Gene Regulation

• An expressed gene is one that is transcribed into RNA

• Not all genes are expressed by every cell

• How does an organism know when to “turn on” or “turn off” a gene?

Typical Gene Regulation• See page 309 in your textbook Figure 12-22

Regulatory sites

promoter

start stop

gene

Promoters- DNA sequence where RNA polymerase will bind

Operator (regulatory sites)- stretches of DNA where proteins can bind to regulate transcription

Start and stop signals- for transcription of gene

Prokaryotic Gene Expression usually uses OPERONS.

OPERONS- are a group of genes that operate together

the Lac operon is a great example!It’s a set of genes needed for E.coli to make proteins so the bacteria can use the sugar lactose as food!

The Lac operon has 1. a promoter (the nucleotide sequence where

RNA Polymerase binds- green)

2. an operator (segment of DNA used to turn gene expression on or off, repressor protein binds here- dotted/shaded region)

3. Three genes for making proteins involved in breaking down lactose

See

page 310

Fig.12-23

How the Lac Operon works or operates…. Lac genes turned OFF by the repressor protein and

turned ON by the presence of Lactose

WITHOUT Lactose- DNA binding protein is bound to the OPERATOR (where protein binds) so RNA polymerase CAN’T WORK (can’t make the mRNA)

WITH Lactose- lactose binds to repressor DNA binding protein, this causes the protein to CHANGE SHAPE and FALL OFF the DNA Operator so RNA polymerase CAN WORK (transcribes the gene)

Without Lactose:

With Lactose:

Lac operon animation

• http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

Eukaryotic Gene Regulation: • Most eukaryotic genes are controlled

individually NOT with operons

• Only a few of the available genes will be expressed in the cells of different tissues (cell specialization/differentiation)

• Complex!

See page 311 in your textbook Figure 12-24

Eukaryotic Gene Regulation

exonexon

enhancer Promoter sequences

TATA Box

exon intron intron

Enhancer sequence- lots of proteins can bind here on different sequences (that’s why eukaryotic gene regulation is more

complex!!)

Promoters- usually just before the TATA Box (where RNA polymerase binds)

TATA Box- helps position RNA polymerase in front of gene

Exons- expressed

Introns- cut out before translation

Proteins that bind to enhancer sequences of a gene can work to:

a. Open up tightly packed chromatin.

b. Attract RNA polymerase.

c. Block access to genes.

Fig. 14.9, p. 229

unit of transcription in a DNA strand

exon intron

mature mRNA transcript

poly-Atail

5’

5’ 3’

3’

(snipped out) (snipped out)

exon exonintron

cap

transcription into pre-mRNA

3’ 5’

RNA Editing

Prokaryotic vs. Eukaryotic Gene Expression

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html#

Hox genes

Development and Differentiation

• Hox genes are master control genes that control the differentiation of cells and tissues in the embryo.

• A mutation in hox gene in fruit flies had a leg growing where antenna should be.