (a)
$\beta$-galactosidase is a structural gene, which carry codes for the synthesis of protein. Mutation in the lac $Y$ gene of $E$. colineeds residues of cytoplasmic enzyme $\beta$-galactosidase. Lactose permease is a membrane protein, which is a major facilitator superfamily. Transacetylase is an enzyme transferring acetyl groups from one compound to another.
Question
Select the two statements out of the four (I-IV) given below about lac operon.
I. Glucose or galactose may bind with the repressor and inactivate it.
II. In the absence of lactose, the repressor binds with the operator region.
III. The z-gene codes for permease.
IV. This was elucidated by Francois Jacob and Jacques Monod.
The correct statements are [CBSE AIPMT 2010]
(a) I and III
(b) I and III
(c) II and IV
(d) I and II
Answer/Explanation
Ans. (c)
Statement II and IV are true about lac operon. In prokaryotes, a hypothesis was given in 1961 to explain the protein synthesis regulation. This hypothesis was given by F Jacob and $\mathrm{J}$ Monod and for this they were awarded Nobel Prize in 1965 , the hypothesis was known by the name of Operon Model.
The operator gene is the segment of DNA, which exercise a contral over transcriptions. In the absence of lactose, the repressor binds with the operator gene.
Question
Differentiation of organs and tissues in a developing organism is associated with [CBSE AIPMT 2007]
(a) developmental mutations
(b) differential expression of genes
(c)lethal mutations
(d) deletion of genes
Answer/Explanation
Ans. (b)
Differentiation of organs and tissues in a developing organism is associated with differential expression of genes. In regulation of gene expression the chromosomal proteins play important role.
The chromosomal proteins are of two types, histones and non-histones. The regulation of gene expression involves an interaction between histones and non-histones.
Question
What does lac’ refer to in what we call the loc operon? [CBSE AIPMT 2003]
(a) Lac insect
(b) The number, $1,00,000$
(c) Lactose
(d) Lactase
Answer/Explanation
Ans. (c)
Lac operon refers to the DNA sequence in the genome of the bacterium E. coli encoding enzymes involved in lactose uptake and metabolism.
Question
$\ln E$. coli, during lactose metabolism repressor binds to [CBSE AIPMT 2002]
(a) regulator gene
(b)operator gene
(c) structural gene
(d) promoter gene
Answer/Explanation
Ans. (b)
In Lac operon, the repressar protein combines with the operator gene to express its functioning.
Question
Jacob and Monod studied lactose metabolism in E. coli and proposed Operon concept. Operon concept applicable for [CBSE AIPMT 2002]
(a) all prokaryotes
(b) all prokaryotes and some eukaryotes
(c) all prokaryotes and all eukaryotes
(d) all prokaryotes and some protozoans
Answer/Explanation
Ans. (b)
Jacob and Monod’s operon concept is basically a theory of gene expression in prokaryotes- though it is of some value in the explanation of eukaryotic gene expression.
Question
In negative operon [CBSE AIPMT 2001]
(a) co-repressor binds with repressor
(b) co-repressor does not bind with repressor
(c) co-repressor binds with inducer
(d) cAMP has negative effect on lac operon
Answer/Explanation
Ans. (a)
In negative operon co-repressor binds with repressor to form repressor co-repressor complex which further binds with operator. Since, the product of the regulator (the repressor) acts by shutting off the transcription of structural genes, it is referred to as a negative control system.
Question
Genes that are involved in turning on or off the transcription of a set of structural genes are called [CBSE AIPMT 1998]
(a) polymorphic genes
(b) operator genes
(c) reductant genes
(d) regulatory genes
Answer/Explanation
Ans. (d)
The switching on and off of an operator is controlled by respressor protein which is coded by the regulator gene $R$.
Question
The wild type $E_{\text {. coli cells are }}$ growing in normal medium with glucose. They are transferred to a medium containing only lactose as sugar. Which of the following changes takes place? m[CBSE AIPMT 1995]
(a) The lac operon is repressed
(b) All operons are induced
(c) The lac operon is induced
(d) E. coli cells stop dividing
Answer/Explanation
Ans. (c)
Inducible genes are the genes which remain inactive or repressed in a cell and can be activated when a certain substrate is to be metabolised. It has been seen when lactose is added to the medium of E.coli the operon is induced and synthesis of enzymes required for degradation of lactose to glucose and galactose starts.
Question
in Escherichia coli lac Operon is induced by [CBSE AIPMT 1994]
(a) lactose
(b) promoter gene
(c) $\beta$-galactosidase
(d) I-gene
Answer/Explanation
Ans. (a)
In E. coli lac operon is induced by adding lactose sugar to the culture.
Question
Binding of specific protein on regulatory DNA sequence can be studied by means of [CBSE AIPMT 1993]
(a) ultra centrifugation
(b) electron microscope
(c) light microscope
(d) X-rays crystallography
Answer/Explanation
Ans. (d)
$X$-ray crystallography is an important technique in molecular biology to analyse the structure and orientation of molecules. It is used to find out 3-D positions of atoms in the molecules of DNA, RNA and proteins, binding of specific protein on regulatory DNA sequences, 3-D structure of haemoglobin, insulin, DNA, proteins, collagen fibre, muscle and actin protein.