MBI- Gene Regulation Flashcards
what are the points at which regulaton takes place
(i) transcriptional level (formation of primary transcript),
(ii) processing level (regulation of splicing),
(iii) transport of mRNA from nucleus to the cytoplasm,
(iv) translational level.
give two examples that environment controls the expression of genes
The genes in a cell are expressed to perform a particular function or a
set of functions. For example, if an enzyme called beta-galactosidase is
synthesised by E. coli, it is used to catalyse the hydrolysis of a
disaccharide, lactose into galactose and glucose; the bacteria use them
as a source of energy. Hence, if the bacteria do not have lactose around
them to be utilised for energy source, they would no longer require the
synthesis of the enzyme beta-galactosidase. Therefore, in simple terms,
it is the metabolic, physiological or environmental conditions that regulate
the expression of genes. The development and differentiation of embryo
into adult organisms are also a result of the coordinated regulation of
expression of several sets of genes
when does gene control take place in most prokaryotes
In prokaryotes, control of the rate of transcriptional initiation is the
predominant site for control of gene expression.
how is gene expression controlled during transcriptional initiation
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).
what is an operator? what is its function
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.
who discovered 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.
what is an operon
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.
what are 4 genes present in lac operon
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).
what do the 4 genes code for
The i gene codes for the repressor
of the lac operon. The z gene codes for beta-galactosidase (β-gal), which
is primarily responsible for the hydrolysis of the disaccharide, lactose
into its monomeric units, galactose and glucose. The y gene codes for
lactose permease, which increases permeability of the cell to β-galactosides. The
a gene encodes a b-galactoside 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
function of the 3 enzymes
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.
functioning of lac operon
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 5.14).
Essentially, regulation of lac operon can also be visualised as regulation
of enzyme synthesis by its substrate.
what is the type of regulation
Regulation of lac operon by repressor is referred to as negative
regulation. Lac operon is under control of positive regulation as well
define inducer
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
