operons Flashcards
define operon
group of genes that are under control of the same regulatory mechanism and are expressed at the same time
define structural gene
codes for synthesis of all proteins except regulatory proteins (factors)
name the proteins produced from the structural gene in the lac operon
- LacZ
- LacY
- LacA
state the role of LacZ
codes for beta galactosidase (hydrolyses lactose)
state the role of LacY
codes for lactose permease (facilitates passage of lactose into cell)
state the role of LacA
codes for transacetylase
describe the role of regulatory genes in the lac operon
codes for transcription factor (repressor protein) that prevents transcription of structural genes in absence of lactose
describe the role of structural genes of the lac operon
codes for proteins involved in lactose digestion
describe the role of the operator region of the lac operon
transcription factor binding site - switches structural genes on/off
describe the role of the promoter region of the lac operon
RNA polymerase binding site to allow transcription
explain how the lac operon works when lactose is absent from the growth medium.
- repressor protein is constantly produced + binds to the operator
- so prevents RNA polymerase binding to promoter section of DNA + starting transcription
explain how the lac operon works when lactose is present in the growth medium.
- lactose binds to repressor protein, causing it to change shape + it no longer binds to operator
- so RNA polymerase can bind to promoter
- 3 structural genes are transcribed + proteins are synthesised
describe the role of cAMP in the control of the lac operon
- binding of RNA polymerase = slow rate of transcription - needs to be higher to produce enzymes to metabolise lactose
- binding of cAMP to CRP allows CRP to bind to promoter & increase rate of transcription of lac operon proteins
- glucose entering E.coli decreases cAMP - transcription of genes for metabolism of lactose reduced
- glucose is preferred substrate
