chapter 11 regulation of gene expression. regulation of gene expression u important for cellular...

Chapter 11

Regulation of

Gene Expression

Regulation of Gene Expression

Important for cellular control and differentiation.

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

Differentiation

Specialization of structure and function of cells

Results from activation/ deactivation of genes

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, prevents transcription.

Gene Structures

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.

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.

Eukaryotic Gene Regulation

Can occur at any stage between DNA and Protein.

DNA packing

DNA is coiled around histones which are then coiled to form supercoil

Less tightly coiled= easier expression

DNA packing

Histones

Chromatin Structure and Expression

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 affecting the control of gene expression.

Long term control from generation to generation.

End of Part 1

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).

Post-transcriptional Control

Alternative RNA Splicing Ex - introns and exons

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

Result – different mRNA and different proteins.

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 a BIG topic in Biology.

About 60% of genes are estimated to have alternative splicing sites.

One “gene” does not equal one polypeptide.

Translation Control

Regulated by the availability of tRNAs, AAs and other protein synthesis factors.

Protein Processing and Degradation

Changes to the protein structure after translation.

Ex: Cleavage Modifications Activation Transport Degradation

Noncoding RNA

Small RNA molecules that are not translated into protein.

Whole new area in gene regulation.

Types of RNA

MicroRNAs or miRNAs. RNA Interference or RNAi using

small interfering RNAs or siRNAs.

RNAi

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

A high percentage of our DNA produces regulatory RNA.

Morphogenesis

The generation of body form How do cells differentiate from

a single celled zygote into a multi-cellular organism?

Induction

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

Homeotic 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.

When things go wrong

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

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.

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

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.

End of Part 2