1. Chapter 11
Gene Expression

2. Role of Gene Expression
Chapter 11
Role of Gene Expression
Gene expression is the activation of a gene that results in transcription and the production of mRNA…and eventually the production of a PROTEIN
Only a fraction of any cell’s genes are expressed at any one time
Remember, EVERY CELL IN YOUR BODY (except red blood cells) has the same DNA
But your kidney cells don’t need to make liver proteins and vice versa
So kidney cells will transcribe KIDNEY proteins, and liver cells will transcribe LIVER proteins

3. Gene Expression in Prokaryotes
Chapter 11
Gene Expression in Prokaryotes
An operon is a series of genes that code for specific products and the regulatory elements that control these genes. In prokaryotes, the structural genes, the promoter, and the operator collectively form an operon.

4. Gene Expression in Prokaryotes, continued
Chapter 11
Gene Expression in Prokaryotes, continued
A promoter is the segment of DNA that is recognized by the enzyme RNA polymerase, which then initiates transcription
Structural genes code for the actual protein
An operator is the segment of DNA that acts as a “switch” by controlling the access of RNA polymerase to the promoter
A repressor may bind to it, thus preventing transcription

5. Gene Expression in Prokaryotes, continued
Chapter 11
Mechanism of lac operon
The prokaryotic lac operon codes for the lactase enzyme
In bacteria, if there is NO lactose present, the lac operon is REPRESSED (no transcription to make lactase)
No point in making a product (in this case, lactase) if it is NOT NEEDED (that would be a waste of energy)

6. Gene Expression in Prokaryotes, continued
Chapter 11
Mechanism of lac operon
Again, the prokaryotic lac operon codes for the lactase enzyme
In bacteria, if lactose IS present, the lac operon is ACTIVE (need to transcribe lactase in order to break down the lactose)
Repressor is removed in the presence of the lactose, which acts as an inducer

7. Gene Expression in Eukaryotes
Chapter 11
Gene Expression in Eukaryotes
Structure of a Eukaryotic Gene
Eukaryotes DO NOT HAVE operons
The genomes of eukaryotes are larger and more complex than those of prokaryotes
Eukaryotic genes are organized into noncoding sections, called introns, and coding sections, called exons
Control After Transcription
In eukaryotes, gene expression can be controlled after transcription—through the removal of introns from pre-mRNA
Remember…introns stay IN the nucleus, but exons EXIT the nucleus and are EXpressed into proteins!

8. Removal of Introns After Transcription

9. Eukaryotic Gene Expression
Many eukaryotic genes have a sequence called the TATA box
The TATA box seems to help position RNA polymerase
Eukaryotic promoters are usually found just before the TATA box, and consist of short DNA sequences

10. Heterochromatin vs. Euchromatin
How tightly the DNA (chromatin) is coiled around the proteins (histones) also affects gene expression
Euchromatin: Uncoiled; certain regions of DNA loosely associated with histones
Genes are easily transcribed
Heterochromatin: Tightly coiled; regions of DNA wrapped tightly around histones
These genes are not easily transcribed

11. Heterochromatin vs. Euchromatin
Because the DNA is not tightly wound up in euchromatin, the “machinery” needed to transcribe genes can fit onto the genes between the histones = TRANSCRIPTION
In heterochromatin, the histones are too close together to allow the transcription “machinery” to position itself on genes ≠ TRANSCRIPTION
NO transcription
Transcription proceeds

12. Gene Expression in Embryonic Development
Chapter 11
Gene Expression in Embryonic Development
The development of cells with specialized functions is called cell differentiation
Examples: Liver cells, muscle cells, white blood cells
As an organism grows and develops, organs and tissues develop to create a characteristic form. This process is called morphogenesis
Both cell differentiation and morphogenesis are governed by gene expression

13. Homeotic Genes
Homeotic genes: Regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis
The “master genes of development”
Homeotic genes control the expression of other genes, thus regulating many aspects of development

14. Hox Genes Hox genes initially discovered in Drosophila (fruit flies)
Hox genes control the segmentation pattern of embryonic fruit flies, which eventually results in the normal development of the flies’ appendages in the correct location
Each Hox gene controls the development of a particular body section

15. Hox Genes Gone Wrong Mutant with legs growing out of head Normal
No legs, but plenty of eyes

16. Hox Genes In Other Organisms
Hox genes seen in Drosophila appear to be conserved (evolutionarily similar) to regulatory genes in other organisms
These similar genes allow for similarity in patterns of body development