Control of gene expressions.docx

All cells in the organism contain the same DNA. Proteins common to all cells of a multicellular organisms= housekeeping protein such as DNA replication, RNA polymerase etc. Different cell types produce different sets if specialised proteins that are responsible for distinctive properties. Some gene...

All cells in the organism contain the same DNA. Proteins common to all cells of a multicellular organisms= housekeeping protein such as DNA replication, RNA polymerase etc. Different cell types produce different sets if specialised proteins that are responsible for distinctive properties. Some genes are turned on in specific organs/cells whereas other genes are turned on in multiple cells/organs but expressed at different levels. Prokaryotes Why are genes switched on or off? A significant amount of energy is required to always always express every gene. More efficient to turn on genes when they are required. Cell can change the expression of its genes in response to external signals. Transcription factors turn gene expresso on or off (activators or repressors) Regulatory sequences can vary in length. Promoter region contains regulatory sequences to which transcription factor can bind. Negative and positive regulation- happens through transcription factors. Negative regulation Repressor binds to the promoter region- competes with RNA polymerase therefore no transcription. Some of repressor proteins can be regulated by binding of ligands so if a ligand binds to the repressor protein, it removes it from promoter region so RNA polymerase can bind and enable transcription. E.g. when the repressor is on its own, its able to recognise the promoter region on DNA sequence and binds to it but when the ligands bind to repressor, causes a confirmational change therefore it’s not able to bind to the DNA anymore so it comes off the promoter region and gene switches on. Oppositely, on its own the repressor can be inactive so its unable to bind to DNA sequence but when the ligand binds to it, it changes conformation so now the repressor can bind to the promoter region and gene switches off. Activators normally help to recruit RNA polymerase by binding to DNA sequences. Positive regulation Activator ON ITS OWN bind to regulatory sequence on promoter region. Recruits RNA polymerase and initiate transcription of gene. Activator can bind a ligand which changes its confirmation and cant bind to DNA anymore therefore RNA polymerase can’t be recruited so gene can’t be transcribed. Oppositely, removal of the ligand from the activator switches the gene off by removing the activator protein when the ligand binds to the activator. Operon= “A group of genes or a segment of DNA that functions as a single transcription unit. It is comprised of an operator, a promoter, and two or more genes that are transcribed into one polycistronic mRNA.” Lactose operon – dual regulation 3 genes that are important. Lactose permease mediates uptake of lactose. (Lac Y) Hydrolysis of lactose to galactose and glucose by beta-galactosidase. (Lac Z) Lac (A)- function unknown. Organisation of Lac operon Contains a separate promoter which drives the expression of inhibitor/ repressor. Another promoter region with the structural genes which are necessary for metabolism for lactose. Has a operator region which transcription factors bind. No lactose present. Repressor is expressed, recognises operator region, and binds to it to inhibit transcription. Lactose present Lactose acts as ligand to repressor, binds to it and changes it confirmation therefore cannot bind to operator region so the genes can then be transcribed. When beta galactosidase cleaves the lactose, it changes a little bit therefore what binds isn’t lactose, its Allolactose. When allolactose binds repressor, repressor can no longer bind to DNA and genes can be transcribed. E-coli prefers glucose over lactose. Sufficient glucose no expression of the Lac operon Insufficient glucose expression of the Lac operon. Positive regulation aspect of the lac operon When glucose is present, there is low levels of cAMP. When glucose is absent, there is high levels of cAMP. cAMP is a ligand and binds to transcription activator which are cyclic AMP receptor protein (CRP)/ catabolite activator protein (CAP). Once cAMP is bound to the transcription activator, it can recognise the DNA sequence and bind to promoter region- helps to recruit RNA polymerase therefore initiate transcription. As long as glucose is available, it will always be preferred carbon source for E-coli. If glucose runs out, cAMP levels increase therefore another source is used. Dual regulation of lac operon by Lac repressors and CRP/CAP If allolactose is absent, it binds to operator sequence and interferes with binding of RNA polymerase therefore the Lac operon is off. If allolactose is present, it binds to repressor, changes confirmation, RNA polymerase binds and initiates transcription. For the transcription to be initiated, RNA polymerase can’t do it by itself as it needs transcription activator. If glucose is present, cAMP levels will be low so it can’t bind to the transcription activators (CAP/CRP) therefore no transcription of genes as it cannot recruit RNA polymerase. If glucose is absent, cAMP levels will be high so it can bind to the transcription activators (CAP/CRP) and initiate transcription. Trp operon- single negative regulation. In E-coli Encodes 5 structural genes necessary for tryptophan biosynthesis in the absence of it in the environment. Has a promoter region and operator sequence within the promoter region. Trp operon is only regulated by trp repressor/ negative regulation. In presence of trp, operon is OFF In absence of trp, operon is ON. Tryptophan repressor is continually synthesised interpedently of the operon and is an example of a repressor which will bind tryptophane as a ligand. Presence of Tryptophan means the operon is switched off. Absence of Tryptophan means the operon is switched off. When there is low tryptophan RNA polymerase binds to promoter region and begins transcription of the 5 structural units which makes up the tryptophan. When there is high tryptophan already Tryptophan acts as a co-repressor and binds to trp repressor which activates it and causes it to bind to promotor. This means RNA polymerase cannot bind to promoter anymore therefore the transcription of the 5 structural units which makes up the tryptophan is not possible anymore. Summary for prokaryotes Eukaryotes Much more complex regulatory switches than in prokaryotes. Combination of multiple repressors and activators can be integrated to regulate expression. Genes can be transcribed individually, no operons. //// ///// ///