1. Section 8.6: Gene Expression and Regulation
Biology

2. Objectives Why do prokaryotes control transcription so carefully?
How do specific sequence control transcription in prokaryotes?
Describe the use of the lac operon to study transcription.
What are different ways that eukaryotes control gene expression?
Describe how the start of transcription requires specific patterns in eukaryotes.
Describe mRNA processing.

3. Prokaryotes
Control of gene expression allows for prokaryotes to respond to stimuli
Allows them to conserve energy and materials
Transcription and translation both occur simultaneously in the cytoplasm
Therefore, the best way to control gene expression is to control transcription (start of protein making process)

4. Genes Genes include more than just the protein-coding sequence
Often include sequences that control its expression
Two major control sequences are:
Promoters
Operators

5. Promoters DNA segment that allows DNA to be transcribed
Helps RNA polymerase find where a gene starts

6. Operators DNA segment that can turn gene “on” or “off”
Can interact with proteins that increase the rate of transcription of stop transcription from occurring.

7. Bacterial Genes Bacteria have less DNA than other organisms
Genes organized into operons
Operon: region of DNA that includes a promoter, an operator, and the genes that code for the protein
Found only in prokaryotes and round worms

8. The lac Operon
One of the first examples of gene regulation in bacteria
Consists three genes, all of which code for enzymes that breaks down lactose
All under the control of a single promoter and operator

9. The lac Operon Turned off when lactose is absent
Saves the cell’s resources
When lactose is present, the lac operon is switched on

11. Gene Regulation In Eukaryotes
Eukaryotes have different sets of genes, and these genes are only utilized in certain types of cells
Many different ways to control gene expression at different points in time
The start of transcription is often the most utilized way to control gene expression

12. Starting Transcription
Many elements work together to control the start of transcription
Types of regulatory DNA Sequences:
Close to the gene
Far away and are looped close
Some are found in almost all eukaryotic cells
Some genes control the transcription of other genes

13. Regulation of Transcription
Regulatory Sequences:
Sequences recognized by transcription factors that bind to the DNA, so that RNA polymerase can bind
Far off regulatory sequences:
If far way, often the DNA will loop so that the regulatory sequence is close
Promoter:
TATA box, that helps start transcription
Enhancers and silencers can also speed up or slow down the rate of transcription

14. Regulation of Transcription
Genes Controlling Genes
Some genes code for proteins that control the expression of other genes
Example: Sonic Hedgehog

15. mRNA Processing mRNA is generally edited prior to leaving the nucleus
Cap: added to the beginning of mRNA
helps bind to ribosome
keeps it from getting broken down
Tail: help mRNA exit the nucleus
Editing of Code:
Introns: nucleotide sequences that do not code for an amino acid
Exons: nucleotide sequence that codes for an amino acid

17. Introns Regulate Gene Expression Protect DNA from mutations
Sometimes different editing can give you a different protein
Seen often in viruses