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Jacob-Monod: The Lac operon
Explore the Jacob-Monod model and its groundbreaking insights into gene expression. Dive into the workings of the Lac Operon, a cluster of genes regulated by a single promoter. Discover how cells control enzyme levels, switch energy sources, and manage gene expression through the interaction of inducer and repressor molecules. Created by Tracy Kim Kovach.
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- This only pertains to E coli correct ? This doesn't happen in the human genome?(11 votes)
- Yes, this type of gene expression is only in prokaryotes and particularly E. coli(19 votes)
- You already have this video under "DNA" - Jacob Monad Lac Operon..
:)(0 votes)- This is a different video (different narrator), but same concept. I personally like the two alternative explanations.(28 votes)
- in the vid she says that the repressor protein inhibits rna polymerase from binding.but later on she says that the rna polymerase is bound. is it a mistake or am i missing something?(3 votes)
- Because of the location of the repressor protein on the operator, it inhibits the RNA polymerase from binding to to the operator. The RNA polymerase is still bound to the DNA strand at the promoter. When the repressor protein is removed, the RNA polymerase, still bound to the promoter, can begin transcription.(17 votes)
- what does she mean by constitutively expressed?
transcribed by default on base line??means that even when lactose is not needed ?
base line??(5 votes)- Yes, constitutive regulation implies that it is always active or an inherit part of its cellular role. In this case, she states that the lacI gene (for repressor protein) is constitutively expressed, or in general terms as you stated always transcribed.(6 votes)
- At3:20, she says "Now when glucose is readily available, the repressor protein..." Did she mean glucose or Lactose?(2 votes)
- She meant glucose. When glucose is readily available the cell does not need to transcribe the genes that are involved in lactose degradation, and this is done by the repressor protein binding onto the operator, which represses transcription of the Lac Z, Lac Y, and Lac A genes.(5 votes)
- At around1:00she says that the genes in an operon might undergo trans-splicing to create single mRNAs that are translated separately. What exactly is trans-splicing and does it even occur in E. coli since mRNA does not undergo processing in prokaryotes?(4 votes)
- At5:30she says decreased lactose in the cell leads to decreased repressor protein binding but it should be either: decreased allolactose binding to repressor protein, or increased repressor protein binding to operator thus blocking Lac Operon transcription.(3 votes)
- So she says that once the inducer binds to the repressor the RNA polymerase moves down the DNA strand and transcribes the genes. My question was how does the RNA polymerase make it back to the site of the regulatory genes - to the spot before the repressor - after the inducer breaks away from the repressor so that a new cycle of gene expression can take place in the future?(3 votes)
- Why don't they talk about CAP and cyclic AMP in the absence of glucose? I thought the duality of lac operon control was one of the most important concepts...(3 votes)
- CAP and cAMP are a part of positive gene expression feedback, Lac Operons, however, are a part of negative gene expression. so they are different things.(1 vote)
- Am I right to assume that the repressor promoter is there to ensure that the repressor protein is continually produced?(3 votes)
Video transcript
Voiceover: So the Jacob-Monod model for gene expression
describes the very first genetic regulatory mechanism
to be understood clearly. When it was first described
by French biologist Francois Jacob and Jacques Monad, who originated the idea that the control of enzyme levels in cells occurs through the regulation of transcription. And they, along with another scientist, shared the 1965 Nobel Prize in Medicine for their work on what
is called the Lac Operon. Now a little bit of background first: An operon is a unit of genomic DNA containing a cluster of
genes that are under control of a single regulatory signal, otherwise known as a promoter. And these genes are co-transcribed into a single mRNA strand and
either translated together or undergo trans-splicing
to create single mRNA's that are translated separately. So, basically, genes in
an operon are expressed either altogether or not at all. Now, the operon that I've drawn here happens to represent the lac operon, and the lac operon is an example of an inducible set of genes
which are responsible for importing and breaking
down the sugar molecule lactose to use as a source of energy. So, in the event that glucose,
which is the ideal source of carbon and energy for a cell, if that's not available then
the cell has sort of a backup source of energy in the form of lactose. And you can see where the
name lac operon comes from because it is named for the
inducer molecule for the operon. And what do I mean by inducer molecule? Well, it is the presence of lactose that actually induces the
transcription of the genes in this lac operon, which I'll
explain in just a little bit. So, there are three coordinately regulated genes contained in the lac operon. You have the lacZ gene, which codes for an enzyme called beta-galactosidase, which is a cytoplasmic enzyme that cleaves lactoce into glucose and galactose. The next gene is the lacY gene, which codes for lactose permease, which is a cytoplasmic membrane protein that transports lactose into the cell. And then finally you have the lacA gene, which codes for
thiogalactoside transacetylase. Now only the lacZ and the lacY gene are actually needed
for lactose catabolism. LacA is not as important in terms of understanding how the lac operon works. Now besides these three structural genes, lacZ, Y, and A, there are two regulatory sequences contained in the lac operon, and they are called the promoter, which promotes the
transcription structural genes if you will, and then also the operator. And there are two other
regulatory sequences that lie just upstream of the lac operon that are genes that encode
for a repressor protein, and then you have the associated promoter for that repressor protein. So, these are the structural genes here, and then here are the regulatory genes. Now, when glucose is readily
available to the cell, the repressor protein is
constitutively expressed, meaning that it is
transcribed at base line and that is just the default. And this regulatory protein
binds to the lac operator, and this interferes with and represses the binding of RNA polymerase which wants to bind here to the lac promoter. And this prevents and
represses the transcription of these genes for lactose metabolism. Now, when glucose is not
readily available to the cell, and an alternate source
of energy is available in the form of lactose,
then things start to change. First, lactose passively enters the cell at a pretty slow rate, and
the metabolite of lactose, called allolactose, then
binds to the repressor, and this alters the confirmation of this repressor protein, or it's
shape, and it causes it to sort of loosen up and
fall off the operator. Now, remember that the
RNA polymerase is bound to the promoter immediately
upstream of the genes. With the repressor now
gone, RNA polymerase is free to sort of,
picture it rolling down to transcribe all the three genes, leading to higher levels
of the encoded proteins. So then you have lactose permease, which allows more lactose
to enter the cell, and then you have more beta-galactosidase which can break down the lactose into galactose and glucose to be used for the cells basic metabolic needs. Now, what happens if both glucose and lactose are present? Which one does the cell prefer? Well, in that case, the
transport of glucose actually blocks the
transport of the inducer, the lac operon, the lactose, in a process that's called inducer exclusion. So, actually the transport
of glucose into the cell leads to the formation of
this protein intermediate that binds to the lactose
permease and prevents it from bringing in any more
lactose into the cell. Then you have decreased lactose, which leads to decreased
repressor protein bindings, so then the repressor protein then sort of latches back onto the operator there and prevents the transcription of the rest of the lac operon genes. Now, there are two key take away points from the lac operon model. The first is to realize
that it is the interaction between the inducer and the repressor molecules that mediate gene expression. And the second idea is that
the cell expends energy to make enzymes only when necessary. So, there are inducible
genes whose transcription is induced when a particular
molecule is present.