A bacterium carries many genes, yet it does not run them all at once. When lactose is around, one set of genes switches on; when it is not, the set stays quiet. A small unit keeps that account: the operon.
1. The operon: a small team of genes
In 1961 Jacob and Monod proposed the operon model to explain how gene action is controlled. In a prokaryote such as E. coli, the unit of gene expression is the operon.
An operon has four parts: the structural gene, the promoter gene, the operator gene and the regulator gene.
The structural gene makes enzymes. The promoter gene, also called the stimulator gene, is where RNA polymerase attaches.
The operator gene, also called the driver gene, controls how much protein the structural genes make. The regulator gene, in turn, controls the operator gene.
The drawing below follows Figure 1.37. It has five points; tap them one by one and match each name to its job.
Figure 1.37tap a point
Four parts of the operon
The regulator gene. It controls the operator gene.
The promoter gene, also called the stimulator gene. Two names, one thing. RNA polymerase attaches here.
The operator (driver) gene. It controls protein production by the structural genes.
The structural genes. They make the protein, that is, the enzymes. They lie one after another and are transcribed into one mRNA.
RNA polymerase. It attaches at the promoter gene.
All five seen. Control runs from regulator to operator to structural genes.
Drawn by hand after the book's figure, not to scale.
The regulator gene often makes a repressor protein. The repressor blocks transcription, and the operon is then inactive.
2. Two operons
Each prokaryote has several operons. The book names two: the lactose operon and the tryptophan operon.
Their conditions run opposite ways. The lactose operon is active when lactose is present. The tryptophan operon is active when tryptophan is absent.
The number of structural genes differs too: three in the lactose operon, five in the tryptophan operon.
Flip the switch on both operons: add the substance, take it away, and see whether the genes run.
operon switchadd and take away
When do the genes run?
The structural genes of one operon lie one after another and are transcribed together into one mRNA. Now drop four features into the right operon's box.
sort thempick a chip, then a box
Two operons
Pick up a chip, then drop it in a box.
Right. Next one.
Not yet. Think: three or five? And is the condition presence or absence?
All in place. Lactose operon: lactose present, three genes. Tryptophan operon: tryptophan absent, five genes.
3. Gene expression in a eukaryote
The process by which a gene takes part in protein synthesis is called the action of the gene.
Gene expression in a eukaryote runs in five steps, in this order: transcription, mRNA processing, translation, post-translation processing and feedback inhibition.
Not every gene works. Genes in euchromatin are active; genes in heterochromatin are not.
4. How prokaryote and eukaryote genes differ
In a prokaryote, closely related genes are transcribed together through operons. In a eukaryote, genes usually lie separately, and each has its own control.
A hormone-responsive eukaryotic gene carries a special sequence near its promoter: the hormone response element.
RNA polymerase comes in one kind in a prokaryote and three kinds in a eukaryote.
In a prokaryote, RNA polymerase reactivates the promoter with the help of one peptide subunit. In a eukaryote, many proteins join the start of transcription, and promoters are of many kinds. Tap the cells of the grid to check.
gridtap a cell
Gene differences
prokaryote
eukaryote
gene arrangement
together in operons
separate
RNA polymerase
one kind
three kinds
initiation
one subunit
many proteins
promoter
-
many kinds
Related prokaryote genes are transcribed together in operons. Eukaryote genes lie separately, each with its own control.
One kind of RNA polymerase in a prokaryote, three kinds in a eukaryote.
In a prokaryote, RNA polymerase reactivates the promoter with one peptide subunit. In a eukaryote, many proteins join the initiation.
Eukaryote promoters are of many kinds. See the row above: many proteins join the initiation as well.
5. Check yourself
Three questions; the first is about the parts of the operon.
check yourselfpick one answer
Question 1
Which describes when the lactose operon and the tryptophan operon work?
Right. The lactose operon works when lactose is present and the tryptophan operon works when tryptophan is absent.
Both conditions are reversed in this answer.
An operon is not always on; it is switched by the presence or absence of a substance.
The tryptophan operon responds to the lack of tryptophan, not to lactose.
For the second, recall the control chain.
check yourselfpick one answer
Question 2
How many structural genes are there in the lactose operon and the tryptophan operon respectively?
Right. The lactose operon has three structural genes and the tryptophan operon has five.
The order is reversed; lactose has three and tryptophan has five.
The tryptophan operon has five structural genes, not three.
Neither of these numbers matches either operon.
In the last one, watch the numbers and the conditions.
check yourselfpick one answer
Question 3
To which gene of the operon does RNA polymerase attach?
The structural gene makes the enzyme; it is not where RNA polymerase binds.
The operator gene controls protein production by the structural genes.
Right. RNA polymerase attaches to the promoter (initiator) gene.