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biology

Operon

Operon is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Operon rather than just read about it. In short: In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product.

Operon — main illustration
Operon — illustration

Key takeaways

  • Operon belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Operon to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Operon from memory before moving on to harder problems.

Reference excerpt

In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product. The result of this is that the genes contained in the operon are either expressed together or not at all. Several genes must be co-transcribed to define an operon. Originally, operons were thought to exist solely in prokaryotes (which includes organelles like plastids that are derived from bacteria), but their discovery in eukaryotes was shown in the early 1990s, and are considered to be rare. In general, expression of prokaryotic operons leads to the generation of polycistronic mRNAs, while eukaryotic operons lead to monocistronic mRNAs. Operons are also found in viruses such as bacteriophages. For example, T7 phages have two operons. The first operon codes for various products, including a special T7 RNA polymerase which can bind to and transcribe the second operon. The second operon includes a lysis gene whose function is to cause the host cell to burst.

History The term "operon" was first proposed in a short paper in the Proceedings of the French Academy of Sciences in 1960. From this paper, the so-called general theory of the operon was developed. This theory suggested that in all cases, genes within an operon are negatively controlled by a repressor acting at a single operator located before the first gene. Later, it was discovered that genes could be positively regulated and also regulated at steps that follow transcription initiation. Therefore, it is not possible to talk of a general regulatory mechanism, because different operons have different mechanisms. Today, the operon is simply defined as a cluster of genes transcribed into a single mRNA molecule. Nevertheless, the development of the concept is considered a landmark event in the history of molecular biology. The first operon to be described was the lac operon in E. coli. The 1965 Nobel Prize in Physiology and Medicine was awarded to François Jacob, André Michel Lwoff and Jacques Monod for their discoveries concerning the operon and virus synthesis.

Overview

Operons occur primarily in prokaryotes but also rarely in some eukaryotes, including nematodes such as C. elegans and the fruit fly, Drosophila melanogaster. rRNA genes often exist in operons that have been found in a range of eukaryotes including chordates. An operon is made up of several structural genes arranged under a common promoter and regulated by a common operator. It is defined as a set of adjacent structural genes, plus the adjacent regulatory signals that affect transcription of the structural genes. The regulators of a given operon, including repressors, corepressors, and activators, are not necessarily coded for by that operon. The location and condition of the regulators, promoter, operator and structural DNA sequences can determine the effects of common mutations. Operons are related to regulons, stimulons and modulons; whereas operons contain a set of genes regulated by the same operator, regulons contain a set of genes under regulation by a single regulatory protein, and stimulons contain a set of genes under regulation by a single cell stimulus. According to its authors, the term "operon" is derived from the verb "to operate".

As a unit of transcription An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for more than one protein). However, the definition of an operon does not require the mRNA to be polycistronic, though in practice, it usually is. Upstream of the structural genes lies a promoter sequence which provides a site for RNA polymerase to bind and initiate transcription. Close to the promoter lies a section of DNA called an operator.

Operons versus clustering of prokaryotic genes All the structural genes of an operon are turned ON or OFF together, due to a single promoter and operator upstream to them, but sometimes more control over the gene expression is needed. To achieve this aspect, some bacterial genes are located near together, but there is a specific promoter for each of them; this is called gene clustering. Usually these genes encode proteins which will work together in the same pathway, such as a metabolic pathway. Gene clustering helps a prokaryotic cell to produce metabolic enzymes in a correct order. In one study, it has been posited that in the Asgard (archaea), ribosomal protein coding genes occur in clusters that are less conserved in their organization than in other Archaea; the closer an Asgard (archaea) is to the eukaryotes, the more dispersed is the arrangement of the ribosomal protein coding genes.

General structure

An operon is made up of 3 basic DNA components:

Promoter – a nucleotide sequence that enables a gene to be transcribed. The promoter is recognized by RNA polymerase, which then initiates transcription. In RNA synthesis, promoters indicate which genes should be used for messenger RNA creation – and, by extension, control which proteins the cell produces. Operator – a segment of DNA to which a repressor binds. It is classically defined in the lac operon as a segment between the promoter and the genes of the operon. The main operator (O1) in the lac operon is located slightly downstream of the promoter; two additional operators, O2 and O3 are located at -82 and +412, respectively. In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes. Structural genes – the genes that are co-regulated by the operon. Not always included within the operon, but important in its function is a regulatory gene, a constantly expressed gene which codes for repressor proteins. The regulatory gene does not need to be in, adjacent to, or even near the operon to control it. An inducer (small molecule) can displace a repressor (protein) from the operator site (DNA), resulting in an uninhibited operon. Alternatively, a corepressor can bind to the repressor to allow its binding to the operator site. A good example of this type of regulation is seen for the trp operon.

… excerpt ends here. Continue reading the full article.

Illustrations

Operon: 1: RNA Polymerase, 2: Repressor, 3: Promoter, 4: Operator, 5: Lactose, 6: lacZ, 7: lacY, 8: lacA. Top: The gene is essentially turned off. There is no lactose to inhibit the repressor, so the repressor binds to the operator, which obstructs the RNA polymerase from binding to the promoter and making lactase. Bottom: The gene is turned on. Lactose is inhibiting the repressor, allowing the RNA polymerase to bind with the promoter, and express the genes, which synthesize lactase. Eventually, the lactase will digest all of the lactose, until there is none to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.
1: RNA Polymerase, 2: Repressor, 3: Promoter, 4: Operator, 5: Lactose, 6: lacZ, 7: lacY, 8: lacA. Top: The gene is essentially turned off. There is no lactose to inhibit the repressor, so the repressor binds to the operator, which obstructs the RNA polymerase from binding to the promoter and making lactase. Bottom: The gene is turned on. Lactose is inhibiting the repressor, allowing the RNA polymerase to bind with the promoter, and express the genes, which synthesize lactase. Eventually, the lactase will digest all of the lactose, until there is none to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.
Operon: Arrangement of genes within the trp operon of three bacterial genomes.
Arrangement of genes within the trp operon of three bacterial genomes.

Worked examples

Example 1 — a first encounter with Operon

Start with the simplest possible case. Write down what Operon claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Operon before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Operon ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Operon

In research
Operon appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Operon in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Operon is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gene expression, Operons, so understanding it makes those chapters shorter.
In everyday life
Look for Operon outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Operon in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Operon means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Operon out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Operon in simple terms?

In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated sep…

Why does Operon matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Operon?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Operon.

Tags

  • Gene expression
  • Operons

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