Transcript Regulation of Gene Expression

SC.912.L.16.6
 All
cells contain a set of genes, which can
be thought of as a set of instructions for
making each of a very large number of
proteins.
 The creation of a protein from its gene is
called gene expression.
 However, for
a given cell not all of these
instructions are actually used, and among
those that are, some are used more than
others or only under certain
circumstances.
 Controlling gene expression is critical to
a cell because it allows it to avoid wasting
energy and raw materials in the synthesis
of proteins it does not need.



Remember, transcription is the process of
making a messenger ribonucleic acid (mRNA)
copy of the deoxyribonucleic acid (DNA) gene.
Transcription can only occur if RNA polymerase
first attaches, or binds, to the DNA.
Controlling this binding process is the major way
that gene expression is controlled, and proteins
are the major controllers of binding.
A
protein molecule that helps regulate
binding can itself be regulated.
 This
usually occurs when some other
molecule binds to the protein, causing the
protein to undergo a structural change, in
other words, to change shape.

In some cases this shape change will help
RNA polymerase to bind to DNA, and in
other cases it will prevent it from doing so.
 Regulation
of gene expression is by
necessity far more complex in eukaryotic
cells (cells with a nucleus) than in
prokaryotic cells.
• eukaryotic cells are larger and more highly
compartmentalized
• multicellular eukaryotes pass through a number of
developmental stages, each requiring different
proteins, on the road to their final differentiated
state.
• multicellular organisms contain many different cell
types, each of which expresses distinct sets of
proteins.
 How
does an organism know whether to
turn a gene on or off?

Operons-a group of genes that operate together

Example: lac operon in E. coli, control genes that code
for proteins that break down lactose for food
• Lac operons are turned on by lactose and turned
off by repressors (a protein that blocks the
transcription of lac genes)
 Many
genes are regulated by repressor
proteins, while others can use proteins to
speed up transcription
Most eukaryotic genes are controlled
individually and have regulatory sequences that
are much more complex than prokaryotic
operons.
 Genes are regulated by enhancer sequences
located at a point where transcription beginsdifferent proteins can bind here causing
transcription to speed up or not take place at all
 Example: TATA box-region of DNA containing
sequence TATATA before the start of
transcription

• Helps position RNA polymerase