Chapter 18

Regulation of

Gene Expression

Regulation of Gene Expression

Important for cellular control and differentiation.

Understanding “expression” is a “hot” area in Biology.

General Mechanisms

1. Regulate Gene Expression

2. Regulate Protein Activity

Operon Model

Jacob and Monod (1961) - Prokaryotic model of gene control.

Always on the National AP Biology exam !

Operon Structure

1. Regulatory Gene

2. Operon Area a. Promoter

b. Operator

c. Structural Genes

Gene Structures

Regulatory Gene

Makes Repressor Protein which may bind to the operator.

Repressor protein blocks transcription.

Promoter

Attachment sequence on the DNA for RNA polymerase to start transcription.

Operator

The "Switch”, binding site for Repressor Protein.

If blocked, will not permit RNA polymerase to pass, preventing transcription.

Structural Genes

Make the enzymes for the metabolic pathway.

Lac Operon

For digesting Lactose. Inducible Operon - only

works (on) when the substrate (lactose) is present.

If no Lactose

Repressor binds to operator. Operon is "off”,

no transcription, no enzymes made

If Lactose is absent

If Lactose is present

Repressor binds to Lactose instead of operator.

Operon is "on”, transcription occurs, enzymes are made.

If Lactose is present

Enzymes

Digest Lactose. When enough Lactose is

digested, the Repressor can bind to the operator and switch the Operon "off”.

Net Result

The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy.

Animation

http://www.biostudio.com/d_%20Lac%20Operon.htm

trp Operon

Makes Tryptophan. Repressible Operon.

If no Tryptophan

Repressor protein is inactive, Operon "on” Tryptophan made.

“Normal” state for the cell.

Tryptophan absent

If Tryptophan present

Repressor protein is active, Operon "off”, no transcription, no enzymes

Result - no Tryptophan made

If Tryptophan present

Repressible Operons

Are examples of Feedback Inhibition.

Result - keeps the substrate at a constant level.

Positive Gene Regulation

Positive increase of the level of transcription.

Uses CAP - Catabolite Activator Protein

Uses cAMP as a secondary cell signal.

CAP - Mechanism

Binds to cAMP. Complex binds to the

Promoter, helping RNA polymerase with transcription.

Result

If the amount of glucose is low (as shown by cAMP) and lactose is present, the lac operon can kick into high gear.

Eukaryotic Gene Regulation

Can occur at any stage between DNA and Protein.

Be prepared to talk about several mechanisms in some detail.

Chromatin Structure

Histone Modifications DNA Methylation Epigenetic Inheritance

Histone Acetylation

Attachment of acetyl groups (-COCH3) to AAs in histones.

Result - DNA held less tightly to the nucleosomes, more accessible for transcription.

DNA Methylation

Addition of methyl groups (-CH3) to DNA bases.

Result - long-term shut-down of DNA transcription.

Ex: Barr bodies genomic imprinting

Epigenetics

Another example of DNA methylation effecting the control of gene expression.

Long term control from generation to generation.

Tends to turn genes “off”.

Do Identical Twins have Identical DNA?

Yes – at the early stages of their lives.

Later – methylation patterns change their DNA and they become less alike with age.

Transcriptional Control Enhancers and Repressors Specific Transcription

Factors Result – affect the

transcription of DNA into mRNA

Enhancers

Areas of DNA that increase transcription.

May be widely separated from the gene (usually upstream).

Posttranscriptional Control

Alternative RNA Processing Ex - introns and exons

Can have choices on which exons to keep and which to discard.

Result – different mRNA and different proteins.

Another Example

Results

Bcl-XL – inhibits apoptosis

Bcl-XS – induces apoptosis

Two different and opposite effects!!

DSCAM Gene

Found in fruit flies Has 100 potential splicing sites. Could produce 38,000 different

polypeptides Many of these polypeptides have

been found

Commentary

Alternative Splicing is going to be a BIG topic in Biology.

About 60% of genes are estimated to have alternative splicing sites. (way to increase the number of our genes)

One “gene” does not equal one polypeptide (or RNA).

Other post transcriptional control points

RNA Transport - moving the mRNA into the cytoplasm.

