Section 8.6: Gene Expression and Regulation

Biology

Objectives1. Why do prokaryotes control transcription so

carefully?

2. How do specific sequence control transcription in prokaryotes?

3. Describe the use of the lac operon to study transcription.

4. What are different ways that eukaryotes control gene expression?

5. Describe how the start of transcription requires specific patterns in eukaryotes.

6. Describe mRNA processing.

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)

Genes

Genes include more than just the protein-coding sequence

Often include sequences that control its expression

Two major control sequences are:

Promoters

Operators

Promoters

DNA segment that allows DNA to be transcribed

Helps RNA polymerase find where a gene starts

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.

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

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

The lac Operon Turned off when lactose is absent

Saves the cell’s resources

When lactose is present, the lac operon is switched on

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

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

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

Regulation of Transcription

Genes Controlling Genes

Some genes code for proteins that control the expression of other genes

Example: Sonic Hedgehog

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

Introns Regulate Gene Expression

Protect DNA from mutations

Sometimes different editing can give you a different protein

Seen often in viruses