Biology 1002B Chapter Notes - Chapter 14: Regulatory Sequence, Operon, Cytogenetics

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6 Mar 2017
Chapter 14: Control of Gene Expression
- Biochemical and cytogenetic analyses have shown that all nucleated cells of a developing
embryo retain essentially the same set of genes that was created in the original single-
celled zygote at fertilization
- Structural and functional differences in cell types result from the presence or absence of
the products resulting from expression of genes
- All gene expression initially results in RNA products made by transcription
o mRNA further directs the synthesis of protein products by translation
o All cells contain genes coding for the rRNA molecules needed for ribosome
function, as well as genes coding for various hemoglobin polypeptides
o Particular hemoglobins are found only in those cells that give rise to red blood
cells in the fetus, newborn, or adult
- When a gene is “turned on,” it’s more likely to be transcribed actively
- The expression of gene products is subject to further controls affecting the processing of
RNA, possible translation into protein, and the activity of the product itself
14.1 Regulation of Gene Expression of Prokaryotic Cells
- Transcription and translation are closely regulated in prokaryotic cells
- Prokaryotic organisms tend to be single celled and relatively simple, with generation
times measured in minutes
- Prokaryotic cells typically undergo rapid and reversible alterations in biochemical
pathways that allow them to adapt quickly to changes in their environment
- A versatile and responsive control system allows the bacterium to make the most efficient
use of the particular array of nutrients and energy sources available at any given time
14.1a) The Operon Is a Unit of Transcription
- For a typical metabolic process, several genes are involved, and they must be regulated in
a coordinated fashion
o Ie. 3 genes encode proteins for the metabolism of lactose by E. coli
o In the absence of lactose, the 3 genes are transcribed very little, whereas in the
presence of lactose, the genes are transcribed quite actively
o The on/off control of these genes is at the level of transcription
- In 1961, François Jacob and Jacques Monod proposed the operon model for the control of
the expression of genes for lactose metabolism in E. coli
- Operon: a cluster of prokaryotic genes and DNA sequences involved in their regulation
- Promoter: a region where the RNA polymerase begins transcription
- Operator: a short segment that is a binding sequence for a regulatory protein (a DNA-
binding protein that binds to a regulatory sequence and affects the expression of an
associated gene or genes)
- Some operons are controlled by a regulatory protein, a repressor, which when bound to
the DNA, reduces the likelihood that genes will be transcribed
- Other operons are controlled by a regulatory protein, an activator, which, when bound to
the DNA, increases the likelihood that genes will be transcribed
- Each operon, which can contain several to many genes, is transcribed as a unit from the
promoter into a single messenger RNA (mRNA), and, as a result, the mRNA contains
codes for several proteins
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- Transcription unit: a region of DNA that transcribes a single primary transcript
- A ribosome translates the entire mRNA from one end to the other, sequentially making
each protein encoded in the mRNA
14.1b) Lac Operon Is Transcribed When an Inducer Inactivates a Repressor
- Jacob and Monod researched the genetic control of lactose metabolism in E. coli through
a series of genetic and biochemical approaches
- Their studies showed that metabolism of lactose as an energy source involves three
genes: lacZ, lacY, and lacA (see figure 14.2)
o These genes are adjacent to one another on the chromosome in the order Z-Y-A
o The genes are transcribed as a unit into a single mRNA starting with the lacZ
gene; the promoter for the transcription unit is upstream of lacZ
- The lacZ gene encodes the enzyme β-galactosidase, which catalyzes the conversion of the
disaccharide sugar, lactose, into the monosaccharide sugars, glucose and galactose
o These sugars are then further metabolized by other enzymes, producing energy for
the cell by glycolysis and Kreb’s cycle
- The lacY gene encodes a permease enzyme that transports lactose actively into the cell,
and the lacA gene encodes a transacetylase enzyme, the function of which is more
relevant to metabolism of compounds other than lactose
- The lac operon was controlled by a regulatory protein that they termed the Lac repressor.
- The Lac repressor is encoded by the regulatory gene lacI, which is nearby but separate
from the lac operon and is synthesized in active form
o When lactose is absent from the medium, the Lac repressor binds to the operator,
thus blocking the RNA polymerase from binding to the promoter
o Repressor binding is a kind of equilibrium; while it is bound to the operator most
of the time, it occasionally comes off
o When the repressor is not bound, polymerase can successfully transcribe
there’s always a low concentration of lac operon gene products in the cell
Figure 14.2: The lacZ, lacY, and lacA genes encode the enzymes taking part in lactose metabolism. The
separate regulatory gene, lacI, encodes the Lac repressor, which plays a pivotal role in the control of the operon.
The promoter binds RNA polymerase, and the operator binds the activated Lac repressor. The transcription
unit, which extends from the transcription initiation site to the transcription termination site, contains the genes.
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