Chapter 16: Gene Expression

Prokaryote Regulation

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Experiments with E. Coli showed that it is capable of regulating the expression of its genes

An operon consists of the following elements

1.  Promoter   -  where RNA polymerase attaches, signalling the start of the gene
2. Operator - where repressor binds, stopping the transcription of that gene
3. Structural Genes - code for an enzyme, transcribed as a unit

Example: the trp Operon controls the production of tryptophan

Tying it altogether - the Lac Operon Gene

E. Coli bacteria can synthesize lactase, which is an enzyme that breaks down lactose. Lactase is only synthesized in the presence of lactose. If there is no lactose in the environment, the gene is repressed. Since they live in the intestinal tract of humans, they must live on whatever the host eats. When you drink milk, you are provided your E. Coli bacteria with lactose.

• E. Coli has three genes (6, 7, 8) that code for lactase.
• It also has a promoter (1) and an operator (2)
• Without lactose, the lac repressor binds to the operator site.
• With lactose (5), the repressor is removed
• Once repressor is removed, RNA polymerase binds to the promoter
• RNA is transcribed, the genes (6,7,8) code for lactase

Repressors turn transcription OFF(tryp operon)
Inducers turn transcription ON (lac operon)

Gene Expression in the Stickleback Fish

Lesson from HHMI - Tha Making of the Fittest
Modeling Regulatory Switches in the Pitx1 Gene

What is a SNP?

single nucleotide polumorphism

Genome Wide Association Studies - used to find SNPs associated with traits (and diseases)

Gene Mutations

Point Mutation - substitute one base for another
Original: A T A C A C
Mutant : T T A C A C

Frameshift Mutation - a base is either added or removed which causes a shift in the reading frame. Many genes affects
Original: A T A C A C A A G C C A
Mutant: A T T A C A C A A G C C A

Silent Mutation - a base is changed but the resulting amino acid is the same as in the non mutant DNA. No outward changes.
Original: A A A C A G
Mutant: A A G C A G

Nonsense Mutation - a codon is changed to a STOP codon
Original: A T A C C C A A A
Mutant: A T T C C C A A A

What happens when you have a nonfunctional protein?

Hemophilia
SIckle Cell Anemia
Cystic Fibrosis
Androgen Insensitity