Control of Gene Expression 19.2 Flashcards
Types of genes
Structural and regulatory genes
Structural genes
Genes that code for metabolic proteins with a structural role
Regulatory genes
Genes that code for proteins that control the expression of a gene.
Housekeeping genes
Genes that code for enzymes that are necessary for metabolic pathways
Why do we regulate genes?
The entire genome of an organism is present in every cell that contains a nucleus which includes genes that aren’t required by that cell so you have to regulate the expression of the genes so that they can be turned on or off depending on the demand of the product.
Ways in which genes are regulated
Transcriptional - 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 started or stopped Post-translational - proteins can be modified after synthesis to change their function
What is chromatin?
DNA is a very long molecule so has to be wrapped around proteins called histones so that the DNA can be packed into the nucleus. It forms a complex called chromatin
Heterochromatin
This is when the DNA is wound tightly around the histones, causing chromosomes to be visible during cell division.
Euchromatin
This is when DNA is wound loosely around the chromatin which is present during interphase.
When in the cell cycle does protein synthesis occur?
During interphase
Why does protein synthesis occur when it does during the cell cycle?
Because DNA can only be replicated if it is in the form of euchromatin as it needs to be loosely wound around histones so that RNA polymerase can reach it. This is why protein synthesis cannot be done with heterochromatin as it is too tightly wound.
Why does DNA wrap around histones?
DNA is negative and histones are positive so they can associate with each other
How can you increase or decrease how tightly wound the chromatin is?
You can modify the histones to increase or decrease the degree of packing. If you add acetyl or phosphate groups, it makes the histones less positive and therefore the DNA won’t wind around it as tightly which means the genes will be transcribed easier. You could add methyl groups which make the histones more hydrophobic and this will make the DNA bind more tightly to the histones so will prevent gene transcription. This regulation is called epigenetics.
Promoters
100 base pairs upstream (before the DNA) that controls the transcription of specific genes that are needed at that time.
How does transcription begin?
Certain proteins called transcription factors to bind to the promoters and allow RNA polymerase to bind so that it can catalyse the formation of phosphodiester bonds between nucleotides and so transcription begins.
How is oestrogen expressed?
- Oestrogen is lipid-soluble so will pass through the cell membrane and nuclear envelope, into the nucleus. 2. The oestrogen then binds to oestrogen receptors which are actually transcription factors 3. The receptors then change shape and detach from the oestrogen. 4. The receptor then binds to the promotor site which allows transcription to occur so the gene can then be expressed.
Constitutive genes
Genes that are expressed all the time but can waste energy
Inducible genes
Genes that are only expressed when they are required.
Operon
An operon is a group of genes that are under the control of the same promoter and are all expressed at the same time. They are much more common in bacteria and prokaryotes.
What is the preferred respiratory substrate for bacteria?
Glucose is the preferred respiratory substrate for bacteria, but if this is not present, then lactose is used. This means different enzymes are needed to metabolism lactose.
What is a lac operon?
A group of 3 structural genes: lacz, lacy and lacA which are involved in the metabolism of lactose.
What do the genes in the lac operon code for?
The genes code for the enzymes: beta galactosidase, lactose permease and transacetylase
What is a repressor protein?
A protein that prevents the transcription of the structural genes in the absence of lactose and this protein is made by a regulatory gene called lac I.
Where does the repressor protein bind?
It will bind to an area on the gene called the operator
What happens in the lac operon when lactose is absent and glucose levels are high?
- The repressor gene will code for a repressor protein
- The repressor protein will bind to the operator region
- This prevents the RNA polymerase from binding to the promoter region so transcription will not occur.
What happens in the lac operon when lactose is present?
- The repressor gene will code for a repressor protein
- Lactose will act as an inducer and bind to the repressor protein
- The shape of the repressor protein will change so it can no longer bind to the operator region
- This means RNA polymerase can bind to the promoter region and transcription can occur.
Role of cAMP
cAMP is cyclic AMP. it is present when lactose levels are high and glucose levels are low. Even though RNA polymerase can bind when lactose is present, it is quite slow so needs to be sped up. This is why cAMP binds to CRP (cAMP receptor protein) and this forms a complex that binds to the CRP site. RNA polymerase can now bind to the promoter region and transcription can happen quicker.
What happens if glucose is brought in when lactose is present?
If glucose levels rise, the cAMP levels will decrease and this will slow down transcription of the genes for the metabolism of lactose. Glucose will then be metabolised again as it is the preferred respiratory substrate.
What happens during post-transcriptional control?
After transcription occurs, you are left with a precursor molecule which is called pre-mRNA. This becomes modified into mature mRNA before it binds to the ribosome. Its modifications include:
- A cap is added to the 5’ end
- A tail is added to the 3’ end
- Both help to stabilise mRNA
- Splicing also occurs where the RNA is cut at specific points by sNRPs so the introns are removed and the exons are joined together.
Translational control
- degradation of mRNA which is where the mRNA will only resist in the cytoplasm if it is strong and will therefore synthesise more protein
- inhibitory proteins can bind to the mRNA to prevent translation as the mRNA wouldn’t be able to bind to the ribosome.
Protein Kinases
These are enzymes that catalyse the addition of phosphate groups to proteins so that their tertiary structure changes and therefore so does their function. Kinases are activated by cAMP binding to their allosteric site
Post-translational control
This means modifying proteins that have already been synthesised:
- adding non-protein groups
- modifying amino acids
- shortening or folding the proteins
- modifying by cAMP so increasing the rate of transcription
What is the difference between RNA polymerase and DNA polymerase
RNA polymerase is the enzyme that catalyses transcription. It makes mRNA and so one strand of DNA is formed. DNA polymerase is an enzyme that catalyses DNA replication. It occurs before cell division and two semi-conservative strands are used.
