Gene Expression - mcvsd

butterbeanscubaBiotechnology

Dec 12, 2012 (4 years and 9 months ago)

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Gene Regulation &
Genetic Engineering

Our passport to the future….


Gene Expression



Gene Expression

is a process by which
a gene’s DNA sequence is turned into
functional protein



Marked by the production of
mRNA

in
a cell



All cells contain the same genome, but
not all genes are transcribed &
translated



Specific cues to express genes include:
temperature changes, nutrients in the
external environment, hormones, or other
complex signals

Gene Control



Gene regulation

is a
process by which genes are
controlled

(turned on and off)
in response to signals



Cells regulate the genes that
are expressed



Prokaryotic & eukaryotic cells
regulate a bit differently



Transcriptional

regulation (common to both cells)
refers to controlling the amount of
mRNA

transcribed.

Transcriptional Regulation



Once the amount of
mRNA

is controlled, the amount of
protein

is indirectly controlled



RNA polymerase binds at the
promoter

(found
“upstream” of gene sequences), which indicates the
beginning site for transcription



Eukaryotic

promoters commonly
have a
CAAT

box

(GGCCAATCT)
about 80 nucleotides upstream of the
start site of gene and a
TATA

box

(TATAAAA) about 30 nucleotides
upstream of start site of gene



These bind transcription factors

Transcription Factors

Transcription factors

are DNA binding proteins that
regulate the binding of RNA polymerase to stimulate
transcription of a gene.



Bind to the TATA & CAAT
boxes to stimulate
transcription



Bind upstream to either
enhance

or
repress

transcription of a gene by
assisting

or
blocking

RNA
polymerase binding

Enhancers



Many regulated genes
contain
enhancers



They are regulatory
DNA
sequences
, about
50 or more base pairs




They are found upstream (occasionally downstream),
and can be bound by
regulatory proteins



Can be located far, but a loop in DNA causes contact



The
regulatory proteins

are called
activators

Activators



Activators

are
DNA
-
binding proteins
that regulate one or
more genes by
increasing the rate of
transcription



They bind to particular enhancers




They interact with transcription factors and RNA
polymerase to
activate

or
enhance
transcription of a gene



Examples include certain hormones

Repressors



A
repressor

is a
DNA
-
binding protein
that regulates the
expression of one or
more genes by
decreasing

the rate
of transcription



Repressors bind
within

the promoter,
preventing

RNA
polymerase from binding



Identifying promoters, enhancer sequences, and
transcription factors is important for making biotech products!

Quick Review

1.
What is the mark of gene expression?

2.
What cues may turn genes on or off?

3.
Define promoter and name 2 regions of
eukaryotic promoters.

4.
Define transcription factor.

5.
Define enhancer.

6.
Define activator.

7.
Define repressor.

Prokaryotic Regulation



Studying bacterial regulation of genes is very
important for many biotechnology
applications
.



Many bacterial genes are organized in arrangements
called
operons
.

Operons



Operons

are clusters of
several
related genes

that are
located together and
controlled by a single
promoter.



Many genes controlling
nutrient metabolism

by
bacteria are organized as
operons.

lac operon



Classic example

of gene regulation =
lac

operon



Consists of 3 genes:



lac z

(encodes ß
-
galactosid
ase
)



lac y

(encodes perme
ase
)



lac a

(encodes acetyl
ase
)



These 3 enzymes are necessary for transport &
breakdown of lactose (important energy source) by
bacterial cells.

lac operon basics



lac for lactose!



Lactose is transported into the
cell using
permease



Lactose is degraded into
glucose + galactose by the
enzyme
ß
-
galactosidase



The function of
acetylase

is not
clear (protection possibly?)

lac operon (on)

lac operon in absence of lactose



Lac operon regulated by protein
lac

repressor
(encoded by
lac i

gene
-

a
regulator gene upstream of operon).



Repressor protein binds to
operator




An
operator

is a segment of DNA that
regulates the activity of the structural genes of
an operon that it is linked to, by interacting with
a specific repressor or activator.



Blocks RNA polymerase from binding to promoter



Blocks transcription of z, y, a genes

lac operon (off)

Inducible Operon



Bacteria grown with lactose; sugars are
inducers

(stimulate transcription by
disabling repressor protein)



Lactose binds to lac repressor (changing
its shape; thus, preventing binding of
repressor to operator)




RNA polymerase binds to promoter to stimulate
transcription of the operon.



This is called an
inducible operon
. (Enzymes needed
when nutrient is present)
animation

Trp operon



trp for tryptophan, an amino
acid (pictured right)



Produces 5 enzymes that
are part of an
anabolic

pathway for the synthesis of
tryptophan (amino acid
pictured above).



Regulator gene codes for
inactive
repressor (enzymes
ordinarily made).

Repressible operon



In presence of tryptophan, the
enzymes are not needed



Tryptophan binds to repressor
(changes shape & is now able to
attach to operator & genes are
not

expressed)



Hence, tryptophan is called a
corepressor
.



This is an example of a
repressible operon

(enzymes
needed when nutrient is not present).



trp operon
animation

Trp operon