Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
Match column-I with column-II
Column-I Column-II
A. Operator site I. Binding site for
RNA polymerase
B. Promoter site II. Binding site for
repressor molecule
C. Structural gene III. Codes for enzyme protein
D. Regulator gene IV. Codes for repressor
molecules
(a) A® (II); B ® (I); C ® (III); D ® (IV)
(b) A® (II); B ® (I); C ® (IV); D ® (III)
(c) A® (IV); B ® (III); C ® (I); D® (II)
(d) A® (II); B ® (III); C ® (I); D® (IV)
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Answer: a
👉In prokaryotes, control of the rate of transcriptional initiation is the
predominant site for control of gene expression. In a transcription unit,
the activity of RNA polymerase at a given promoter is in turn regulated
by interaction with accessory proteins, which affect its ability to recognise
start sites. These regulatory proteins can act both positively (activators)
and negatively (repressors). The accessibility of promoter regions of
prokaryotic DNA is in many cases regulated by the interaction of proteins
with sequences termed operators. The operator region is adjacent to the
promoter elements in most operons and in most cases the sequences of
the operator bind a repressor protein. Each operon has its specific
operator and specific repressor. For example, lac operator is present
only in the lac operon and it interacts specifically with lac repressor only.
👉The Lac operon
The elucidation of the lac operon was also a result of a close association
between a geneticist, Francois Jacob and a biochemist, Jacque Monod. They
were the first to elucidate a transcriptionally regulated system. In lac operon
(here lac refers to lactose), a polycistronic structural gene is regulated by a
common promoter and regulatory genes. Such arrangement is very common
in bacteria and is referred to as operon. To name few such examples, lac
operon, trp operon, ara operon, his operon, val operon, etc.
The lac operon consists of one regulatory gene (the i gene – here the
term i does not refer to inducer, rather it is derived from the word inhibitor)
and three structural genes (z, y, and a). The i gene codes for the repressor
of the lac operon. The z gene codes for beta-galactosidase (b-gal), which
is primarily responsible for the hydrolysis of the disaccharide, lactose
into its monomeric units, galactose and glucose. The y gene codes for
permease, which increases permeability of the cell to b-galactosides. The
a gene encodes a transacetylase. Hence, all the three gene products in
lac operon are required for metabolism of lactose. In most other operons
as well, the genes present in the operon are needed together to function
in the same or related metabolic pathway (Figure 6.14).
👉Lactose is the substrate for the enzyme beta-galactosidase and it
regulates switching on and off of the operon. Hence, it is termed as inducer.
In the absence of a preferred carbon source such as glucose, if lactose is
provided in the growth medium of the bacteria, the lactose is transported
into the cells through the action of permease (Remember, a very low level
of expression of lac operon has to be present in the cell all the time,
otherwise lactose cannot enter the cells). The lactose then induces the
operon in the following manner.
👉The repressor of the operon is synthesised (all-the-time – constitutively)
from the i gene. The repressor protein binds to the operator region of the
operon and prevents RNA polymerase from transcribing the operon. In
the presence of an inducer, such as lactose or allolactose, the repressor is
inactivated by interaction with the inducer. This allows RNA polymerase
access to the promoter and transcription proceeds (Figure 6.14).
Essentially, regulation of lac operon can also be visualised as regulation
of enzyme synthesis by its substrate.
👉Remember, glucose or galactose cannot act as inducers for lac
operon. Can you think for how long the lac operon would be expressed
in the presence of lactose?
👉Regulation of lac operon by repressor is referred to as negative
regulation. Lac operon is under control of positive regulation as well.