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Section 8.6: Gene Expression
and Regulation
Biology
Objectives
1. Why do prokaryotes control transcription so carefully?
2. How do specific sequence control transcription in
prokaryotes?
3. Describe the use of the lac operon to study transcription.
4. What are different ways that eukaryotes control gene
expression?
5. Describe how the start of transcription requires specific
patterns in eukaryotes.
6. Describe mRNA processing.
Prokaryotes
 Control of gene expression allows for prokaryotes to
respond to stimuli
 Allows them to conserve energy and materials
 Transcription and translation both occur
simultaneously in the cytoplasm
 Therefore, the best way to control gene expression is
to control transcription (start of protein making
process)
Genes
 Genes include more than just the protein-coding
sequence
 Often include sequences that control its expression
 Two major control sequences are:
 Promoters
 Operators
Promoters
 DNA segment that allows DNA to be transcribed
 Helps RNA polymerase find where a gene starts
Operators
 DNA segment that can turn gene “on” or “off ”
 Can interact with proteins that increase the rate of
transcription of stop transcription from occurring.
Bacterial Genes
 Bacteria have less DNA than other organisms
 Genes organized into operons
 Operon: region of DNA that includes a promoter, an
operator, and the genes that code for the protein
 Found only in prokaryotes and round worms
The lac Operon
 One of the first examples of gene regulation in bacteria
 Consists three genes, all of which code for enzymes that
breaks down lactose
 All under the control of a single promoter and operator
The lac Operon
 Turned off when lactose is absent
 Saves the cell’s resources
 When lactose is present, the lac operon is switched on
Gene Regulation In
Eukaryotes
 Eukaryotes have different sets of genes, and these genes
are only utilized in certain types of cells
 Many different ways to control gene expression at
different points in time
 The start of transcription is often the most utilized way to
control gene expression
Starting Transcription
 Many elements work together to control the start of
transcription
 Types of regulatory DNA Sequences:
 Close to the gene
 Far away and are looped close
 Some are found in almost all eukaryotic cells
 Some genes control the transcription of other genes
Regulation of Transcription
 Regulatory Sequences:
 Sequences recognized by transcription factors that bind to
the DNA, so that RNA polymerase can bind
 Far off regulatory sequences:
 If far way, often the DNA will loop so that the regulatory
sequence is close
 Promoter:
 TATA box, that helps start transcription
 Enhancers and silencers can also speed up or slow down
the rate of transcription
Regulation of Transcription
 Genes Controlling Genes
 Some genes code for proteins that control the expression
of other genes
 Example: Sonic Hedgehog
mRNA Processing
 mRNA is generally edited prior to leaving the nucleus
 Cap: added to the beginning of mRNA
 helps bind to ribosome
 keeps it from getting broken down
 Tail: help mRNA exit the nucleus
 Editing of Code:
 Introns: nucleotide sequences that do not code for an
amino acid
 Exons: nucleotide sequence that codes for an amino acid
Introns
 Regulate Gene Expression
 Protect DNA from mutations
 Sometimes different editing can give you a different
protein
 Seen often in viruses