RNA Degradation - breaking down old mRNA.

Translation Control

Regulated by the availability of initiation factors.

Availability of tRNAs, AAs and other protein synthesis factors. (review Chapter 17).

Protein Processing and Degradation

Changes to the protein structure after translation.

Ex: Cleavage Modifications Activation Transport Degradation

Protein Degradation

By Proteosomes using Ubiquitin to mark the protein.

Noncoding RNA

Small RNA molecules that are not translated into protein.

Whole new area in gene regulation.

Ex - RNAi

Types of RNA

MicroRNAs or miRNAs. RNA Interference or RNAi using

small interfering RNAs or siRNAs.

Both made from RNA molecule that is diced into double stranded (ds) segments.

RNAi

siRNAs or miRNAs can interact with mRNA and destroy the mRNA or block transcription.

A high percentage of our DNA produces regulatory RNA.

Morphogensis

The generation of body form is a prime example of gene expression control.

How do cells differentiate from a single celled zygote into a multi-cellular organism?

Clues?

Some of the clues are already in the egg.

Cytoplasmic determinants – chemicals in the egg that signal embryo development.

Made by Maternal genes, not the embryo’s.

Induction

Cell to cell signaling of neighboring cells gives position and clues to development of the embryo.

Fruit Fly Studies

Have contributed a great deal of information on how an egg develops into an embryo and the embryo into the adult.

Homeotic (Hox) Genes

Any of the “master” regulatory genes that control placement of the body parts.

Usually contain “homeobox” sequences of DNA (180 bases) that are highly conserved between organisms.

Comment

Evolution is strongly tied to gene regulation. Why?

What happens if you mutate the homeotic genes?

Stay tuned for more “evo-devo” links in the future.

When things go wrong

Example case

Bicoid (two tailed) – gene that controls the development of a head area in fruit flies.

Gene produces a protein gradient across the embryo.

Result

Head area develops where Bicoid protein levels are highest.

If no bicoid gradient – get two tails.

Other Genes Control the development of

segments and the other axis of the body.

Gene Expression and Cancer

Cancer - loss of the genetic control of cell division.

Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes).

Proto-oncogenes Normal genes for cell growth and

cell division factors. Genetic changes may turn them

into oncogenes (cancer genes). Ex: Gene Amplification,

Translocations, Transpositions, Point Mutations

Proto-oncogenes

Tumor-Suppressor Genes

Genes that inhibit cell division.

Ex - p53, p21

Cancer Examples

RAS - a G protein. When mutated, causes an

increase in cell division by over-stimulating protein kinases.

Several mutations known.

Cancer Examples

p53 - involved with several DNA repair genes and “checking” genes.

When damaged (e.g. cigarette smoke), can’t inhibit cell division or cause damaged cells to apoptose.

Carcinogens

Agents that cause cancer. Ex: radiation, chemicals Most work by altering the

DNA, or interfering with control or repair mechanisms.

Multistep Hypothesis

Cancer is the result of several control mechanisms breaking down (usually).

Ex: Colorectal Cancer requires 4 to 5 mutations before cancer starts.

Colorectal Cancer

News Flash

Severe damage to a chromosome that causes it to “shatter” can lead to immediate cancer.

Doesn’t always take a long time and multiple steps.

Can Cancer be Inherited?

Cancer is caused by genetic changes but is not inherited.

However, oncogenes can be inherited.

Multistep model suggests that this puts a person “closer” to developing cancer.

Example – BRAC1

BRAC1 is a tumor suppressor gene linked with breast cancer.

Normal BRAC1 – 2% risk. Abnormal BRAC1 – 60% risk. Runs in families. Some will

have breasts removed to avoid cancer risk.

Homework Read Chapter 20 Lab – Gel Electrophoresis.

Lab report – 2/9 New Discussion Forum –

articles found under “labs”. Chapter 18 – Fri. 2/10 No broadcast Mon. 2/6

Summary Know Operons Be able to discuss several

control mechanisms of gene expression.

Be familiar with gene expression and development of organisms.

Summary How control of DNA can lead

to cancer.