Control Control of gene of gene

expressioexpressionn

Control Control of gene of gene

expressioexpressionn

Every cell has at least one chromosome consisting of a

DNA molecule. Each chromosome contains genes – pieces of a DNA molecule – that are recipes for proteins. Genes can be turned “on”

or expressed, or turned “off” not expressed. It is a waste of resources (energy) to express all genes at all

times.

Prokaryotes

- expression of genes as needed

- dictated by environment

Prokaryotic Control Prokaryotic Control MechanismsMechanisms

Prokaryotic Control Prokaryotic Control MechanismsMechanisms

The Operon

Prokaryotes control gene expression through operons

An operon is a group of genes that contains all of the protein recipes needed for one particular function

Ex: There are 5 enzymes “recipes”

needed for the production of the amino

acid tryptophan

Bacteria often group together geneswith related functions ex. enzymes in a biosynthesis pathway Transcription of these genes iscontrolled by a single promoter when transcribed, read as 1 unit & asingle mRNA is madeOperon operator, promoter & genes they control

Overview of an OperonOverview of an OperonOverview of an OperonOverview of an Operon

Regulator Gene (upstream of rest of operon; recipe for a protein called repressor protein) Promoter (where RNA Polymerase attaches) Operator Gene (“on/off” switch for operon) Structural Genes (contains recipes for

proteins)

Regulator Gene

Promoter Operator

GeneStructural Genes

There are 2 types of negative control operons:

Inducible Operons - are normally turned “off” - an inducer turns them “on” Repressible Operons - are normally turned “on” - a repressor/corepressor turn “off”

Lactose operon

What happens when lactose is present?Need to make lactose-digesting enzymes

What happens when lactose is absent?

No need to make lactose-digesting enzymes… waste of cell’s energy & resources

When the inducer – lactose – is absent, repressor protein binds to the operator gene preventing RNA Polymerase from transcribing the structural genes. The

operon is turned “off”.

Lactose operon

When lactose enters the environment, it binds to the repressor protein and

inactivates it. This turns the operon “on” because nothing is attached to the

operator gene so RNA Polymerase is “induced” and can now transcribe the

structural genes.

lac operonlac operonlac operonlac operon

The lac operon is an example of an inducible operon.

The inducer which turns the operon “on” is lactose. So when lactose is present, the operon is “on” and when it is absent, the

operon is turned “off”.

trp operontrp operontrp operontrp operonThis operon responds to the cell’s need for the amino acid tryptophan.

This operon is usually turned “on” as the cell needs a lot of trptophan.

trp operontrp operon What if the cell begins to stockpile this amino acid? Don’t need to make tryptophan-building enzymes…therefore the operon needs to be turned off.

Tryptophan binds allosterically to repressor protein

trp operontrp operon

The trp operon is a repressible operon (normally “on” but can be turned “off”)

This is an example of “feedback inhibition”.

Operon summary

Repressible operon usually functions in anabolic pathways synthesizing end products when end product is present cell allocatesresources to other usesInducible operon usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient isavailable cell avoids making proteins that have nothing to do

Control of Gene ExpressionControl of Gene Expressioninin

EukaryotesEukaryotes

Control of Gene ExpressionControl of Gene Expressioninin

EukaryotesEukaryotes

The BIG Questions…

How are genes turned on & off ineukaryotes?

How do cells with the same genesdifferentiate to perform completely

different, specialized functions?

prokaryotes use operons to regulate gene transcription, however eukaryotes do not

The controls that act on gene expression are much more complex in eukaryotes than in prokaryotes. A major difference is the presence of a nuclear membrane which prevents the simultaneous transcription and translation that occurs in prokaryotes (which is why control of genes in prokaryotes really has to be done by turning transcription on or off)

Whereas, in prokaryotes, control of transcriptional initiation is the major

point of regulation,

In eukaryotic cells, the ability to express biologically active proteins comes under

regulation at several points.

The control points of gene expression can occur at any step in the pathway from gene to

functional protein

Control of Eukaryotic Control of Eukaryotic Gene ExpressionGene Expression

Control of Eukaryotic Control of Eukaryotic Gene ExpressionGene Expression

The first point is called

DNA packing

Imagine you have been given a string 3 feet long, which represents an unwound, human chromosome.

The chromosome (DNA molecule) is wound around a group of 8 positively

charged histone proteins (bind tightly to negatively charged DNA)

forming what is called a nucleosome – the basic unit of DNA packing.

Another histone links adjacent nucleosomes. These nucleosomes

resemble beads on a string.

