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Chapter 9

BIL 250 Chapter 9: Control of Gene Expression


Department
Biology
Course Code
BIL 250
Professor
Skromne Isaac
Chapter
9

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Control of Gene Expression
Chapter 8:
Control of gene expression means transcription and translation. In this chapter we will cover the
process from transcription to protein. The steps that will be covered are listed below:
1. Transcription in both Eukaryotes and Prokaryotes
2. Processing in Eukaryotes
3. Localization control 3UTR
4. Translation control
5. mRNA degradation (Poly-A tail and 3UTR
6. Protein activity control (methylation)
We must know the elements in the DNA sequence or the words on a list work together to make a
masterpiece.
An Overview of Gene Expression:
NOTE: That even the simples single celled bacterium use its gene selectively for example
switching genes ON or OFF to make an enzyme, etc.
That the difference between an information-
processing nerve cell and a white blood cell for
example are so extreme that is difficult to imagine
that the two cells contain the same DNA. That all cells
of a multicellular organism contain the same genome
That cell differentiation is the product of gene
expression, thus all cells in our body contain the
same genome and have the same DNA
A typical differentiated cell express ONLY about
HALF the genes in its total repertoire
Figure 8-1:
A neuron and a liver cell share the same genome
Both of these mammalian cells contain the same genome, but they express many different
RNAs and proteins.
The cell picks and chooses what genes must be expressed depending on its specialty.
Gene Expression: is a complex process by which cells selectively direct the synthesis of the many
thousands of proteins and RNAs encoded in their genome.
Cell Differentiation: arises because cells make and accumulate different sets of RNA and protein
molecules, meaning they express different genes.
The Different cell types of a multicellular
organism contain the same DNA.
NOTE:
That the genome from a differentiated cell is made to
direct the development of a complete organism
That the chromosomes of the differentiated cell were
altered irreversibly during development, they would
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not be able to accomplish this feat. Ex: on FIGURE 8-2:
That these experiments all show that DNA is specialized cell types of multicellular organisms still
contains the entire set of instructions needed to form a whole organism such as the example of the
calf.
That the various cell types of an organism differ NOT BECAUSE THEUS CONTAIN DIFFERENT GENES,
BUT BECAUSE THEY EXPRESS THEM DIFFERENTLY.
Different cell types produce different sets of proteins:
NOTE: That an analysis was performed by 2-D gel electrophoresis or nowadays it s done by a
newer faster and more modern method called mass spectrometry.
This last technique is more sensitive and it enables the detection of even proteins that are produced in
minor quantities. Thus, both techniques revealed that many proteins are common to all cells such as:
Housekeeping genes: DNA replication, DNA transcription, Protein synthesis, Metabolism
enzymes. These types of cells pick and choose what they need to express everywhere in the cell.
Cell specific genes: Oxygen carriers, Hormone synthesis, Neurotransmission, etc. these types of
genes are expressed only in specific cells. They are responsible for the cells distinctive properties.
Also, by cataloging a cells RNA, including mRNA, from its nucleotides we can determine the expression of
different collection of genes in each cell types that causes the large variation seen in the size, shape,
behavior, and function of differentiated cells.
A cell can change the expression of its gens in response to the external signals:
NOTE: The specialized cells in a multicellular organism are capable of altering their patterns of
gene expression in response to extracellular cues.
A great example would be that of a liver and its relation to cortisol.
If the liver cell is exposed to the steroid hormone the production of certain hormone is increased.
It is released when the body is under stress ex: intense exercise, starvation…
Cortisol signals liver cells to boost production of glucose from amino acids and other molecules
When released it induce enzymes that convert tyrosine to glucose for ATP and body use
When the hormone is absent these enzyme undergo a resting level.
Thus, other cells act differently to cortisol, such as fat cells. These scenarios explain on the differentiation
of cells and their different functions have different effect and get affected differently. Thus, giving each
cell its distinctive character
Gene Expression can be regulated at various steps from DNA to RNA to Proteins.
NOTE: There are many steps in the pathway leading from DNA to Protein, and all of them can in
principle be regulated and controlled by the cell.
1. Controlling when and how often a given gene is transcribed
2. Controlling how an RNA transcript is spliced or processed
3. Selecting which mRNA re exported
from the nucleus to the cytosol
4. Regulating how quickly certain
mRNA molecules are degraded
5. Selecting which mRnas are
translated into protein
6. Regulating how rapidly specific
proteins are destroyed
Nonetheless, there are many other
ways that the genes can be regulated
and expressed.
VIEW FIGURE 8-3:
NOTE: The figure shows a
Eukaryotic gene expression
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pathway can be controlled at several different steps.
The help of many enzymes, proteins, and complex mechanisms can control the
translation.
BUT FOR MOST GENES STEP 1 TRANSCRIPTION IS THE MAIN SITE OF CONTROL
because transcriptional control can ensure that no UNNECESSARY INTERMEDIATES
are synthesized.
How Transcriptional Switches Work? ----- STEP I
NOTE: the binding of proteins to a specific DNA sequences controls
Transcription
DNA sequences: contains Promoter, Operators
(Prokaryotes), and enhancers (Eukaryotes).
PROKARYOTES:
They have a 5 UTR at and 3UTR Untraslated Regions used for
translation. These regions embed a coding region shown in green
They carry a promotor, this promotor embedes an operator that is
garded by 2
promotor
sequences at -
35 and -10 upstream of start transcription (+1).
EUKARYOTES:
They are more complexed than PROK and have more complexed transcriptional machine.
They have a TATA BOX. An enhancer that activate the promotor usually lovcated between genes, introns,
downstream or upstream they are located everywhere. A repressor that halt trascription. And a
promotor that is ALWAYS found at the 5 OF )N)T)AT)ON S)TE.
Proteins: Such as Transcriptional Regulators. Three elements of transcriptional gene regulation
Regulatory gene sequence
Transcriptional regulator
o DNA binding domain
o Activator/repressor domain
NOTE: The transcription regulatory has 2 domains: One act as repressor / activator
The other act as DNA binding domain.
Method to identify DNA binding proteins: Electrophoretic Mobility Shift Assay (EMSA):
Experiment was conducted to identify DNA biding.
The DNA was radioactivaated and ran on a electrphrosis.
DNA binding domain
RNA pol.
Activator/Repressor domain
ACTIVITY
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