regulation of gene expression · gene expression.notebook february 29, 2012 when the proteins are...
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Gene Expression.notebook February 29, 2012
Regulation: the mechanism by which protein synthesis is increased, decreased, initiated or stopped• Prevents a cell from wasting energy transcribing and translating genes when they are not needed• Provides the potential for cellular differentiation
Regulation of Gene Expression: What turns DNA molecules on and off?
Gene Expression in ProkaryotesDefault = ONProkaryotes are sensitive to their environment, and their genetic activity is controlled by specific proteins that interact directly with their DNA to quickly adjust to environmental changes. The default setting for prokaryotes appears to be on: allowing for the continual synthesis of protein to occur, whereas in eukaryotes the system is normally off until activated.
Gene Expression.notebook February 29, 2012
Operon Hypothesis
An operon is a selfregulating series of genes that work in together. An operon includes a special segment of genes that are regulators of the protein synthesis, but do not code for protein, called the promoter and operator. These segments overlap, and their interaction determines whether the process will start and when it will stop. RNA polymerase must create RNA by moving along the chromosome and “reading” the genes in the process of transcription.
Because there is no nucleus to separate the processes of transcription and translation, when bacterial genes are transcribed, their transcripts can immediately be translated.
Gene Expression.notebook February 29, 2012
Regulator• a portion of the DNA molecule that codes for repressor proteins
repressor proteins can repress or block mRNA transcription• blocks RNA polymerase
prevents it from binding or moving down the DNA• binds to the operator site so transcription cannot occur
Promoter• an initiation site on the DNA which the RNA polymerase enzyme bonds to in order to initiate transcription
Operator• positioned within the promoter, or between the promoter and structural genes (genes that code for polypeptides)• the site on the DNA molecule that the repressor can bind to
when the repressor is bound to the operator, RNA polymerase cannot bind to the promoter, and transcription is turned offwhen the repressor is removed, RNA polymerase can bind to the promoter, and transcription is turned on
repressor
Gene Expression.notebook February 29, 2012
*a repressor is an allosteric enzyme – the ability of the repressor to bind depends on another molecule called an effector, which could activate or inactivate the repressor protein
Back
Gene Expression.notebook February 29, 2012
For Example.... The lac operon
E. coli bacteria in the intestines of mammals can use the energy supplied by lactose in order to grow by breaking the bonds between the two sugars
βgalactosidase (βg) is the enzyme used to break down lactose. But bacteria only produce it when they need to, so they must regulate the production of βg using a negative control system.
The Lactose Utilization Operon is an example of an Inducible Operon
Gene Expression.notebook February 29, 2012
§ When E. coli are grown in a medium containing lactose, βg is produced§ When E. coli are grown in a medium that does not contain lactose, βg is not produced
The gene for βg is part of an operon (cluster of genes under the control of one promotor and one operator)
Gene Expression.notebook February 29, 2012
This operon is induced in the presence of lactose:• bacteria takes in lactose and converts it to allolactose (an effector)• allolactose binds to the repressor protein• the repressor protein changes shape and cannot bind to DNA• RNA polymerase is able to bind to the DNA, and mRNA can be made
Allolactose is an inducer (effector) because it inactivates the repressor Repressor
Substrate
Gene Expression.notebook February 29, 2012
• If tryptophan is available, the enzyme needed to create it doesnt need to be made.
Another Example....The tryp operon
The Tryptophan Synthesis Operon is an example of an Repressor Operon
• Repressor proteins can stop production of trypmaking enzyme, but they are inactive (unable to bind to DNA) if there is no tryp around
Gene Expression.notebook February 29, 2012
Tryptophan is called a corepressor (effector) it is a small molecule that cooperates with the repressor protein to switch the operon off.
If tryptophan is available, it binds to the repressor, causing a shape change. This activates the repressor, and it can now bind to the DNA on the operator site = synthesis of the enzyme is blocked
SubstrateRepressor
Gene Expression.notebook February 29, 2012
Humans have about 42,000 genes that code for proteins, so not all are required at any one time. Our cells have ways to control the transcription and translation of genes.
Gene Expression in EukaryotesDefault = OFF
Genes that code for proteins that are constantly required in basic life function for the cell are not turned "off" they are always being transcribed.
These genes are known as housekeeping genes.
Gene Expression.notebook February 29, 2012
When the proteins are needed, our cells turn on our genes. Unlike prokaryotes, multiple generegulating mechanisms operate in the nucleus before and after RNA transcription, and in the cytoplasm both before and after translation. Regulation can occur at any point in the synthesis of a protein.
Controlling Gene Expression
Pre-Transcription
Transcription
Post Transcription
Translation
Post-Translation
Gene Expression.notebook February 29, 2012
Pre-transcriptional Control:• Because certain sections of the DNA molecule are more condensed (think about the way that the DNA strand is stored), they are less accessible, and as a result are more difficult to transcribe• The more “loosely packed” sections are more likely to be transcribed
Gene Expression.notebook February 29, 2012
Transcriptional Control:• There are no operons in eukaryotic cells• Gene expression relies on DNA promoters and transcription factors (regulator proteins) which together create an initiation complex• An initiation complex is the proteinprotein interaction that allows RNA polymerase to bind to the promoter sequence of each gene
Gene Expression.notebook February 29, 2012
Post-transcriptional Control:• The cell controls the rate at which the premRNA is processed into mRNA• This can be done by preventing the addition of the 5` cap (if it is absent, enzymes will degrade the mRNA) or the polyA tail (if it is absent, it will remain in the nucleus)
Gene Expression.notebook February 29, 2012
Translational Control:• Once mRNA is in the cytoplasm, regulatory proteins can inhibit the ability of the ribosome to attach to the mRNA and prevent translation.
October 8, 2009
Gene Expression.notebook February 29, 2012
Post-Translational Control:• The polypeptide chain is modified chemically or the cell varies its rate at which it becomes a functional protein. The polypeptide may also be degraded by the cell before it becomes a functional protein
Reading p. 272-280
Gene Expression.notebook February 29, 2012