6.1.1 (b) Flashcards
What are the different levels of gene regulation?
- Transcriptional: genes turned on or off
- Post-transcriptional: mRNA modified to regulate translation and the types of protein produced
- Translational: translations stopped or started
- Post-translational: proteins modified after synthesis – changing their function
What are the 3 methods of transciptional control?
Chromatin Remodelling
Histone Modification
Lac operon
What is is chromatin and what are the different types?
Chromatin is a DNA/protein complex, where DNA is wound around a protein called histone in eukaryotes to fit into a cell nucleus
Heterochromatin: tightly wound DNA causing chromosomes visible during cell division
Euchromatin: loosely wound DNA present during interphase
How does chromatin remodelling work?
Gene transcription cannot occur on heterochromatins as RNA polymerase cannot access genes (explaining why protein synthesis does not occur during cell division, but it does during interphase as RNA polymerase can access genes in euchromatin)
- Regulates the formation of proteins for cell division, also ensures there is no energy-taxing protein synthesis during cell division
What is the process of histone modification?
DNA is negatively charged, and histones are positively charged. Histones can be modified to increase/decrease how tightly DNA is bound to histone
- Adding acetyl (acetylation) or after phosphorylation, the histone becomes less positive, so DNA repels slightly and is not as tightly bound
- Adding methyl (methylation) makes histones more hydrophobic, they bind tighter together and DNA coils more tightly
Define epigenetics
Study of phenotypic changes to organisms due to modification of gene expression instead of the alteration of genetic code
Define:
Operon
Structural Genes
Operon: Group of genes controlled by the same regulatory mechanism & expressed at the same time. Lac stands for lactose in Lac Operon
Structural Genes: Genes that code for proteins not involved in DNA regulation
- 3 in Lac operon: lacZ, lacy and lacA
- These 3 genes code for β-galactosidase, lactose permease, lactose transacetylase (enzymes involved in lactose metabolism)
Define:
regulatory gene
(and repressor protein)
Operator
Promoter
Regulatory Genes: Genes that code for proteins involved in DNA regulation
- LacI in the Lac operon – it codes for repressor proteins: proteins which stop the transcription of lacZ, Y, A in the absence of lactose – positioned next to the promoter
Operator: DNA sequence where the repressor protein binds to
Promoter: DNA sequence where RNA polymerase binds to
What occurs at a Lac Operon in the presence of glucose alone, and lactose alone?
In the presence of glucose
- LacI is expressed, producing the repressor proteins which bind to the operator
- Down regulation occurs: RNA polymerase is blocked from binding to the promoter so the structural gene transcription cannot occur
In the presence of lactose and not glucose
- Lactose binds to the repressor protein causing a conformational change (change in protein shape) which releases the repressor from the operator.
- RNA polymerase can then bind to the promoter and transcription of the structural genes can occur
What is the role of cyclic AMP in a Lac operon?
RNA polymerase binding to the promoter gives a slow rate of transcription
To increase this (for more efficient lactose metabolism), cAMP receptor protein (CRP) binds to cAMP which binds to RNA polymerase which is bound to the operator
- Glucose into E.coli decreases cAMP levels, thus decreasing the transcription of lactose metabolising genes
- If glucose and lactose are present, the preferred respiratory substrate, glucose, is metabolised so lactose is released from the repressor protein
What are the stages of post-transcriptional/pre-translational control?
- After transcription there is a precursor molecule, pre-mRNA (has a mix of introns and exons so is not ready for translation)
- To modify, a cap (modified nucleotide) is added to the 5’ end
- RNA splicing then occurs, which removes all the introns (non-coding DNA)
-
Polyadenylation occurs to add an adenine tail
- The cap and tail help stabilise mRNA and delay degradation in the cytoplasm
-
mRNA editing can also occur: the nucleotide sequence can be changed by base addition, deletion or substitution (same effect as point mutation), synthesising different proteins with different functions
- Increases the no. of proteins that can be produced from one mRNA
What is the purpose of translational control and what are the methods?
To stop translation
- Degrade mRNA*
- The more resistant the molecule, the longer it will last in the cytoplasm = more proteins. Vice versa
- Inhibitory proteins*
- They bind to mRNA preventing mRNA from binding to ribosomes and subsequently stopping protein synthesis
To begin translation
Initiation factors
- Aid the binding of mRNA to ribosomes
- This is done by phosphorylation – protein kinases are the enzymes which catalyse the addition of phosphates to proteins
- Phosphorylation causes tertiary structure change and thus a functional change
- Protein kinases are activated by cAMP
What are the methods for post-translational control?
Involved modifying polypeptide chains to make proteins for specific functions
Ways to do it:
- Add prosthetic group (carbohydrate, lipid and phosphates) to make e.g. glycoproteins, lipoproteins for specific functions such as cell signalling
- Modify amino acids to make bonds – such as disulphide bridges
- Fold or shorten the protein to its tertiary or quaternary structure
- Modify the protein with cAMP
- E.g. cAMP + CRP
- E.g. cAMP + kinases
