Trimester 3
8th Grade
Control of Gene Expression – Chp 11
11.1 Proteins interacting w/ DNA turn Prokaryotic genes on or off in response to environmental
changes
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Gene Regulation: turning on and off of genes
Gene Expression: overall process of information flow from genes to proteins
The control of gene expression allows cells to produce specific kinds of proteins when and
where they are needed
Our earlier understanding of gene control came from the study of E. Coli
A cluster of genes with related functions, along with control sequences, is called an operon
With few exceptions, operons exist only in Prokaryotes
When an E. Coli encounters Lactose, all enzymes needed for its metabolism are made at once
using the Lactose operon
The Lactose (lac) operon includes
1) Three adjacent lactose-utilization genes
2) A promoter sequence where RNA polymerase binds and initiates transcription
of all 3 lactose genes and
3) An operator sequence where a repressor can bind and block RNA polymerase
action
Regulation of the Lac operon
o A regulatory gene, located outside the operon, codes for a repressor protein
o In the absence of lactose, the repressor binds to the operator and prevents RNA
polymerase action
o Lactose inactivates the repressor, so
 The operator is unblocked
 RNA Polymerase can bind to the promoter
 All 3 genes of the operon are transcribed
There are 2 types of repressor-controlled operons
o In the Lac Operon, the repressor is
 Active when alone
 Inactive when bound to Lactose
o In the trp bacterial operon, the repressor is
 Inactive when alone
 Active when bound to the amino acid Tryptophan (Trp)
Another type of operon control involves activators, proteins that turn operons on by
o Binding to DNA and
o Making it easier for RNA polymerase to bind to the promoter
Activators help control a wide variety of operons
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Trimester 3
8th Grade
11.2 Chromosome Structure & Chemical Modifications can affect Gene Expression
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Differentiation
o Involves cell specialization, in structure and function, and
o Is controlled by turning specific sets of genes on or off
Almost all of the cells in an organism contain an identical genome
The differences btwn cell types are
o Not due to the presence of different genes but instead
o Due to selective gene expression
Eukaryotic chromosomes undergo multiple levels of folding and coiling, called DNA packaging
Chemical modification of DNA bases or histone proteins can result in epigenetic inheritance
o Certain enzymes add a methyl group to DNA bases, without changing the sequence of
the bases
o Individual genes are usually more methylated in cells in which the genes are not
expressed. Once methylated, genes usually stay that way through successive cell
divisions in an individual
o Removal of the extra methyl groups can turn on some of these genes
o Inheritance of traits transmitted by mechanisms not directly involving the nucleotide
sequence is called epigenetic inheritance. These modifications can be reversed.
11.3 Complex Assemblies of Proteins Control Eukaryotic Transcription
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Eukaryotic RNA polymerase requires the assistance of proteins called Transcription Factors.
Transcription Factors include
o Activator proteins, which bind to DNA sequences called enhancers and initiate gene
transcription. The binding of the activators leads to bending of the DNA
o Other transcription factor proteins interact with the bound activators, which then
collectively bind as a complex at the gene’s promoter
RNA polymerase then attends to the promoter and transcription begins
Silencers are repressor proteins that
o May bind to DNA sequences
o And inhibit transcription
11.4 Eukaryotic RNA may be spliced in more than one way
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Alternative RNA splicing
o Produces different mRNAs from the same transcript
o Results in the production of more than one polypeptide from the same gene and
11.5 Small RNAs play multiple roles in controlling gene expression
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MicroRNAs (miRNAs) can bind to complementary sequences on mRNA molecules either
o Degrading the target mRNA or
o Blocking its translation
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Trimester 3
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8th Grade
RNA Interference (RNAi) is the use of miRNA to artificially control gene expression by injecting
miRNAs into a cell to turn off a specific gene sequence
11.6 Later Stages of Gene Expression are also subject to regulation
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After mRNA is fully processed and transported to cytoplasm, gene expression can still be
regulated by
o Breakdown of mRNA
o Initiation of translation
o Protein activation
o Protein breakdown
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