workshop materials
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Materials and images in this page were adapted from
Di Giuseppe, M., Vavitsas, A., Ritter, B., Fraser D., Arora, A., and Lisser, B. (2003) Biology 12 Textbook, p. 255-258. Nelson Thomson Learning, Toronto.
Di Giuseppe, M., Vavitsas, A., Ritter, B., Fraser D., Arora, A., and Lisser, B. (2003) Biology 12 Textbook, p. 255-258. Nelson Thomson Learning, Toronto.
Gene expression
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.
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.
How Do Genes 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.
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 eukaryotes
Genes can be regulated during transcription (transcriptional), after transcription (post-transcriptional), during translation (translational), or after translation (post-translational)
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
Photo credit:
http://web.nchu.edu.tw/~jhliu/index_e.htm
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
Photo credit:
http://web.nchu.edu.tw/~jhliu/index_e.htm
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.
Photo credit:
http://www.ks.uiuc.edu/Research/methylation/
Photo credit:
http://www.ks.uiuc.edu/Research/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
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)
Gene regulation and 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.
Photo credit:
http://www.dreamstime.com/royalty-free-stock-photography-cancer-cause-hiding-dna-strand-image9429447
Two types of genes have significant control on the rate of mitosis.
- Tumor-Supressor 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.
Photo credit:
http://www.dreamstime.com/royalty-free-stock-photography-cancer-cause-hiding-dna-strand-image9429447
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.
Photo credit:
http://www.medicalobserver.com.au/news/bladder-cancer
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.
Photo credit:
http://www.medicalobserver.com.au/news/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.
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
If lactose is not present, the LacI protein binds to the operator sequences. This does not allow RNA polymerase to bind and therefore transcription does not occur
lazY codes for β-galactosidase permease
lacA codes for an unknown transacetylase that participates in lactose decomposition
*Lac I is a repressor protein
If lactose is not present, the LacI protein binds to the operator sequences. This does not allow RNA polymerase to bind and therefore transcription does not occur
When lactose is present, it binds to the LacI protein. This changes the protein's shoe and does not allow it to bind to the operator sequence. Thus, RNA polymerase can bind to the promoter region and transcription of the lac genes occurs and lactose can decompose.
Lactose in this case acts as an Inducer molecule, inducing the decomposition
Lactose in this case acts as an Inducer molecule, inducing the decomposition
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 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.