Control of Gene Expression
Objectives:
1. Describe how cells are composed of
genes and gene products that control
cell activity
2. How do regulatory genes conserve
resources and maintain certain levels of
protein production
3. How gene expression can be controlled
by the intron-exon patterns
Role of Gene Expression
• Cells have ways to control which genes
make proteins and only when the proteins
are needed
• The turning on of a gene is called gene
expression
– This is the turning on of translation which leads
to transcription (you should know the meaning
of these words! )
• All of the genetic material in an individual is
called the genome
Gene Expression in Prokaryotes
• Two scientists studying Escherichia coli (E.
coli) bacteria in the 1960’s figured out how
gene expression was controlled in the
digestion of a specific sugar called lactose
• They found that when the E. coli is around
lactose the bacterial will make three
enzymes (remember enzymes are proteins)
• These enzymes are controlled by three
factors found in the DNA of the bacteria
Gene Expression in Prokaryotes
• When genes code for proteins they are called
structural genes. In E. coli the structural genes
to digest lactose are located next to one another
• Promoters:
– Initiates transcription (remember from last
chapter?)
• Operator: “switch” that controls the ability of
RNA polymerase to move transcribe the genes
• Together the promoter and operator and the
genes that code for the protein are called the
OPERON
Gene Expression in Prokaryotes
• In E. coli the operon that breaks down
lactose was named the lac operon
• The lac operon is only functional when
bacteria is exposed to lactose
Operon “Turned off”
• Without lactose around a “repressor”
protein connects to the operon and stops
transcription
Operon “Turned On”
• When lactose is in the bacteria it
attaches to the repressor protein
changing the shape of the protein and
making it release from the operon
• Now the operon is “switched on” and the
RNA polymerase can make enzymes to
digest the lactose
Graphic Organizer
Turing the Operon “Off”
Turning the Operon “On”
1. A gene that codes for a repressor
is activated
2. A repressor is produced
3. A repressor attaches to the
operator
4. This does not allow RNA
polymerase to attach and turns
the off the lac operon
1. Lactose binds to repressor protein
2. The repressor is removed
3. The operator is free to be bound
by RNA polymerase
4. RNA polymerase binds to open
operator frame and the lac
operon is turn on
Gene Expression in Eukaryotes
• Eukaryotes are VERY different from
prokaryotes
– DNA is in long strands instead of circles
– Genome is much larger
– Organisms are multicellular instead of single
cells
– More complex gene expression
– Operons not found in eukaryotes!
Structure of a Eukaryote Gene
• Recall that DNA coils before mitosis and
meiosis.
• Once these events are over the DNA
uncoils only in regions where the genes
need to be expressed.
• Like prokaryotes, eukaryotes also have
promoters signaling for the RNA
polymerase to bind there.
Structure of a Eukaryote Gene
• Beyond the promoter are two structures
called introns and exons
– Introns – sections of DNA that get
transcribed but not translated, they do not
really code for amino acids
– Exons – sections of DNA that get transcribed
AND translated, these are the actual genes
– Scientists are not 100% sure of why we have
introns but several hypothesis exist
Control After Transcription
• Eukaryotes control gene expression by
modifying RNA after transcription
1. Pre-mRNA is formed (combination of
introns and exons)
2. Introns are removed from pre-mRNA
3. Exons are spliced together
4. mRNA leaves the nucleus and begins
translation once attached to a ribosome
Control at the onset of
Transcription
• Transcription factors are regulatory
proteins in eukaryotic cells
– When they bind to DNA they allow for the
RNA polymerase to bind to the promoter
and start transcribing DNA into RNA
• Enhancers are usually located several
thousand nucleotides away from the
gene.
– They change the shape of DNA and allow for
transcription to take place