Controlling Gene Expression Flashcards
The Four stages of how gene expressions can be controlled
- Transcriptional Control
- Post Transcriptional Control
- Translational Control
- Post Translational Control
What happens in transcriptional control (Transcription Factors in Eukaryotes)
- Transcription Factors bind with specific DNA sequences, the right complexes of transcription factors is required for transcription to begin
- Increasing/ decreasing the production of transcription factors will control the transcription of other genes
- RNA polymerase is not a transcription factor
What happens in transcription Control (Chromatin remodelling) (2)
- DNA associates with histone proteins forming chromatin
- DNA wraps around a bundle of 8 histones which is called a nucleosome
- Methylation causes nucleosomes to pack tightly together = transcription factors cannot bind to DNA and gene cannot be expressed
- Acetylation and Phosphorylation causes nucleosomes to be loosely packed and transcription can bind to DNA
What happens in transcriptional control (The Lac OPeron) (3)
- Operon are a group of genes which are under control of a same regulatory mechanism. Composed of structural genes and control sites ( Promoter and Operator Regions)
- Promotor is where RNA polymerase binds for transcription
- Operator controls genes switching on or off, if nothing is bounded to operator RNA polymerase can bind to promotor. However if a repressor protein is bound to operator the binding site for RNA polymerase to bind with proctor is partially blocked
- Structural Gene codes for protein
What is Down Regulation
- The repressor Protein is constantly produced and binds to the operator. This will prevent RNA polymerase binding to DNA and beginning transcription.
What is the role of Cycle AMP
- To up-regulate the rate of transcription to produce the required amount of enzymes to metabolise lactose.
- By using a cAMP (secondary messenger)
- If glucose levels are low = cAMP is produced and is attached to CAP, this binds DNA, CAP allows RNA polymerase to bind to promoter and transcription will occur
- If glucose levels are high = cAMP is not made and CAP cannot be used to bind DNA. Transcription is at low levels
What is an operon
- A group of genes which are under control of the same regulatory mechanism
What is operon composed of?
- Structural genes and controlling regions
- Controlling Regions (Promotor and Operator)
- Promotor = RNA polymerase binds to this
- Operator = Controls genes from switching on and off, if a repressor protein binds to it this will partially block the binding site for RNA polymerase
- Structural genes codes for a protein
What are the enzymes that E.Coli synthesises for?
- beta galactosidase which hydrolises lactose into glucose and galactose
- beta galactose permease which takes up lactose
Examples of respiratory substrate of E.Coli
- Glucose
- Lactose
What is the function of Beta Galactosidase
- Hydrolises Lactose into Glucose and Galactose
What is the function of Beta Lactose Permease
Enables the cell to take up lactose which is embedded on the cell membrane and creates a channel
Describe the events when lactose is the only respiratory substrate available
- Lactose will passively enter the cell
- Lactose will bind to the repressor protein, this causes the repressor to change shape and release from the operator
- Structural gene can now be transcribed
- Beta galactosidase and Beta lactose permease is formed
What happens in Post - transcriptional control
- Intron is taken out of DNA to become pre mRNA
- In the beginning, transcription produces a molecule called pre-mRNA to form mature RNA
- Cap and tail are added, ‘splicing’ by spliceosomes which remove introns
- RNA can be edited by ‘splicing’ (via sub,del or ins|) meaning a single strand of DNA produces strands of mRNA
How is splicing activated
Through cAMP which triggers a protein to be made to cause splicing
What does the cap do in post transcriptional control
- Allows ribosomes to bind to mRNA better
- Helps to stabilise mRNA and delay degradation in cytoplasm
What happens in translational control if you don’t want it to occur
- Switching translation on and off
- If we don’t want translation to occur mechanisms can be:
- mRNA degradation
- binding of inhibitory proteins and this will stop mRNA binding to the ribosomes and synthesis of proteins
What happens in translational control if you do want it to occur? ( phosphorylation and protein kinases)
-If we do want translation to occur the mechanism would be:
-Activation if initiation factors which will allow mRNA to bind to ribosomes.
- This is done by phosphorylation (adding a phosphate group to the protein will change the tertiary structure and function of the protein|)
- This chemical process is done by protein kinase which will catalyse this and is a regulator of cell activity.
What happens in post-translational control?
- Modifying the protein
- Addition of non - protein groups like carbohydrate chains and lipids for cell signalling
- Modify amino acids and formation of bonds such as disulphide bridges
- Protein folding or shortening (changing structure)
- Modification of cAMP e.g. lac operon cAMP binds to cAMP repressor protein to increase rate of transcription in repressor genes.
What are homeobox genes?
- Ancient, highly conserved regions of DNA found in animals, plants and fungi
- 180 base pairs long
- Codes for homeodomains = protein segments that act as transcription factors.
Function of Homeobox genes
- Able to switch genes on and off = regulatory proteins
- They control the development of the body plan
What are hox genes?
- Are one specific group of homeobox genes only found in animals.
What is a homeodomain?
- part of the protein in homeobox genes which binds to DNA and switches other genes on and off.
What are hox genes responsible for?
- Are responsible for body positioning
- The order in which the genes appear along the chromosomes is the order when their effects are expressed in the organism
