12-5 GENE REGULATION
I. GENE REGULATION: AN
EXAMPLE
1.
E. Coli (bacteria) gives
a good example of
how genes are able to
regulate being
expressed
2.
Operon- a group of
genes that operate
(work) together
a) E. Coli use lactose
(milk sugar) as a food
source
b) To eat the lactose the
genes to eat the sugar
must be turned on
c) Lac Operon- genes
that allow E. coli to
eat lactose
3.
4.
a)
b)
Repressor- turns off genes
Operator- area of a chromosome in an operon
that the repressor binds to when the operon is
“turned off”
Lac genes are turned off by the repressor
Lac genes are turned on by the presence of lactose
Repressor
5.
6.
7.
Many genes are
regulated by repressor
proteins
Some genes use
proteins that speed up
transcription.
Sometimes regulation
occurs at the level of
protein synthesis.
Discuss w/ Partner:

1.
2.
3.
4.
Pg. 310 fig. 12-23
When is the repressor protein bound to the
operator?
Can transcription occur when the repressor is
bound to the operator?
Why or why not?
How does the presence of lactose help start
transcription of the lac genes?
Answer:
1.
2.
3.
4.
When is the repressor protein bound to the
operator? When lactose is not present
Can transcription occur when the repressor is
bound to the operator? No
Why or why not? The repressor protein blocks
RNA polymerase from binding to the promoter
How does the presence of lactose help start
transcription of the lac genes? Lactose binds to
the repressor protein, causing it to release from
the operator site, and RNA polymerase can bind
to the promter
II. EUKARYOTIC GENE
REGULATION
Operons are not found in eukaryotic
organisms …..but…
2. Most eukaryotic genes:
a) are controlled individually
b) have regulatory sequences that are
much more complex than those of the
lac operon.
1.
3.
4.
TATA Box- a short region of DNA that helps
start transcription
Helps RNA polymerase bind to DNA
Discuss w/ Partner:

1.
2.
3.
4.
Pg. 309 fig. 12-22
What codon sequence would you expect to
find in the mRNA at the place where
transcription starts?
At the place where transcription ends?
What kinds of molecules bind to the
regulatory sites of genes?
What is the action of these proteins on genes?
Answer
1.
2.
3.
4.
What codon sequence would you expect to find
in the mRNA at the place where transcription
starts? AUG, the start codon
At the place where transcription ends? Any one of
the three stop codons- UAA,UAG, or UGA
What kinds of molecules bind to the regulatory
sites of genes? DNA –binding proteins
What is the action of these proteins on genes?
They turn genes off or on
III. DEVELOPMENT AND
DIFFERENTIATION
1.
2.
Differentiation- as
cells grow & divide
they become
specialized in structure
and function with the
help of hox genes
Hox genes- control the
differentiation of cells
and tissues in the
embryo
3.
4.
Careful control of
expression in hox
genes is essential for
normal development
All hox genes are
descended from the
genes of common
ancestors.
Each color reflects genes on a chromosome
that form similar structures in different species