MODULE 6 SECTION 1 - CELLULAR CONTROL Flashcards
What are the different levels in which gene expression can be controlled?
- transcriptional level.
- post-transcriptional level.
- post-translational level.
Transcriptional level control (transcription factors).
Gene expression can be controlled at this level by altering the rate of transcription of genes.
Transcription factors:
- Proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription.
- Transcription factors that start transcription are called activators.
- Transcription factors that stop transcription are called repressors.
- Shape of the factors determines whether it can bind to DNA or not. Shape can sometimes be altered by binding of some molecules like sugar/hormone.
- Means that amount of some molecules in an environment/cell can control synthesis of some proteins by affecting transcription factor binding.
Eukaryotes:
- Transcription factors bind to specific DNA sites near the start of their target genes.
- RNA polymerase binds to them, initiating transcription.
Prokaryotes:
- Control of gene expression often involves transcription factors binding to operons.
Operons
- Operons are a section of DNA that contain a cluster of structural genes (that are all transcribed together), control elements (promoter and operator regions), and sometimes a regulatory gene.
- Promoter comes before operator.
- Structural genes: codes for useful proteins such as enzymes.
- Control elements: Contains a promoter region where RNA polymerase binds to, and an operator region where transcription factors can bind.
- Regulatory gene: Codes for transcription factors (activators or repressors).
Example of an operon - Lac operon in E coli
Operons are an example of transcriptional level control of gene expression.
E coli bacterium respires glucose, but can respire lactose if glucose is no available. The genes that code for the enzymes required to respire lactose are found on the lac operon.
Sturcture of the lac operon:
3 structural genes (code for the proteins which allow E coli to respire lactose:
- lac Z (codes for beta-galactosidase)
- lac Y (codes for lactose permease)
- lac A.
Regulatory gene lacl:
- codes for protein called lac repressor (transcription factor).
Promoter region:
- Where RNA polymerase binds to.
Operator region:
- Where lac repressor binds to.
Lac operon - abscence of lactose
- Regulatory gene lacl codes for and produces lac repressor (transcription factor).
- Lac repressor binds to the operator region.
- This blocks RNA polymerase from binding to the promoter region.
- The structural genes are not transcribed and the proteins are not coded for and produced.
Lac operon- presence of lactose
- Lactose binds to lac repressor, causing it’s tertiary structure to change shape.
- Change in shape means that the lac repressor can no longer bind to the operator region.
- RNA polymerase is able to bind to the promoter region and begin transcription of the structural genes - lacZ, lacY, lacA.
- They code for beta-galactosidase, lactose permease.
What is the function of lactose permease?
Lactose permease is a membrane protein that transports lactose into the cell.
Post-transcriptional level control
Careful, post-TRANSCRIPTIONAL
mRNA in eukaryotic cells are edited after transcription.
- Introns: DNA sequences that do not code for amino acids.
- Exons: DNA sequences that do code for amino acids.
Primary/pre mRNA:
- mRNA strands containing both introns and exons.
Splicing:
- Primary mRNA undergoes a process called splicing, where introns are removed and exons are joined together by spliceosomes.
- This forms mature-mRNA. (strands containing only exons).
- Occurs inside the nucleus,
- Mature mRNA leaves the nucleus via nuclear pores for the next stage of protein synthesis (translation).
Post-translational level control
Careful, post-TRASLATIONAL
Some proteins are not functional straight after they have been synthesised - they have to be activated to become a functional protein.
Protein activation is controlled by molecules such as sugars and hormones.
Post-translational level control example - cAMP
- Some protein activation control molecules (such as hormones) work by binding to specific complementary protein receptors on the cell surface membrane, and triggering the production of cAMP inside the cell.
- cAMP activates proteins inside the cell by changing their 3D structure.
- E.g it can cause the change in the shape of the active site of an enzyme, making it become more or less active.
Example - activation of protein kinase A (PKA) by cAMP:
- Protein kinase A (PKA) is an enzyme made of 4 subunits.
- When cAMP is not bound, the 4 subunits are bound together and are inactive.
- When cAMP binds, it causes a change in the enzyme’s 3D structure, releasing the active subunits.
- PKA is now active.
cAMP is a second messenger.
What is a body plan?
The general structure of an organism.
The particular positioning of the body parts.
What are homeobox genes?
- They are regulatory genes that control body plan development.
Regulatory genes?
Genes that code for transcription factors