The “nucleosome”

string is further coiled to produce a

thicker structure

called chromatin

Some of the chromatin is tightly packed – it contains genes that are seldom used (genes are not transcribed).

This tightly packed chromatin is called heterochromatin.

Genes that are transcribed (used) are more loosely packed into areas that “loop” out – called looped domains –

and form what is known as euchromatin (true chromatin).

Chromatin structure affects the

availability of genes for transcription

In humans, 97% of the DNA is heterochromatin (does not encode

proteins or RNA)!

Some of this heterochromatin consists of repetitive sequences called

satellite DNA.

Satellite DNA is usually found at the centromeres and telomeres and

abnormally long sequences can cause a variety of genetic diseases such as

Fragile X syndrome.

Fragile X syndrome

most common form of inherited mental retardationdefect in X chromosomemutation of FMR1 gene causing manyrepeats of CGG triplet in promoter region:

200+ copies normal = 6-40 CGG repeats

The final product of DNA packing

is the chromosome

(shown here as a metaphase

chromosome).

2. Modifying the 2. Modifying the ChromatinChromatin

2. Modifying the 2. Modifying the ChromatinChromatin

DNA Methylation – attaching methyl (CH3) groups turns “off” genes.

Example is Barr Body (extra X chromosome)

2. Modifying the Chromatin…2. Modifying the Chromatin…

Histone acetylation activates genes = on

attachment of acetyl groups (–COCH3) to certain amino acids of histone proteins neutralizes their positive charges and they no longer bind to neighboring nucleosomes they change shape & grip DNA less tightly = unwinding DNA transcription proteins have easier access to genes

3. Transcriptional 3. Transcriptional ControlControl

3. Transcriptional 3. Transcriptional ControlControl

DNA must be unpacked and if

gene is methylated, the

methyl group must be

removed. Proteins called transcription

factors assist in this job.

4. mRNA Processing4. mRNA Processing4. mRNA Processing4. mRNA ProcessingIntrons must be cut

out, exons sewed together, and a

GTP cap and poly A tail added.

Of course, the newly formed

mRNA can also be destroyed at this

point if the cell has changed its

mind.

5. mRNA Leaves 5. mRNA Leaves NucleusNucleus

5. mRNA Leaves 5. mRNA Leaves NucleusNucleus

A fully processed mRNA must leave

the nucleus in order to be translated into protein. The large nuclear pores must be opened for its

passage. Again, the cell can change its mind and not open

the pores.

6. 6. TranslationTranslation

6. 6. TranslationTranslationTranslation requires that

ribosomes participate. Some mRNAs have an “open later” label (they are called masked mRNAs) and they are not translated right

away. Examples are plant proteins needed to

photosynthesis (“go” given in day).

This step can also be blocked by regulatory proteins that prevent the attachment of ribosomes to the mRNA.

7. 7. PosttranscriptioPosttranscriptio

n Controlsn Controls

7. 7. PosttranscriptioPosttranscriptio

n Controlsn ControlsFinally, the new protein must be

“folded” to become active. The cell can

withhold this folding or even

degrade (breakdown) the

protein at this step.

Molecular BiologyMolecular Biologyofof

CancerCancer

Molecular BiologyMolecular Biologyofof

CancerCancer

Cancer is a Genetic Cancer is a Genetic DiseaseDisease

Cancer is a Genetic Cancer is a Genetic DiseaseDisease

Normally the cell controls whether or not it divides to form more cells. Some of this control comes from

proteins called growth factors. These growth factors come from

other cells and stimulate a “target cell” to divide.

All cells contain genes called

proto-oncogenes

which give the internal signal to leave G1 of cell cycle and begin S.

Proto-oncogenes can be defective or they can mutate. This promotes excessive cell

division.

Such genes are now called

oncogenes

The cell now divides continually

But there are still safeguards

called

tumor suppressor genes

It is the job of these genes to produce a protein that inhibits cell division.

An example of a tumor suppressor gene is p53

The loss or mutation of a tumor suppressor produces the same effect as an oncogene.

p53 gene

“Guardian of the Genome” the “anti-cancer gene” after DNA damage is detected, p53 initiates:

DNA repair growth arrest

apoptosis – cell suicide

almost all cancers have mutations in p53

Multiple mutations, however, are required for the development of cancer.

This is called the

“multiple-hit” hypothesis:• several changes must occur at DNA level for cell

to become fully cancerous• including at least 1 active oncogene & mutation

or loss of several tumor-suppressor genes

Some of these mutations can be inherited, resulting in a predisposition to certain

types of cancer