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Chapter 8

Microbiology and Immunology 2500A/B Chapter Notes - Chapter 8: Lac Repressor, Lac Operon, Regulatory Sequence

Microbiology and Immunology
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Topic 18 Reading
Chapter 8 - Control of Gene Expression
Nearly all the cells of a multicellular organism contain the same genome. Cell differentiation is
instead achieved by changes in gene expression.
An Overview of Gene Expression
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: that is, they express different genes.
The Different Cell Types of a Multicellular Organism Contain the Same
The evidence that cells have the ability to change which genes they express without altering
the nucleotide sequence of their DNA comes from experiments in which the genome from a
differentiated cell is made to direct the development of a complete organism.
Therefore, differentiated cells contain all the genetic instructions necessary to direct the
formation of a complete organism.
The various cell types of an organism therefore differ not because they contain different
genes, but because they express them differently.
Different Cell Types Produce Different Sets of Proteins
Many proteins are common to all the cells of a multicellular organism.
These housekeeping proteins include, for example, the structural proteins of chromosomes,
RNA polymerases, DNA repair enzymes, ribosomal proteins, enzymes involved in glycolysis
and other basic metabolic processes, and many of the proteins that form the cytoskeleton.
Each different cell type also produces specialized proteins that are responsible for the cell’s
distinctive properties.
This gene expression can be studied by mass spectrometry or electrophoresis to identify the
proteins, or RNA cataloging to identify the mRNAs expressed in that cell.
A Cell Can Change the Expression of it’s Genes in Response to
External Signals
The specialized cells in a multicellular organism are capable of alter- ing their patterns of gene
expression in response to extracellular cues.
Gene Expression Can be Regulated At Various Steps from DNA to RNA
to Protein
A cell can control the proteins it contains by:
I. Controlling when and how often a given gene is transcribed,
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II. Controlling how an RNA transcript is spliced or otherwise processed,
III. Selecting which mRNAs are exported from the nucleus to the cytosol,
IV. Regulating how quickly certain mRNA molecules are degraded,
V. Selecting which mRNAs are translated into protein by ribosomes, or
VI. Regulating how rapidly specific proteins are destroyed after they have been made; in
addition, the activity of individual proteins can be further regulated in a variety of ways.
For most genes, transcriptional control is the most commonly used regulation method. This
makes sense because only transcriptional control can ensure that no unnecessary
intermediates are synthesized.
How Transcriptional Switches Work
Transcription Regulators Bind to Regulatory DNA Sequences
I. The promoter region of a gene binds the enzyme RNA polymerase and correctly orients the
enzyme to begin its task of making an RNA copy of the gene.
II. The promoters of both bacterial and eukaryotic genes include a transcription initiation site,
where RNA synthesis begins, plus a sequence of approximately 50 nucleotide pairs that
extends upstream from the initiation site.
A. This upstream region contains sites that are required for the RNA polymerase to
recognize the promoter, although they do not bind to RNA polymerase directly.
B. Instead, these sequences contain recognition sites for proteins that associate with the
active polymerase—sigma factor in bacteria or the general transcription factors in
III. In addition to the promoter, bacterial & eukaryotic genes, have regulatory DNA sequences
that are used to switch the gene on or off.
A. To have any effect, these sequences must be recognized by proteins called transcription
B. It is the binding of a transcription regulator to a regulatory DNA sequence that acts as
the switch to control transcription.
IV. Proteins that recognize a specific nucleotide sequence do so because the surface of the
protein fits tightly against the surface features of the DNA double helix in that region.
A. Because these surface features will vary depending on the nucleotide sequence,
different DNA-binding proteins will recognize different nucleotide sequences.
B. In most cases, the protein inserts into the major groove of the DNA helix and makes a
series of intimate molecular contacts with the nucleotide pairs within the groove. Many
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transcription regulators form hydrogen bonds (as shown above), ionic bonds, and
hydrophobic interactions with individual bases in the major groove.
C. Many transcription regulators bind to the DNA helix as dimers. Such dimerization roughly
doubles the area of contact with the DNA, thereby greatly increasing the strength and
specificity of the protein–DNA interaction.
Transcriptional Switches Allow Cells to Respond to Changes in Their
-The simplest and best understood examples of gene regulation occur in bacteria and in the
viruses that infect them.
-Bacteria regulate the expression of many of their genes according to the food sources that
are available in the environment. For example, in E. coli, five genes code for enzymes that
manufacture the amino acid tryptophan.
-These genes are arranged in a cluster on the chromosome and are transcribed from a single
promoter as one long mRNA molecule; such coordinately transcribed clusters are called
-Within the operon’s promoter is a short DNA sequence, called the operator, that is recognized
by a transcription regulator.
-When this regulator binds to the operator, it blocks access of RNA polymerase to the
promoter, preventing transcription of the operon and production of the tryptophan-
producing enzymes.
-The transcription regulator is known as the tryptophan repressor —> the repressor can
bind to DNA only if it has also bound several molecules of tryptophan.
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