Download Gene Regulation I. Gene regulation: The ability of an organism to

Gene Regulation
I. Gene regulation: The ability of an organism to control which genes are transcribed in response to
the environment.
II. Prokaryote gene regulation
A: Operon: Section of DNA that contains the genes for the proteins needed for a specific
metabolic pathway.
1. Operon consists of:
a. Operator: Segment of DNA that acts as an on/off switch
b. Promoter: Where RNA polymerase first binds to the DNA
c. Regulatory gene: gene involved in controlling the expression of one or more
other genes. A regulator gene may encode a protein, or it may
work at the level of RNA.
d. Genes coding for protein: These genes code for proteins.
B. The trp operon
1. In bacteria tryptophan synthesis occurs in 5 steps, each step catalyzed by a specific
enzyme.
2. Five genes that code for these enzymes are clustered together.
3. Group of DNA that controls if enzymes are transcribed. This is called the tryptophan
(trp) operon.
4. These genes are normally repressed.
5. When tryptophan is present the cell has no need to produce it and therefore the trp
repressor genes turn off.
6. Tryptophan is an inhibitor—negative feedback—stops the enzyme from making
tryptophan
Trp operon “on”
Low levels of tryptophan
Trp operon “off”
High levels of tryptophan.
C. The lac operon
1. When lactose is present E. coli makes enzymes that enable it to use lactose as an
energy source.
2. Lactose (lac) operon contains:
a. Promoter
b. Operator
c. Regulatory gene
d. 3 enzyme genes that control lac digestion
3. When an inducer is present (allolactose) it binds with the repressor and inactivates it.
4. lac operon is called an inducer operon because transcription is turned on by an
inducer.
5. This is an example of positive feedback
III. Eukaryote gene regulation
A. Controlling transcription
1. Transcription factors: Ensure that a gene is used at the right time and that proteins
are made in the right amount.
2. 2 Sets of transcription factors
a. One set forms complexes that guide and stabilize the binding of RNA
polymerase to a promoter
b. One set includes regulatory proteins that control rate of transcription.
1. Activator: Protein that folds DNA so enhancer sites are close to the
complex and increase rate of gene transcription
2. Repressor: Binds to specific sites of DNA and prevents binding of
activators
B. Hox genes
1. Differentiation: Process through which cell become specialized in structure and
function.
2. Hox genes: Determine the body plan of an organism
a. Code for transcription factors and are active in zone of embryo that are in the
same order as the genes on the chromosome.
1. Control what body part will develop in a given location.
2. Similar clusters of Hox genes that control body plans have been found
in all animals.
C. RNA interference
1. Small pieces of double-stranded RNA in cytoplasm of cell are cut by enzyme called
dicer.
a. Resulting double-stranded RNA are called interfering RNA
b. Interfering RNA and protein complexes bind to sequence specific section of
mRNA in cytoplasm and cause the mRNA in this region to be cut and thus
prevent its translation.