Chapter 11

GENE EXPRESSION

Gene Expression in Eukaryotes

Gene expression – the activation of a gene that results in the formation of a gene

Transcription causes expression Important for cellular control and

differentiation.

General Mechanisms

1. Regulate Gene Expression

2. Regulate Enzyme Activity

Initiating Transcription

Operon Model

Jacob and Monod (1961) - Prokaryotic model of gene control.

Operon Structure

1. Regulatory Gene

2. Operon Area a. Promoter

b. Operator

c. Structural Genes

Regulatory Gene

Makes Repressor Protein which may bind to the operator.

Promoter

Attachment sequence on the DNA for RNA polymerase.

Operator

The "Switch”, binding site for Repressor Protein.

If blocked, will not permit RNA polymerase to pass, preventing transcription.

Structural Genes

Make the enzymes for the metabolic pathway.

Lac Operon

For digesting Lactose. Inducible Operon - only

works (on) when the substrate (lactose) is present.

If no Lactose

Repressor binds to operator. Operon is "off”,

no transcription, no enzymes made

If Lactose is absent

If Lactose is present

Repressor binds to Lactose instead of operator.

Operon is "on”, transcription occurs, enzymes are made.

If Lactose is present

Enzymes

Digest Lactose. When enough Lactose is

digested, the Repressor can bind to the operator and switch the Operon "off”.

Net Result

The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy.

Repressible Operons

Are examples of Feedback Inhibition.

Result - keeps the substrate at a constant level.

CELL DIFFERENTIATION Development of specialized cells Homeotic Genes – determine

where certain anatomical structures such as appendages will occur Ex HOX genes –control position

front/back/top/bottom

Gene Expression and Cancer

Cancer - loss of the genetic control of cell division.

Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes).

Proto-oncogenes Normal genes for cell growth and

cell division factors. Genetic changes may turn them

into oncogenes (cancer genes). Ex: Gene Amplification,

Translocations, Transpositions, Point Mutations

Proto-oncogenes

Tumor-Suppressor Genes

Genes that inhibit cell division.

Ex - p53, p21

Cancer Examples

p53 - involved with several DNA repair genes and “checking” genes.

When damaged (e.g. cigarette smoke), can’t inhibit cell division or cause damaged cells to apoptose.