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Controls Over Genes
More on Transcription
Promoters are regions on DNA that show
where RNA Polymerase must bind to begin the
Transcription of RNA
Called the TATA box
Transcription factors are also involved (proteins
that mediate the binding of RNA polymerase)
Specific base sequences act as signals to stop
Called the termination signal
mRNA Processing
After the DNA is transcribed into
RNA, editing must be done to the
nucleotide chain to make the RNA
functional
Introns, non-functional segments
of DNA are snipped out of the
chain (RNA splicing)
mRNA Editing
Exons, segments of DNA that code for
proteins, are then rejoined by the enzyme
ligase
A guanine triphosphate cap is added to
the 5” end of the newly copied mRNA
A poly A tail is added to the 3’ end of the
RNA
The newly processed mRNA can then
leave the nucleus
Result of Transcription
CAP
New Transcript
Tail
mRNA Transcript
•mRNA leaves the nucleus through
its pores and goes to the
ribosomes
Why Control Gene
Expression?
Some genes are “on” (being transcribed)
almost all the time
Called housekeeping genes
Examples: ribosome components, enzyme for
basic metabolic pathways
Many genes are only turned on when they
are needed
Why Control?
Transcribing genes that are not needed is
a waste of energy and may interfere with
the status of the cell
Regulation
Respond to a range of stimuli
Prokaryotes respond to external stimuli
(food, enzymes turned on)
Eukaryotes also respond to internal stimuli
(hormones, growth factors)
Regulation
Developmentally regulated
Multicellular organisms progress through
developmental stages
Different genes expressed at different times
during development
Cell specialization
Different genes expressed in different cells
The strategy behind
regulation..
Gene control is control over amount of
gene produced (RNA or protein) in cell
Multiple ways to control the amount of
gene product in a cell
Controlling gene product
amount
1. Rate of transcription – rate mRNA is
produced; faster produced = more
product
2. mRNA degradation – rate mRNA is
broken down; faster broken down = less
product
Controlling gene product
amount
3. mRNA processing – capping, splicing; slower
processing = less product
4. Translation – rate of translation or # of
ribosomes translating; fast/more = more
product
Although control probably involves all of these, the
most understood are changes in the rate of
transcription
Gene Control – lac operon
Lac operon is a gene in bacteria
Bacteria have 3 genes in a row (operon)
that involve breaking down lactose for
energy
In order to be efficient, these genes
should not be expressed unless lactose is
present
Lac Operon - vocab
Regulatory protein – control transcription,
translation, and gene products by interacting
with DNA, RNA, or proteins
Repressor – protein that binds with an operator
on prokaryotic DNA to prevent transcription
Operator – short base sequence between a
promoter and genes; a binding site for
repressors
Lac operon – vocab.
Promoter – piece of DNA where RNA
polymerase can bind and start
transcription
Negative control – regulatory protein that
slows down gene activity
Positive control – regulatory protein that
enhances gene activity
Lac operon vocab.
Operon – a promoter and a pair of
operators that control a bacterial gene
Activator – protein that exerts positive
control over an operon
Figure 15.3a
Page 241
operator
regulatory
gene
transcription,
translation
operator
gene 1
gene 2
promoter
lactose operon
repressor protein
gene 3
Lac operon
Goal 1 – transcription low when lactose is
absent
Lac I (gene upstream from operon)
produces a repressor which binds to
promoter region
Binding of repressor prevents RNA
polymerase from binding and transcribing
genes
Lac operon
Goal 2 – increase transcription when
lactose is present
Allolactose will bind to the repressor,
changing its conformation and causing it
to fall off the promoter site
Promoter site now available for RNA
polymerase to bind; transcription of lac
genes begins
Lac operon
Goal 3 – turn off transcription when
lactose is used up
Allolactose metabolizes, freeing up the
repressor
The free repressor is available to bind the
promoter site and stop transcription
Control of lac operon
Negative control – glucose present repressor inactivates the lac operon
Positive control – lactose present –
activator protein (called CAP) makes
promoter more favorable for RNA
polymerase to bind and begin
transcription
Low Lactose
Repressor binds to operator
Binding blocks promoter
Transcription is blocked
Figure 15.3b
Page 241
High Lactose
allolactose
lactose
mRNA
operator
promoter
operator
RNA
polymerase
gene 1
Figure 15.3c
Page 241
Most Genes Are Turned Off
Cells of a multicelled organism rarely
use more than 5-10 percent of their
genes at any given time
The remaining genes are selectively
expressed
Homeotic Genes
Occur in all eukaryotes
Master genes that control development of
body parts
Encode homeodomains (regulatory
proteins)
Homeobox sequence can bind to
promoters and enhancers
X Chromosome Inactivation
In female mammals, in all cells one of the two X
chromosomes is completely inactivated
Inactivation is random
Inactivated chromosome can be observed in the
interphase nucleus as Barr body
Genes on the inactivated chromosome are not
expressed