Transcript 13.4 Gene Regulation and Expression

How Does A Cell Know?
Which Gene To Express
&
Which Gene Should Stay
Silent?
› @To conserve resources, prokaryotes regulate
their activities, producing only those genes
necessary for the cell to function.@
› It would be wasteful for a bacterium to produce
enzymes that are needed to make a molecule that
is readily available from its environment. (why
waste energy if you already have food?)
•
What is an Operon?
• Group of Genes That Operate
Together
•
For Example:
– E. coli ferments lactose
• To Do That It Needs Three Enzymes
(Proteins), It Makes Them All At Once!
–3 Genes Turned On & Off Together. This is
known as the lac Operon (lactose Operon)
@The lac Operon
Regulates Lactose Metabolism@
– It Turns On Only When Lactose Is Present &
Glucose is Absent.
–
Lactose is a Disaccharide
–
A Combination of Galactose & Glucose
To Ferment Lactose E. coli Must:
1. Transport Lactose Across Cell Membrane
2. Separate The Two Sugars
› The first is a promoter (P), which is a site where
RNA-polymerase can bind to begin transcription.
› The other region is called the operator (O),
which is where the lac repressor can bind to
DNA.
Key Concept:
The lac Genes Are:
Turned Off By Repressors
And
Turned On By The Presence Of
Lactose
› Most Eukaryotic Genes Are Controlled
Individually And Have Regulatory
Sequences That Are Much More Complex
Than Prokaryotic Gene Regulation

For years, biologists wondered why cells
that contain lots of small RNA molecules,
only a few dozen bases long, and don’t
belong to any of the major groups of RNA
(mRNA, tRNA, or rRNA)
› @Blocking gene expression by means of an
miRNA silencing complex is known as RNA
interference (RNAi).@
› As an embryo develops, different sets of genes
are regulated by transcription factors, enhancers,
and repressors.
› Gene regulation helps cells undergo
differentiation, becoming
specialized in structure
and function.
› Edward B. Lewis was the first to show that a
specific group of genes controls the identities of
body parts in the embryo of the common fruit fly.
› Lewis found that a mutation in one of these
genes actually resulted in a fly with a leg growing
out of its head in place of an antenna!
› These master control genes, homeobox genes,
activate genes important in cell development
•
Hox Genes
– @Control Organ &
Tissue Development In
The Embryo@
– Mutations Lead To
Major Changes
• Drosophila With Legs In
Place of Antennae
Hox Genes Present In All Eukaryotes
– Shows Common Ancestry
– Pax 6 hox gene
• Controls eye growth in Drosophila, Mice
& Man
• Pax 6 from Mouse Placed In Knee
Development Sequence Of Drosophila
Developed Into Eye Tissue.
Common Ancestor >600M Years Ago
› Nearly all animals,
share the same basic
tools for building the
different parts of the
body.
› Common patterns of
genetic control exist
because all these
genes have descended
from the genes of
common ancestors.
› @In prokaryotes and eukaryotes,
environmental factors can influence gene
expression.@ Ex: temperature, salinity,
nutrient availability
› Ex: The lac operon in E. coli is switched on only
when lactose is the only food source in the
bacteria’s environment.
rRNA
2. tRNA
3. mRNA
4. miRNA
1.

The Lac operon turns off

The Lac operon turns on
miRNA
2. Homeobox Genes
3. Operator
4. Promoter
1.