SBI4U: MOLECULAR GENETICS

CONTROL MECHANISMS IN PROKARYOTES & EUKARYOTES

Melanie Tavone

Curriculum Expectations

D3.3 explain the steps involved in the process of protein synthesis and how genetic expression is controlled in prokaryotes and eukaryotes by regulatory proteins (e.g., the role of operons in prokaryotic cells; the mechanism of gene expression in eukaryotic cells)

Agenda

• Gene Expression Overview• Expression in Eukaryotes with Animation• Expression in Prokaryotes: lac Operon with Animation• Expression in Prokaryotes: trp Operon with Animation• Quizzes on SMARTBoard• Isolation, Cloning & Transformation of DNA Lab

42,000 genes exist that code for proteins in humans

Not all of these proteins are required at all times

Example: A cell needs to produce insulin only when the concentration of glucose is high in the extracellular fluid. Producing insulin all the time is inefficient to the cell

So how do cells maintain their efficiency?

Some genes are always being transcribed and translated.

These are referred to as housekeeping genes

All other genes, are regulated by a cell process called gene regulation. Gene regulation has the ability to turn on or off specific genes depending on the requirements of an organism.

Control in EukaryotesGenes can be regulated during transcription (transcriptional), after transcription (post-transcriptional), during translation (translational), or after translation (post-translational)

Type of Control Description Examples

Transcriptional Regulates which genes are transcribed

Access to promoters, activator/repressor proteins, methylation

Post-transcriptional Controls the availability of mRNA to the ribsosomes

Alternative splicing, masking proteins, degradation

Translational Controls how fast mRNA is translated to protein

Variation of the polyA tail

Post-translational Control when and how long proteins will be functional

Processing- usually chemically modified, tagged for degradation

Transcriptional Regulation

• DNA is wrapped around histone proteins

• the promoter region on DNA are not accessible for proteins to initiate transcription

• An activator molecule binds to a sequence upstream from the promoter and subsequently signals a remodelling complex to bind to the promoter region

• The activator molecule can also signal enzymes to add an acetyl group to the histones

Transcription Inhibition

Transcription can be inhibited altogether by the addition of methyl groups to certain DNA bases. This is called methylation. Methyl groups are added to the cytosine bases of the promoter region, inhibiting the promoter. This is called silencing.

METHYLATION

Post-Transcriptional Regulation

The introns, non-coding regions can be excised from the DNA. The exons are the coding regions which can now be used to code for the protein products of the genes. The exons can exist in multiple combinations which allows for genetic variation

Translational Regulation The length of the poly A tail can affect the amount of

time it takes for translation to occur The cell can produce enzymes that lengthen or shorten

the poly-A tail

Post-Translational Regulation

The amount of functional protein can be limited

• Processing (inactive precursor polypeptides produced that require further processing to become active)

• Chemical modification (addition of chemical groups; the presence or absence can put the protein “on hold”)

• Degradation (polypeptides may be tagged with ubiquitin and degraded by degradation mechanisms)

http://highered.mcgraw-hill.com/sites/007337797x/student_view0/chapter13/animation_quiz_-_control_of_gene_expression_in_eukaryotes.html

Gene Regulation & CANCER Cancer is caused by mutations in DNA

that cause cells to experience uncontrolled reproduction (mitosis).

Two types of genes have significant control on the rate of mitosis. Tumor Suppressor Genes (TSG) code for

proteins/enzymes that tend to slow down mitosis

Proto-Oncogenes (POG) code for proteins/enzymes that tend to speed up mitosis.

Mutations in DNA that inhibit the expression of TSG and/or activate POG increase rates of mitosis, leading to tumors and cancer.

HRas: A Human Proto-Oncogene• The HRas gene on human

chromosome 11 is a POG.• The HRas gene codes for the

H-Ras protein that regulates mitosis and apoptosis.

• Cells with too much active H-Ras protein undergo rapid mitosis, lower cell-cell adhesion, and reduced apoptosis.

• Mutations in the HRas gene itself or its promoter region are linked to development of bladder cancer.

Control in Prokaryotes

• Prokaryotes control gene expression for the same purpose as eukaryotes.

• However, the way they achieve genetic regulation is different

• E.coli bacterial cells may obtain the glucose (and galactose) they need for growth from lactose.

• Lactose is a disaccharide made up of glucose and galactose

• Bacteria such as E. coli produce the enzyme β-galactosidase that is responsible for decomposing lactose into glucose and galactose

• Therefore, E.coli does not need to produce β-galactosidase enzyme when lactose is not present

• The gene for β-galactosidase is part of an operon. • An operon is located on the bacterial chromosome along

with structural genes (coding for protein), a promoter and an operator (DNA sequences that regulate the expression of the structural genes)

• The operator is composed of regulatory sequences of DNA to which a repressor protein binds.

The lac Operon• lacZ codes for β-galactosidase • lazY codes for β-galactosidase permease• lacA codes for an unknown transacetylase that participates in lactose

decomposition

*Lac I is a repressor protein

http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

http://www.phschool.com/science/biology_place/biocoach/lacoperon/quiz.html

Animation of the lac Operon & Quiz

The trp Operon

E.coli cells also need to manufacture tryptophan if they cannot absorb it through their diet

When these cells need to synthesize tryptophan they utilizes the trp operon

Unlike the lac operon, when tryptophan is present, the trp operon is repressed

When tryptophan is not present, the repressor protein is in an inactive state and cannot bind to the operator site. This allows RNA polymerase to bind to the promoter, and transcription of genes 1-5 proceeds.

When tryptophan is present it binds to the repressor and activates it. This complex now binds to the trp operator and transcription of genes 1-5 is blocked . Thus, tryptophan acts as a co-repressor.

http://highered.mcgraw-hill.com/sites/007337797x/student_view0/chapter13/animation_quiz_-_the_tryptophan_repressor.html

Animation of trp Operon & Quiz

SUMMARY

http://www.txtwriter.com/Backgrounders/Genetech/GEpage07.html