Theme 3 module 2

Question 1

Overview of e.coli

Answer 1

when E. coli cells are in an
environment that contains both glucose and the disaccharide lactose, bacteria will metabolize allthe glucose before switching to utilizing lactose as
a fuel source
This is an example of a highly
regulated process. In fact, this metabolic shift in nutrient source utilization is tightly regulated in
bacteria such that they are able to quickly upregulate the expression of genes that produce lactose-metabolizing enzymes when glucose is no longer available, and lactose is the predominant nutrient source
the control of enzyme production is regulated at the transcriptional level

Question 2

How is this metabolic shift enabled?

Answer 2

As E. coli cells utilize the nutrients that are available in the environment, the bacterial cells are able to detect
environmental cues that facilitate this transition from glucose to lactose metabolism

Question 3

Two important cues can be detected by E. coli cells

Answer 3

Changes in glucose levels
and the presence of lactose

Question 4

As E. coli cells transition from glucose to lactose metabolism

Answer 4

there is a concomitant increase in the
amount of detectable beta-galactosidase and
lactose permease proteins
but are soon expressed as they are required for the transport and metabolism of lactose

Question 5

Two gene products are not
expressed

Answer 5

until glucose is fully depleted from the
growth medium

Question 6

Once glucose is depleted

Answer 6

the bacterial cells are starting to
utilize lactose, that there is also an increase in the
expression of the beta-galactosidase and lactose
permease proteins

Question 7

The expression of beta-galactosidase and lactose permease occurs

Answer 7

in response to lactose
metabolism in a medium where glucose nutrients
have been depleted
These proteins are
produced in response to environmental cues and
allows for the effective digestion of lactose

Question 8

lactose permease

Answer 8

is a transport protein that sits in
the bacterial cell membrane and allows for the
transport of lactose into the bacterial cells

Question 9

beta-galactosidase protein

Answer 9

is the cytoplasmically
situated bacterial enzyme that cleaves the imported lactose into glucose and galactose

Question 10

The expression of these two proteins is linked based on their functional relatedness

Answer 10

A key advantage to the organization of the prokaryotic genome is that groups of related genes with similar functions
can often be found clustered together into operons
This is markedly different from
eukaryotes in which each gene has its own promoter and enhancers
This organizational
difference in prokaryotes leads to the ability to
control the transcription of the whole gene cluster
as one unit

Question 11

operon model

Answer 11

basic model for this type of control of gene expression in bacteria was discovered in 1961 by Francois Jacob and Jacques Monod
In bacteria, groups of functionally related genes are organized into
transcriptional units along the bacterial
chromosome. These gene clusters are often coordinately controlled by a single “on-off switch” that can control the transcription of the clustered
genes. This coordination is mediated by an operon

Question 12

The bacterial operon

Answer 12

consists of a promoter, an
operator (or on-off switch), and the coordinated gene cluster whose products will function in a common
pathway or cellular response

Question 13

operator

Answer 13

a sequence of nucleotides near the start of the operon that can be regulated to allow or inhibit transcription

Question 14

When the operator is not bound to any
transcriptional inhibitor

Answer 14

the RNA polymerase
can attach to the promoter and transcribe the genes
in the operon

Question 15

transcription in bacteria

Answer 15

can give rise to
one long mRNA molecule (or polycistronic mRNA)
that can code for many proteins

Question 16

During translation,
bacterial cells can then

Answer 16

produce separate
polypeptides because the polycistronic mRNA is
punctuated with start and stop codons that signal
where the coding sequence for each polypeptide
begins and ends

Question 17

lac operon

Answer 17

model of transcriptional regulation in prokaryotes
regulation of beta galactosidase and lactose permease
Regulatory sequences of transcription exist within
the lac operon
These include the promoter that
binds the RNA polymerase complex, and the operator (lacO) which is the binding site for a repressor protein that is expressed by the lacI coding sequence
Two main structural genes
code for the primary proteins that are needed to
facilitate lactose metabolism. These include the
lacY gene which codes for a lactose permease
transport protein which will embed itself in the cell
membrane and allow for the import of lactose into
the bacterial cell, and the lacZ gene which codes
for B-galactosidase that will be able to cleave the
disaccharide lactose into glucose and galactose