18 Gene Expression in Bacteria Flashcards
Cells a very selective
- about genes they express
- how strongly they are expressed
- when they are expressed
Gene expression
A gene product is actively being synthesized
Efficient use of resources
Survival
Information flow
DNA —> RNA —> Protein —> activated protein
Transcriptional control
When the cell does not production mRNA for specific enzymes
By utilizing regulatory protein
Preventing RNA polymerase from binding to
Translational control
Allows cell to prevent the translation of an mRNA molecule that has already been transcribed
Regulatory molecules can speed up mRNA degradation
Speed and efficiency of three controls
- translational slow but efficient
- transcription faster but more resource
- post-transcription fastest but most energetic
What is lactose made of?
Lactose is a disaccharide made up of one molecule of glucose and one molecule of galactose
What enzyme breaks down lactose?
β-galactosidase - into glucose and galactose
Where does the glucose from lactose go to after lactose breakdown?
Into the glycolytic pathway
What does the cell do with the galactose?
Other enzymes convert galactose to a substance that can also be processed in the glycolytic pathway
What two scientist introduced the lactose metabolism in E. coli as a model system?
Jacque Monod and François Jacob
What is an inducer?
An inducer is a small molecule that triggers transcription of a specific gene
What does lactose induce in E. coli?
Lactose is an inducer for the β-galactosidase gene
Does E. coli produce β-galactosidase when both glucose and lactose are present.
No - only produced when lactose is present with not glucose
What is a mutagen?
X-rays, UV light, chemicals that damage DNA and increase mutation rates.
What two steps were taken to find the particular trait desired for research in lactose metabolism?
- Generate a large number of individuals with mutations at random locations in their genomes. Used mutagen
- Screen the treated individuals for mutants with defects in the process or biochemical pathway in question - in this case, defects in lactose metabolism
Steps in Replica Plating to find Lactose Metabolism Mutants
- Mutants with defects in lactose metabolism are desired, mutagenized bacteria are spread on a “master plate” filled with a gelatinous growth medium containing glucose but not lactose
- A block covered with a piece of sterilized velvet is pressed onto the master plate. Some cells from each colony on the mater plate are transferred to the velvet.
- The velvet is pressed onto a plate called a replica plate that contains medium that differs from the master plate by a single component. In this case, the second medium has only lactose. Cells from the velvet stick to the replica plate’s surface, producing an exact copy of the location of the colonies on the master plate.
- After the transferred cells grow, compare the colonies on the replica plate with those on the master plate. In this example, colonies that grow on the master plate but are missing on the replica are mutant that cannot metabolize lactose.
What are constitutive mutants?
Cells that are abnormal because they produce a product at all times.
LacZ
Codes for β-galactosidase
LacY
Codes for galactosidase permease
LacI
Regulates gene expression
- in normal cells, when lactose is not present, lacI prevents the transcription of lacZ and lacY
- defective mutation on this genes causes the constitutive production of β-galactosidase and galactosidase permease.
Negative control of transcription
Occurs when a regulatory protein called a depressor binds to DNA and shuts down transcription
Positive control of transcription
Occurs when a regulatory protein called a activator binds to DNA and triggers transcription.
What is an operon?
An operon is a set of coordinately regulated bacterial genes that are transcribed together into one mRNA.
