Transcript Controlling Gene Expression

Controlling Gene Expression
Control Mechanisms
Determine when to make more proteins
and when to stop making more
 Cell has mechanisms to control
transcription and translation
 Housekeeping genes are genes that are
always needed, and are constantly
synthesizing proteins (switched on)

There are four levels of control:
◦ transcriptional (controls transcription from
DNA to mRNA)
◦ posttranscriptional (controls the removal
of introns)
◦ translational (controls rate that mRNA is
activated through ribosomes)
◦ posttranslational (affects the rate proteins
can leave the cell)
The lac Operon
An example of control mechanism is the
lac operon
 Operon: a cluster of genes under the
control of one promoter and one
operator


Operator: regulatory sequence of DNA
to which a repressor protein binds
The lac Operon in action!

http://www.youtube.com/watch?v=oBwtx
dI1zvk
Lactose:
* disaccharide found in milk, broken down by E. coli
•B-galactosidase: enzyme responsible for the degradation of
lactose
The lac operon is a cluster of three genes that code for the metabolism of
lactose:
lacZ codes for B-galactosidase
lacY codes for B-galactosidase permease which lets lactose cross the cell
membrane
lacA codes for transacetylase; unknown function.
Repressed State
Repressed State

LacI protein (blue block) is a
repressor protein that blocks the
transcription of lacZ; binds to the lactose
operator and inhibits RNA polymerase by
covering the promoter site (switch is of
when there is no lactose in the system).
Repressed State

If there is no lactose present, lac operon
genes are not transcribed or translated.
Lactose is an effector/inducer; if it is
present, it induces the removal of the
repressor and allows the lac genes to be
transcribed and translated. (switch is on
if there is lactose in the system)
Induced State

When lactose binds to LacI protein, it
changes and the new complex cannot
bind to the operator of the lac operon.
This results in RNA polymerase being
able to bind to the DNA and start
protein synthesis.
Induced State

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