13.4 Gene Regulation and Expression

THINK ABOUT IT

How Does A Cell Know? Which Gene To Express

& Which Gene Should Stay

Silent?

Prokaryotic Gene Regulation

› @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?)

Prokaryotic Gene Regulation• 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 @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

Membrane2. Separate The Two Sugars

Promoters and Operators › 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.

The Lac Repressor Turns Transcription Off

Lactose Turns the Operon “On”

Gene Regulation: lac Operon

Key Concept:

The lac Genes Are:Turned Off By Repressors

And

Turned On By The Presence Of Lactose

Eukaryotic Cell Specialization › Most Eukaryotic Genes Are

Controlled Individually And Have Regulatory Sequences That Are Much More Complex Than Prokaryotic Gene Regulation

RNA Interference

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)

RNA Interference

› @Blocking gene expression by means of an miRNA silencing complex is known as RNA interference (RNAi).@

Genetic Control of Development› 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.

Homeotic Genes › 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

Regulation & Development

• Hox Genes– @Control Organ &

Tissue Development In The Embryo@

– Mutations Lead To Major Changes• Drosophila With Legs In Place of Antennae

Regulation & Development

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

Regulation & Development

Homeobox and Hox Genes › 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.

Environmental Influences › @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.

This type of RNA is used to block gene expression

1. rRNA2. tRNA3. mRNA4. miRNA

In the Lac operon, if lactose is not present what happens to the operon?

The Lac operon turns off

In the Lac operon, if lactose is present what happens to the operon?

The Lac operon turns on

These determine factors like presence of wings or legs

1. miRNA2. Homeobox Genes3. Operator4. Promoter