Unit 5: Gene Regulation Flashcards
Why does gene expression need to be regulated?
To conserve materials and energy
Only genes that are specific to the cell and needed at the particular time should be expressed
(Note that multicellular organisms have unique cell types that share the same genome - they express different sets of genes)
What are the two ways to control metabolic pathways?
Feedback inhibition - using a molecule involved in the pathway to inhibit enzymes earlier in the pathway
Genetic mechanisms - using molecules involved in the pathway to regulate the production of the enzymes themselves (i.e. gene expression)
What are the parts of an operon?
Transcription unit (the promoter and two or more related genes) and operator (switch)
How do operons help with coordinate control of genes?
In prokaryotes, related genes can be under the control of a single promoter and operator. That means that all of them are expressed or none of them are.
Identify and describe the two ways of controlling operons
Positive control: uses an activator to increase gene expression
Negative control: uses a repressor to decrease gene expression
Identify and describe the two types of negative control of operons
Repressible operons: repressor only works when a corepressor is present
Inducible operons: inducer stops the activity of the repressor
Describe the trp operon
(not its control - just the operon / gene products)
Produces enzymes related to the synthesis of tryptophan, an amino acid that is required by cells to build protein
When should the trp operon be expressed and why?
Tryptophan (trp) is an amino acid that is necessary for building proteins
If trp is present in the cell’s environment already, it does NOT make sense to build more.
Ergo - trp operon should NOT be expressed
If trp is not present in the cell’s environment, it needs to build some
Describe the regulation of the trp operon
Trp repressor is natively inactive, so the trp genes are expressed
BUT
Trp acts as a corepressor. So when trp binds to repressor, it becomes active and prevents trp operon from being expressed
Describe the lac operon
(not its control - just the operon / gene products)
Produces enzymes related to the breakdown of lactose (a disaccharide) into monosaccharides (including glucose)
When should the lac operon be expressed and why?
Lactose is a disaccharide that can be broken down into glucose, providing the cell with energy
If lactose is NOT present in the cell’s environment, then it does not make sense to build the enzymes to break it down
If lactose is present but glucose is not, the cell needs to get glucose and it can do this by breaking down lactose. Therefore, the operon is expressed
If lactose and glucose are both present, the cell should use glucose first so it still doesn’t make sense to express the lac genes
Describe the negative regulation of the lac operon
Lac repressor is natively active, preventing the lac genes from being expressed
BUT
Allolactose (which is present if lactose is present) acts as an inducer. When allolactose binds to the repressor, the repressor becomes inactive and the operon is no longer blocked, thus the genes are expressed
What is the relationship between cAMP and glucose?
Glucose is used in glycolysis to make ATP from ADP
ADP, AMP (adenosine monophosphate) and cAMP (cyclic AMP) are in chemical equilibrium
Therefore when glucose levels are high, ADP is converted to ATP and the cell’s cAMP levels drop
When glucose levels are low, ADP is at high concentration which also allows the cell’s cAMP levels to be high
What does it mean if a cell has a high cAMP level?
High cAMP = High ADP = Low ATP
This means the cell has low energy levels (likely due to a lack of enough glucose in the environment)
Describe the positive regulation of the lac operon
RNA polymerase only binds to the lac operon promoter weakly, even in the absence of the repressor
When cAMP is present, it binds to CAP, which is an activator
CAP+cAMP binds to the promoter to increase RNA polymerase’s ability to bind to (and initiate the transcription of) the lac operon
Describe the activity of the lac operon in a cell whose environment is:
+glu +lac
Lac repressor will be inactive, allowing the operon to function
CAP will be inactive (due to low cAMP levels), minimizing the amount that the operon is expressed
Thus the lac operon will be expressed at low levels
Describe the activity of the lac operon in a cell whose environment is:
-glu +lac
Lac repressor will be inactive, allowing the operon to function
CAP will be active (due to high cAMP levels), increasing the amount that the operon is expressed
Thus the lac operon will be expressed at high levels
Describe the activity of the lac operon in a cell whose environment is:
+glu -lac
Lac repressor will be active, preventing the lac from functioning
Thus the lac operon will not be expressed
Describe the activity of the lac operon in a cell whose environment is:
-glu -lac
Die, probably.
IDK
What type of cells have operons?
Prokaryotes
Operons are generall NOT FOUND IN EUKARYOTES!
Explain the difficulty in coordinately controlling genes in eukaryotes
Related genes (ex: genes that make enzymes involved in the same biochemical pathway) are usually found on separate areas of the genome (different parts of a chromosome or entirely separate chromosomes)
Coordinate control requires activating related genes that may be in completely different areas
Define / describe
Differential Gene Expression
In multicellular organisms, different cell types contain the same genomes but have different gene expression
How is gene expression usually controlled in eukaryotes?
By regulating when genes are transcribed, because:
1. It is the first step in gene expression and
2. It can be controlled by external factors
List two ways eukaryotic cells can regulate transcription
- Changing how loose / condensed the DNA is (more condensed = less accessible = less gene expression)
- Changing which transcription factors are present in the nucleus
What determines how condensed chromatin is in eukaryotes?
Histone proteins
Modification to the histone tails can increase or decrease their attraction to each other
Ex:
* acetylation causes histones to become less attracted, thus loosening up
* methylation can cause histones to become more attracted and more coiled (but not always…)
Define
Epigenetics
Inheritance of traits based NOT on the DNA sequence itself, but on how the DNA is expressed
Why is epigenetics important?
Two reasons
- It allows specialized cells to pass on characteristics of that cell type to its offspring; Ex: a white blood cell can divide to make two white blood cells which continue to behave like white blood cells
- It can contribute to an offspring’s traits; Ex: a woman that experiences severe hunger can have a child whose metabolism is slower and more efficient — this is an emerging area of research!
Define
Control elements
Regions of DNA upstream from the promoter that can bind to transcription factors, helping to regulate transcription in eukaryotic cells
How do eukaryotic organisms exert combinatorial control over gene expression?
Even though related genes may be on different regions of chromosomes or even different chromosomes altogether, they can be controlled by the same environmental changes
Related genes often have the same set of control elements
This means that the presence of transcription factors that would cause expression of one gene would also cause the expression of other genes that have the same combination of control elements
What are the two categories of transcription factors?
General TFs - needed for the transcription of any gene
Specific TFs - needed for the transcription of certain genes
* It is the combination of specific TFs that allow the expression of genes that are more specialized or specific to certain cell types
List the ways in which eukaryotic cells can regulate the amount of a certain protein it has
- Before transcription - either by changing the TFs present or by changing how condensed the DNA is
- After transcription - post transcriptional modifications can alter how much the mRNA will be translated
- Before translation - regulatory proteins can prevent mRNA from binding to ribosome
- By degrading the mRNA so that more proteins are not made from it
- By degrading the formed protein after it has been made
Define
Ubiquitin
A small protein “tag” that gets added to proteins to mark them for destruction. Ubiquitylated proteins are digested by proteases (enzymes that break down proteins) in the cell
