Control of gene expression Flashcards
19.2
Gene expression can be controlled at…
4 stages: transcriptional control, post-transcriptional control, translational control and post-translational control
- Transcriptional control in eukaryotes
Chromatin remodelling
Transcriptional factors definition and result of increasing/decreasing
Proteins that bind to specific DNA sequences-promoter regions. The right complex of transcriptional factors are needed for it to begin.
Increasing/decreasing production of specific transcription factors, transcription of other genes can be controlled.
- Transcriptional control in eukaryotes
Transcriptional factors
Chromatin remodelling explanation
DNA wraps twice around a bundle of 8 histones (due to DNA being negatively charged and histones positively) to form a chromatin sunbit structure=nucleosome
- methylation of DNA and histones causes nucleosomes to pack tightly together meaning transcription factors cannot bind to the DNA and the genes are not expressed (heterochromatin)
-acetylation and phosphorylation reduce the positive charges on the histones to result in looser packing of nucleosomes, allowing transcription factors to bind the DNA (euchromatin)
3.Transcriptional control in prokaryotes
the Lac Operon
Operon definition
A group of genes that are under the control of the same regulatory mechanism comprised of structural genes and control sites
What is broken down into what to be used as an energy source in the bacterial cell?
Lactase breaks down the substrate lactose
Promoter region
The area to which RNA polymerase binds for transcription
Control sites
Promoter and operator regions are regulatory genes as they don’t code for polypeptides
Operator region
Controls switching on or off of structural genes. If a repressor protein is bound to the operator then the binding site for the RNA is partially blocked and so the structural genes cannot be read. (Vice versa)
E.coli codes for the synthesis of 2 enzymes…Controlled by..
B galactoside permease (lactose permease)
B galactoside (lactase)
..the lac operon
Lactase is an.. and why?
An inducible enzyme and means can only be synthesized when lactose is present and prevents bacteria from wasting energy and materials
Structure of lac operon
Promoter
Operator
Structural gene-lacZ
Structural gene- lacY
Structural gene-lacA
to left is promoter for regulatory gene
regulatory gene lacl that codes for lac repressor protein
two binding sites -operator and effector molecule (lactose)
Lactose is present (in the medium the bacterium is growing in)
1.Uptake of lactose by the bacterium
2.The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site
3.RNA polymerase is then able to bind to the promoter region and transcription takes place
4.mRNA from all three structural genes is translated
5.Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium
Lactose is absent (in the medium the bacterium is growing in)
1.Regulatory gene is transcribed and translated to produce lac repressor protein
2.Lac repressor protein binds to the operator region upstream of lacZ
3.Due to presence of repressor protein RNA polymerase is unable to bind to the promoter region
4.Transcription of the structural genes does not take place
5.No lactase enzyme is synthesized
At which levels can genes be regulated in what ways
Transcription-genes can be turned on or off
Post-transcriptional- mRNA can be modified which regulates translation and the types of proteins produced
Translational-translation can be stopped or started
Post-translational- proteins can be modified after synthesis which changes their functions
1.Post-transcriptional/pre-translational control RNA processing (cap and tail)
A cap (a modified nucleotide) is added to the 5’ end and a tail ( a long chain of adenine nucleotides) is added to the 3’ end.
-help to stabilise mRNA to ribosomes
2.Post-transcriptional/pre-existing control RNA editing
Nucleotide sequence of some mRNA molecules can also be changed through base addition, deletion or substitution. (same effect as point mutation and result in synthesis of different proteins which may have different functions) = increases range of proteins that can be produced from a single mRNA molecule/gene
1.Post-transcriptional/pre-translational control RNA processing (splicing)
Occurs where the RNA is cut at specific points-the introns (non-coding DNA) are removed and exons (coding DNA) are joined together
1.Translational control (degradation of mRNA)
The more resistant the molecule the longer it will last in the cytoplasm meaning a greater quantity of protein synthesised.
2.Translational control (binding of inhibitory proteins to mRNA..)
Prevents it binding to ribosomes and the synthesis of proteins
3.Translational control (activation of inhibition factors which aid the binding of mRNA..)
To ribosomes (eggs of many organisms produce large quantities of mRNA which are not required until after fertilisation, at which point initiation factors are activated)
Definition protein kinases
Are enzymes that catalyse the addition of phosphate groups to proteins.
-addition of phosphate group changes the tertiary structure and so function of protein
-many enzymes are activated by phosphorylation and so makes them important regulators of cell activity
-themselves activated by secondary messenger cAMP.
Post-transcriptional control
-Addition of non-protein groups such as carbohydrate chains, lipids, phosphates
-Modifying amino acids and the formation of bonds such as disulfide bridges
-Folding or shortening of proteins
-Modification by cAMP- for example, in the lac operon cAMP binds to the cAMP receptor protein increasing the rate of transcription of the structural genes
