6.1.2 regulation of gene expression Flashcards
define exon
coding/expressed region of DNA
define intron
non-coding region of DNA
define operon
group of genes that function as single transcription unit
define transcription factor
protein or short non-coding RNA which can combine with specific site on length of DNA to inhibit or activate transcription of gene
regulation of gene expression at transcriptional level: in prokaryotes
- enzymes that catalyse metabolic reactions involved in basic cellular functions are synthesised at consistent rate
- enzymes which may only be needed under certain conditions are synthesised according to needs of cell
eg. lac operon
regulation of gene expression at transcriptional level: the lac operon (prokaryotes)
- e. coli normally metabolises glucose as respiratory substrate
- if glucose is absent & disaccharide lactose is present, lactose induces production of 2 enzymes:
1. lactose permease = allows lactose to enter bacterial cell
2. beta-galactosidase = hydrolyses lactose to glucose & galactose
what does the lac operon consist of
- length of DNA (~6000 base pairs long) containing operator region lac0 next to structural genes lacZ & lacY
- lacZ codes for beta-galactosidase
- lacY codes for lactose permease
- P (promoter region) is next to lac0
- RNA polymerase binds to P to begin transcription of structural genes: lacZ & lacY
- operator region & promoter region = control sites
- regulatory gene (I) codes for repressor protein (lacI)
what happens when regulatory gene expressed
- repressor protein produced
- binds to operator = prevents RNA polymerase binding to promoter region
- prevents lacZZ & lacY from being transcribed = enzymes for lactose metabolism aren’t made (‘off’)
what happens when lactose is added to the culture medium (& glucose is used up)
- molecules of lactose bind to LacI repressor protein molecules
- alters shape of LacI repressor protein = preventing it binding to operator
- RNA polymerase molecule can now bind to promoter region & begin transcribing structure genes into mRNA
- these are then translated into 2 enzymes
regulation of gene expression at transcriptional level: transcription factors (in eukaryotes)
- transcription factors act within the cells nucleus to control which genes are activated/inhibited
- slide along part of DNA molecule & seek/bind to specific promoter regions
- may aid/inhibit attachment of RNA polymerase to DNA & activate/suppress transcription of gene
- essential for regulation of gene expression
- some involved in regulation of cell cycle
post-transcriptional gene regulation: introns & exons
- introns = non-coding regions of DNA in a gene (not expressed)
- exons = coding/expressed regions of DNA
- all DNA of gene (introns & exons) transcribed = primary mRNA
- primary mRNA edited = RNA introns removed
- remaining mRNA exons are joined together, producing mature mRNA
- endonuclease may be involved in editing/splicing process
post-translational level of gene regulation
- involves activation of proteins
- many enzymes activated by phosphorylation
- cyclic AMP is an important second messenger involved in activation of enzymes
post-translational level of gene regulation: outline the 6 steps by which cAMP activates enzymes (may also stimulate transcription)
- signalling molecule (eg. protein hormone glucagon) binds to receptor on plasma membrane of target cell
- activates transmembrane protein, which activates G protein
- activated G protein activates adenyl cyclase enzymes
- activated adenyl cyclase enzymes catalyse formation of cAMP molecules from ATP
- cAMP activates PKA (protein kinase A)
- activated PKA catalyses phosphorylation of many proteins, hydrolysing ATP in process
- phosphorylation activates many enzymes in cytoplasm - PKA may phosphorylate another protein
- this enters nucleus & acts as transcription factor, to regulate transcription
