control of gene expression at the transcriptional level (prokaryotes)

Gene expression can be controlled by the transcriptional, post-transcriptional and post-translational levels.

This flashcard set is about transcriptional level

transcriptional level

regulates gene expression by controlling the rate of transcription

what does the control of gene expression in prokaryotes involve?

involves operons

operon

(section of DNA that contains) a group of genes that are expressed together and are controlled by a promoter

[operons are more common in prokaryotes]

example of operon and its role and why important

the lac operon (found in E.coli [a bacteria])

→ it controls the metabolism of lactose

→It allows bacteria to use lactose as an energy source when glucose is not available

[-E.coli respires glucose but uses lactose when glucose is not available]

parts of the lac operon

regulatory gene- codes for the repressor protein [lac I]

promoter- the site where RNA polymerase binds to [tip: both have p]

operator- the site where the repressor protein binds to

structural gene- genes that code for enzymes that break down lactose [lac Z, Y and A]

[ tip: po → promoter, operator ]

[this is for lac operon. other operon use diff proteins and their structural genes code for diff proteins for their function]

structural genes and regulatory genes

→ which genes are involved in gene regulation and which are not

regulatory genes are involved

structural genes are not involved

lac operon is transcribed when E.coli is respiring lactose

how does lac operon function when lactose is absent? [/ glucose present]

-the lac operon is switched off [structural genes are not transcribed]

-the regulatory gene produces a repressor protein

-repressor protein binds to operator

-this blocks the RNA polymerase from binding to the promoter so transcription cannot take place

-the RNA polymerase cannot transcribe the structural genes

-The enzymes for lactose metabolism aren't produced

how does lac operon function when lactose is present? [/ glucose not present]

-lactose binds to the repressor protein

-this causes the repressor protein to change shape so it cannot bind to the operator

-RNA polymerase binds to promoter and transcribes the structural genes

-The enzymes for lactose metabolism are produced

difference between regulatory gene and structural gene

regulatory

produces repressor protein [/ transcription factor]

regulatory gene controls the expression of structural gene→ product switches structural gene on or off

structural

produces enzymes [/ proteins]

what does lactose act as?

lactose acts as an inducer [binds to the repressor protein and inactivates it]

when is the rate of transcription increased?

the rate of transcription increases when glucose level is low which increases cAMP level

how is rate of transcription increased by cAMP?

-when glucose level is low, cAMP level increases

-cAMP binds to the CRP (cAMP receptor protein)

-the CRP-cAMP complex upregulates the transcription of the lac operon

what happens when there’s a high level of glucose? [/glucose and lactose present]

-when glucose level is high, cAMP level decreases

-CRP no longer binds to cAMP

-The CRP–cAMP complex is not formed

-this downregulates the transcription of the lac operon

when both glucose and lactose are present, why is glucose used?

bcs glucose is a more efficient energy source. It can be directly used in glycolysis. Less energy is required to break down glucose than lactose.